Troubleshooting and

Repairing LCD TVs

By John Preher

www.preher-tech.com

1

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2

Dedication

This book is dedicated to my wife Lindsay , my daughter Alana and my son Kobin. With out

the support of my wonderful family this book would have never been possible.

I would also like to dedicate this book to Jestine Yong, Author and electronics technician.

Thank you for all your support and inspiration.

3

Contents

Liquid Crystals....6

Liquid Crystal Displays....10

The Power Supply Board....15

The Inverter Board....22

The Main Board....26

The Controller/T-Con Board....34

The LCD Driver Board....35

The Standby Circuit....36

The Liquid Crystal Display In Depth....37

Tools....41

Test Equipment....50

Schematic Diagrams....58

Understanding and Testing Resistors....59

Understanding and Testing Capacitors....63

Understanding and Testing Inductors....73

Understanding and Testing Transistors....77

Understanding and Testing Diodes....90

Understanding and Testing Bridge Rectifiers....103

Understanding and Testing LEDs....110

Understanding and Testing Switching Transformers....113

Understanding and Testing Opto-Isolators....117

Understanding and Testing Voltage Regulators....123

Understanding and Testing Switches....126

Understanding and Testing Fuses and Varistors....128

Some Testing Tips....131

Useful Formulas....132

How To Disassemble An LCD TV....133

Voltage Test Points....136

The Tap Test....144

Freeze Spray and Hair Dryers....146

Connection Problems....149

PSU (Power Supply Unit) Failures....152

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Inverter Board Failures....158

Main Board Failures....163

No Video....169

No Audio....171

OSD/Menu Failure....172

White Screen....173

Rainbow Screen....175

Screen Flashes Then TV Shuts Down....176

No Back light....177

Lines In Picture....178

Cracked Panel....179

LCD TV Repair Case Histories....180

Safety....186

Conclusion....190

Further Reading....191

Websites and free download(LCD TV Display Failure Symptoms and

Possible Causes.)....194

Part, Tool and Test Equipment Distributors....195

5

Liquid Crystals

Liquid crystals are a type of matter that posses properties between those of

standard liquid and those that are more like solid crystal. Liquid crystal

may flow like a liquid but at the molecular level it is oriented in a way that

normally represents a solid crystal.

Liquid crystals molecules are rod like and can be aligned when electricity

is applied to them.

Liquid Crystals

6

Each sub-pixel or cell of an LCD panel is made up of liquid crystal

molecules suspended between two transparent electrodes and two

polarizing filters. The axis of polarity for the two filters are perpendicular

to each other, so without the liquid crystals between them light passing

through one of the filters would be blocked by the next.

Make up of an LCD panel

7

Before an electrical current is applied the molecules are in a “relaxed”

state. When voltage is applied the molecules align themselves with the

electrodes. The electrodes are treated in a manner that causes the crystals

to align in a helical structure. This type is called Twisted Nematic (TN) and

is one of the most common types in LCD TVs.

Twisted nematic (TN)

Twisted nematic displays contain liquid crystal elements which twist and

untwist at varying degrees allowing light to pass through. When no

voltage is applied to a TN liquid crystal cell, the light is polarized to pass

through the cell. In proportion to the voltage applied, the LC cells twist up

to 90 degrees changing the polarization and blocking the light's path. By

properly adjusting the level of the voltage almost any gray level or

transmission can be achieved.

Other Types Include,

In-plane switching (IPS)

In-plane switching is an LCD technology which aligns the liquid crystal

cells in a horizontal direction. In this method, the electrical field is applied

through each end of the crystal, but this requires two transistors for each

cell instead of the single transistor needed for a standard thin-film

transistor (TFT) display. This results in blocking more transmission area,

which requires a brighter back light, which usually consumes more power.

Advanced fringe field switching (AFFS)

Advanced fringe field switching is a similar technology to IPS or S-IPS

offering superior performance and color, besides high luminosity.

8

Light passing through an LCD panels first polarizing filter is rotated as it

passes through the liquid crystal, allowing it to pass through the second

polarizing filter.

When no voltage is applied to the molecules in the helical (TN) structure,

they will untwist so as to not rotate light passing through them from the

first filter, this will cause the second polarizing filter to block this light.

9

Liquid Crystal Displays

A liquid crystal display contains a light source/back lighting, a liquid

crystal panel and circuits that drive the panel.

LCD Display

Back lighting

The light source is found at the very rear of the display and it contains a

group of thin CCFLs or Cold Cathode Fluorescent Lamps.

10

11

Various CCFLs

The light from the back light passes through a diffusion plate to make sure

that the light distribution across the display is uniform and then brightness

enhancement film. Note that some newer TVs use LED back or side

lighting and do not have a large bulky back light and do not require an

inverter board. Next the light passes through the LCD panel which is

made up of millions of cells. As stated earlier the cells will control the flow

of light through the display to create full color images.

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The LCD Panel

The LCD Panel Contains Multiple Layers. First a polarizing filter followed

by the color filters, TFT sheet, Liquid crystal and then a second polarizing

filter (the order can be slightly different depending on manufacturer and

panel type).

13

As we just said the LCD panel contains millions of cells ( depending on the

screen size) and three cells make up a pixel. Each cell is colored either red

green or blue. A complete pixel is made up of one green, one red and one

blue cell. Each cell is controlled by a TFT or Thin Film Transistor that

provides accurate control of each cell and makes for a clear picture.

Close look at pixels and cells (sub-pixels)

14

The Power Supply Board

Typical LCD TV PSU (Power Supply Unit)

LCD TVs Use a switching type power supply also known as an SMPS or Switch

Mode Power Supply. The power supply converts the mains AC supply into DC

(Direct Current) voltages that can be used by the circuits within the LCD TV.

AC (Alternating Current) at the mains voltage (usually 110-120V in USA) enters

the PSU and first passes through over current protection(main fuse), transient

voltage suppression(varistor), EMI (Electromagnetic Interference) filtering, in rush

current limiting(NTC thermistor) and PFC (Power Factor Correction) circuitry.

Then the AC voltage is rectified by the bridge rectifier. Note if Active PFC is used,

the active PFC circuit will be located after the bridge rectifier and before the

reservoir capacitor.

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Passive PFC Described

The simplest way to control harmonic current is to use a filter, filters are designed

that pass current only at line frequency (50z or 60 Hz). This filter reduces the

harmonic current, which means that the non-linear device now looks like a linear

load. At this point the power factor can be brought to near unity(1), using

capacitors or inductors as required. This filter requires large value high current

inductors, which are bulky and expensive. Passive PFC needs an inductor larger

than the inductor in an active PFC, but costs less.

Active PFC Described

Active power factor correction (active PFC) uses a more complex electronic

circuit to control the amount of power drawn by a load in order to obtain a power

factor as close as possible to unity(1). Usually the active PFC circuit controls the

input current of the load so that the current waveform is proportional to the mains

voltage waveform (a sine wave). The purpose of making the power factor as close

to unity(1) as possible is to make the circuit that is power factor corrected appear

purely resistive. In this case the voltage and current are in phase and the reactive

power consumption is zero. This allows the most efficient delivery of electrical

power from the power company to the consumer. Some types of active PFC are

Boost,Buck and Buck-boost. Active power factor correction circuits can be single

stage or multistage. In the case of a SMPS, a boost converter is inserted between

the bridge rectifier and the primary side filter capacitor(reservoir capacitor). The

boost converter attempts to maintain a constant DC bus voltage on its output while

drawing a current that is constantly in phase with and at the same frequency as the

line voltage.

The AC voltage Is now Rectified , output from the Bridge rectifier is a pulsed DC

voltage which is then “smoothed” by the reservoir capacitor also called the

primary side filter capacitor.

Now let's talk about the power MOSFET/s. In LCD TVs you will commonly find

two power MOSFETs in the typical half bridge topology.

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Basic Half Bridge Topology

The MOSFET is the switch in the SMPS, it is turned on by the power IC that

sends a square wave signal to the gates of the power MOSFETs in the half bridge,

turning them on and off alternately at a high frequency. When the first of the

power MOSFETs(Q1) is turned on it allows current to flow through the primary

winding of the switching transformer to the center of the voltage divider formed

by C1 and C2 . When this MOSFET switches off the second MOSFET(Q2) is

switched on and the flow of current reverses, going from the center of the voltage

divider to ground through the second MOSFET and then the process repeats. This

action causes a changing magnetic flux to exist in the transformers core which

induces voltages across the secondary windings of the switching transformer,

which steps down the voltage in this case, to AC voltages which are then again

rectified by either ultra fast recovery or schottky diodes, then filtered by secondary

side filter capacitors and inductors also called chokes because they inhibit or

“choke” high frequency changes in current. Now the rectified and filtered

secondary voltages may or may not then be further regulated by voltage regulator

ICs or regulation circuits found on other circuits or on the secondary side of the

PSU. Please note that not all LCD TVs will use the half bridge topology. Some

will only have one power MOSFET or FET using a quasi-resonant topology and

some will have the power IC and MOSFET/s integrated into a single package.

Most power supplies in LCD TVs you encounter will be fairly similar and will all

be SMPS, with a little studying you will see how different ones you encounter

work because they will be based on the same principles.

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Bottom of PSU showing SMD power IC/half bridge driver

PSU with active PFC

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The output of the PSU is kept stable by utilizing feedback. At least one of the

secondary voltages must be monitored, this is done by a few circuits. First the

sampling circuit which is normally made up of a few resistors. The voltage from

the sampling circuit is then used as the reference for an adjustable shunt regulator

IC, this is the error detection circuit which monitors the sampled voltage taken

from the sampling circuit and coltrols the current flow through the LED of an

opto-isolator which has an output signal that is amplified and then taken to the

power ICs feedback pin so that the power IC can then alter the mark to space ratio

of the square wave signal to the MOSFET/s causing a regulation of the output

voltage increasing or decreasing the output or even shutting down the TV

depending on the signal received from the opto-isolator. This process is called

Pulse Width Modulation or PWM. The power IC is sometimes referred to as the

PWM(Pulse Width Modulator). If the load on the power supply causes the

secondary voltages to drop then the power IC increases the MOSFET/s drive

signals duty cycle or you could say the ratio of marks to spaces increases.

Mark to space ratio

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PWM

Some PSUs will use a different type of feed back implementing a secondary

winding on the primary side that is used for feedback, still the over all principle is

the same and the process is still PWM.

Basic SMPS Block Diagram

To go completely In depth into the workings of the SMPS is beyond the scope of

this book, but you should now have a good idea of how an SMPS in an LCD TV

works.

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I do suggest you read more on SMPS and how to repair them because it will

definitely speed up your troubleshooting time. We will go over the SMPS much

more in this book but I would still like to recommend you read “Troubleshooting

and Repairing Switch Mode Power Supplies” By Jestine Yong.

This book is packed with all you need to know to completely understand how to

troubleshoot SMPS quickly.

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The Inverter Board

The inverter board is responsible for stepping up and converting the low voltage

DC supplied by one of the outputs from the SMPS into a high voltage, roughly

1500V-1800V AC for striking(start up) and 500V-1000V AC to run the CCFL

lamps that provide the back lighting for the LCD panel.

For many years designers have used a buck/royer inverter topology to strike and

supply power to the CCFLs. This topology is basically a combination of a step

down buck regulator, a royer oscillator and a step up transformer.

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Buck Royer circuit simplified

The buck regulator is made up of a power transistor, buck choke, buck diode, buck

coil, power inductor, a PWM or inverter IC and a capacitor.

Simplified Buck Converter

23

The royer oscillator consists of two transistors, capacitor, HVT (High Voltage

Transformer) and a capacitor in series with the lamp called the ballast capacitor.

The buck royer inverter supplies a high voltage AC to drive the CCFL lamps.

I don't think you will come across to many LCD TVs with the Buck/Royer style

inverter circuit but it is still good to understand how different circuits work as you

will see similar circuits employed in different parts of the TV and other electronics

devices you decide to repair. The majority of inverter circuits you will encounter

in LCD TVs will implement a different type of inverter circuit such as the direct

drive topology.

Direct drive topology

The direct drive inverter uses a simple topology that optimizes performance, has a

reduced cost and lowers component count by eliminating the buck choke, buck

diode, resonant capacitors and transistors found in a Buck/Royer oscillator. The

direct drive topology uses an inverter IC(PWM) to drive a pair of MOSFETs

connected to a HVT primary winding, switching the MOSFETs on and off at

separate times, allowing current to flow through the primary windings center tap

and back and forth through the primary winding and one of the MOSFETs to

ground. The type of direct drive inverter we just discussed may also be referred to

as a push pull circuit.

Another common type of inverter you will see is the full bridge inverter.

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Full bridge topology

The full bridge inverter is like the direct drive inverter I showed you earlier only

the center tapped primary is no longer required. The MOSFETs are in a classical

H-Bridge topology that is used to reverse the current flow through the primary

winding of the high voltage transformer. You will commonly find this type of

inverter in today's LCD TVs. Note that some TVs you will encounter will have the

PSU and the inverter integrated into one board. You may also encounter some

other inverter topologies like the half bridge inverter.

25

The Main Board

As the Name implies the main board has many functions within the LCD TV.

You may also hear this board referred to as the A/D board, logic board, digital

board and also the scaler board. The purpose of the main board is to take the input

video and audio signals convert the analogue video signal into a digital signal that

the controller board can send to the driver ICs which then the TFTs in the panel

and control the picture. The audio is taken to the audio processor than an audio

amplifier which then drives the speakers. Sometimes all of the video and audio

inputs will be found on the main board and sometimes they will be found on a

separate board called the jack pack, which connects to the main board via a ribbon

cable or FFC (Flat Flexible Cable). This board may also house the audio processor

and audio amplifier ICs and corresponding circuits.

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LCD TV jack pack/Input or signal board

27

Next we will discuss important components found on the main board.

VPU(Video Processing Unit)

The video processing unit is a highly integrated circuit that includes a

CPU(Central Processing Unit), HD(High Def.)/SD(Standard Def.) video and audio

decoder, NTSC video decoder,OSD(On Screen Display)comb filter a video scaler

and de-interlacer. To fully explain the VPU is beyond the scope of this book and I

suggest you do further studying if you want o understand the VPU better. The

main thing for you to understand is it converts video information into digital

signals that can be sent by the LVDS IC to the controller/T-Con board.

VPU on LCD TV main board

MCU(Micro Controller Unit)

A micro controller is a small computer made up in a single integrated circuit

consisting of a fairly simple CPU(Central Processing Unit) along with support

functions like crystal oscillator, timers, watchdog timer, serial and analog I/O etc.

Program memory like NOR flash or OTP ROM can be included on chip as well as

small amounts of RAM. The MCU carries out small dedicated tasks within the

LCD TV.

28

MCU on LCD TV main board

EEPROM(Electronically Erasable Programmable Read Only Memory)

EEPROMs are a type of non-volatile memory used in electronic devices. Just as

the name implies an EEPROM can be erased and programmed with electrical

signals. EEPROMs are used to store information such as user adjustable settings

and preferences among other things. When you make for instance a brightness

adjustment the MCU may store this information in an external EEPROM.

EEPROMS on LCD TV main board

29

Voltage Regulator ICs

Voltage regulator ICs provide a constant stable voltage for the ICs and other

circuits found on the main board.

Flash Memory

Flash memory is non-volatile and it is a specific type of EEPROM that is erased

and programmed in large blocks. Flash memory costs much less than byte-

programmable memory EEPROM and so is dominant wherever a large amount of

non-volatile memory is needed. The LCD TV software is usually stored on flash

memory and this software can sometimes be upgraded through a USB port or

memory card reader on your TV.

30

Audio Processor

The audio processor receives digital and analogue audio signals input to the TV

and converts them into a signal that can be used by the audio amplifier to drive the

speakers and also to decode and send audio to peripheral devices.

Audio Amplifier

The audio amplifier as the name implies is responsible for receiving the signal

from the output of the audio processor which is small in amplitude and using it to

drive a signal with larger amplitude but the same modulations through the TV

speakers.

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LVDS(Low Voltage Differential Signaling)IC

The LVDS IC uses low voltage differential signaling to send the video signal from

the main board to the T-Con/LCD controller board . LVDS is a differential

signaling system, meaning that it transmits two different voltages that are

compared at the receiving end. LVDS uses this difference in voltage to encode the

video signal.

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Crystals

The function of the crystal is in combination with other components to create an

electrical signal with a very precise frequency. This frequency is used to provide a

stable clock signal for an IC. The most common type you will see in the LCD TV

is the quartz crystal oscillator.

Again remember that not all LCD TVS are the same. In this book I am giving you

examples of what is commonly seen in LCD TVs on the market today. You will

find that some TVs will incorporate different technologies on their main boards. It

is up to you to do further studying as needed like reading the service manual and

going over the schematics for the TV you are working on if possible, but always

be learning about electronics and electronics circuits so can quickly identify them

when you see them and troubleshoot them quickly as you know how they operate.

33

The LCD Controller Board

The LCD Controller or T-Con PCB receives the LVDS signal from the Main

Board which it processes into TFT Drive Signals and then through the

driver board controls the LCD Panel driver ICs.On the T-con PCB you will find

Dynamic Ram IC’s which are High Speed Storage Devices used to store data until

it is time to be addressed.12V is usually supplied to the T-con Board through the

cable from the main board to the T-con board. This voltage is easily measured at

the picofuse on the T-con board.

LCD controller board

34

The LCD Driver Board

The LCD driver board is directly bonded to the LCD panel by flexible printed

circuit(FPC). The driver board directs the signal from the LCD controller to the

driver ICs which are mounted directly to the FPC that bonds the driver board to

the LCD panel and on FPC down the side of the panel. The mounting of the driver

IC on the FPC is often referred to as COF(Chip On Film) or TCP(Tape Carrier

Package). Sometimes you will see different configurations like the T-Con/LCD

controller board and driver board can be integrated into one board.

35

The Standby Circuit

The standby circuit is used to supply power to the MCU and other components in

the LCD TV when the TV is off, this is why it is called standby mode. Really the

TV is not off completely unless it is unplugged. This is how you are able to turn

the TV on when the TV is in standby mode. When you push the power button on

the remote control or on the keyboard located on the TV a signal is sent to the

MCU that tells the MCU to send an “on” or “start up” signal to the power IC so it

will start driving the power MOSFETs which causes the TV to turn on. The

standby circuit is found on the SMPS board and is easily located by it's small

switching transformer. The usual standby voltage is 5V DC. The standby power

supply is an SMPS usually with the PWM and MOSFET integrated into a single

standby power IC, small switching transformer, secondary diode, filter capacitors,

feedback circuit etc. It is a fully functional SMPS only really small, an SMPS

within an SMPS.

Click on the link below to see a repair I did on an LCD TV with a standby circuit failure:

http://preher-tech.com/Documents/Sharp%20Aquos%20LC32D43U%20no

%20power.pdf

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The Liquid Crystal Display In Depth

As we stated before the Liquid Crystal Display contains many layers. A back light,

polarizing filters, color filters, TFT layer and liquid crystal. The very back of the

panel is a back light which contains multiple CCFL lamps. Some newer TVs use

LEDs for back lighting or edge lighting with a light guide, allowing light to evenly

illuminate the entire picture even though the light source is around the edge and

not directly behind. The light passes through the actual LCD panel that contains all

the tiny red, green and blue cells that make up the pixels allowing the picture the

display produces to be illuminated and seen.

Panel Types

Passive Matrix-

Passive matrix panels use a simple grid to address a particular pixel in the display.

As the number of pixels and the corresponding columns and rows of the grid

increase this type of display becomes infeasible. Slow response times and bad

contrast are typical with this type of display.

37

Active Matrix-

Modern LCD TVs use the active matrix structure. The matrix is made up with TFT

(Thin Film Transistors). Each cell within a pixel has its own dedicated transistor.

This allows each cell to be activated individually.

Active matrix addressed displays are brighter, sharper and generally have better

response times not to mention producing better images than passive matrix

addressed displays of the same size.

Response Time

Response time is the amount of time it takes for a liquid crystal cell to change

from activated or white to inactive or black and then return to white. Basically it

refers to the speed of the liquid crystal cells and how fast they can change from

one state to another and so how fast the images can be refreshed on the screen. The

faster the response time the better. This reduces the effect of trailing or ghosting

that can be caused by slow response times. Typical response times are from 4ms-

16ms.

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Contrast Ratio

Contrast ratio is the ratio of the TVs brightest white it can display in comparison to

it's darkest black.

Viewing Angle

the viewing angle of the TV is literally the angle at which it is best viewed from.

Usually the horizontal and vertical viewing angles will be listed in the users

manual. Ideally a TV would have a viewing angle of 180 degrees both horizontally

and vertically, which would mean it could be viewed even if you were standing at

the very side or looking at it from the very top or bottom. Modern LCD TVs have

a wide viewing angle, usually around 170 degrees horizontally , vertical viewing

angle can vary. When a TV has a small viewing angle you will notice the picture

fade and the colors distort as you move up and down or side to side relative to the

TV.

Resolution

The resolution of an LCD TV is the number of distinct pixels it can display. It is

simply the physical number of columns and rows of pixels creating the display.

LCD TVs commonly display the following resolutions.

SDTV(Standard Definition TV): 480i

EDTV(Enhanced Definition TV): 480p(720 x 480)

HDTV(High Definition TV): 720p(1280 x 720)

HDTV: 1080i (1920 x 1080)

HDTV: 1080p (1920 x 1080)

The i stands for interlaced scan. This means for each frame you have two “fields”

during the first field the display is scanned for part of frame and then skips a piece

of that same frame then scans another piece until the end of that field , then the

process repeats filling in the parts that were missed in the first field scan. The two

fields together make up one frame.

39

The p stands for progressive scan. This is when the scan starts at the top of the

panel and drives every necessary cell all the way down the screen completing an

entire frame in one sweep as appose to two.

40

Tools

Let's discuss some tools that are necessary for repairing LCD TVs and some that

will make repairing LCD TVs much easier and reduce your troubleshooting time.

Long Nose Pliers

Long nose pliers come are great for all sorts of things including helping to remove

41

and place/mount components in places that our fingers can't fit.

Diagonal Cutters

Diagonal cutters really come in handy. Good for cutting of a strip of solder wick

and always being used to clip off components leads after soldering them in place.

Nut Driver

42

Screw Drivers

Tweezers

43

Tweezers come in handy, especially when you are removing or replacing SMD

components.

Soldering Kit

You will want to put together a soldering kit that includes such things as solder,

solder wick, solder tip tinner/cleaner, heat sink compound, a solder sucker some

dental picks and a “solder aid kit”.

44

Optical Visor With Light

An optical visor is something I just could not do without, I use them to find bad

solder connections on PCBs and I basically wear them the whole time I am

working, when soldering and to look at part values for instance on SMD

components, it would be very difficult to work with out these and it is important

that they have a light so you can keep your hands free for things like a soldering

iron and solder etc. Without proper lighting and magnification it would not be

possible to even see a lot of the connection problems I have found while I was

wearing them.

45

Example of solder cracks that you might not see without a light and magnifier

Variable temperature soldering station with LED display

A good variable temperature soldering station is an essential if you plan on doing

component level repairs on LCD TVs. I recommend spending the money to get a

nice station with variable temperature like the one in the above photo. It will

surely pay for itself in just a few repairs.

46

Solder Tip Cleaner

SMD rework station

47

An SMD rework station is not a must for doing LCD TV repair, but if you decide

to take on lot's of SMD level repair within the LCD TV this will surely make your

life much easier.

Chip Quick SMD removal kit

Chip Quick is one of my favorite products. This is what I choose to use over an

SMD rework station for the amount of SMD work I find myself doing. I have even

removed and replaced tsop (thin small-outline package) flash ICs with 50 pins

with this stuff and other products form their website, it is amazing.

48

Complete Electronics Tool Kit

All the tools listed are just some of the most important tools needed, but I

personally feel the more tools the better and I am known for carrying a large

amount with me most of the time. You may want to think about purchasing one of

the complete electronics tool kits like in the photo above. You can purchase these

kits from various electronics distributors online.

49

Test Equipment

DMM(Digital Multimeter)-

DMMs in general have less effect on the circuit being tested than an analogue

meter and few circuits are effected by having this type of meter connected to them.

Most DMMs have a constant input resistance of 10M ohms or more.

Digital Multimeter

The DMM is probably the piece of test equipment you will find yourself using

most as it has so many features. Many DMMs include settings for resistance, DC

and AC voltage, frequency and duty cycle, DC and AC current, capacitance,

continuity, transistor hFE, temperature, diode testing and more. Purchasing a

quality DMM is definitely a wise investment.

50

Analogue Meter-

The analogue meter is also a very useful tool for the electronics technician. The

voltages used for testing by most analogue meters are larger than with a DMM and

so they can be used to check for certain component failures that the low voltages

used with most DMMs cannot.

Analogue Meter

Analogue meters are great for testing MOSFETs, BJTs(Bipolar Junction

Transistors), opto-isolators, capacitors, LEDs and other components as well.

Analogue meters are very reasonably priced and you can find a pretty nice one

online for under $30 USD.

51

The ESR Meter-

The ESR meter is an important tool for the electronics technician or hobbyist.

Electrolytic capacitors that have increases in ESR are often the reason electronic

devices fail. Bad electrolytic filter capacitors in the SMPS can cause all sorts of

problems in LCD TVs including no or dim display, flickering display, no power no

audio etc. There is also a lot of electrolytic capacitors on the main board which can

also cause many problems like loss of OSD, artifacts/glitches in the picture no

video etc. when the capacitors fail.

ESR stands for Equivalent Series Resistance and is and effective resistance that is

used to describe the resistive aspect of the impedance of certain electrical

components. The theoretical treatment of capacitors assumes they are perfect

components contributing only capacitance to a circuit, but all physical devices are

constructed of material with some resistance to electricity. This means a capacitor

has a resistance as well as a capacitance. Capacitors also exhibit reactance which

we will not discuss.

Most electrolytic capacitors have a low ESR to begin with, depending on a

specific capacitor and the average ESR can usually be found on the spec. sheet for

a certain capacitor if you can find one. Most likely you will refer to the chart of

common ESR values that will come with your ESR meter or you can also find

them listed on the internet. Normally with a bad electrolytic capacitor the ESR is

quite higher than the typical value listed for a new capacitor. A capacitor ESR rises

over time as they are exposed to and or dissipate heat, because they contain a

liquid electrolyte when

52

they get hot the liquid expands and is vented out of the capacitor and also the

electrolyte can break down and go through chemical changes over time and

exposure to heat also causing the ESR to increase.

The ESR meter is so valuable because it allows you to quickly check the many

electrolytic capacitors found in LCD TVs and other electronic devices and very

often you can test them in circuit all though if you ever doubt the reading it never

hurts to test the capacitors out of circuit and this is a good practice with all

components or at least to unsolder and lift one of the components leads from the

circuit.

The Ring Tester-

The ring tester is an inexpensive and effective way to test any high Q(quality

factor) inductive component. In LCD TV repair the ring tester is very useful for

testing switching transformers in the SMPS and HV transformers on the inverter

board.

The components in many circuit like the SMPS and inverter board contain low

loss(high Q) resonant circuits. The ring test got its name from the fact that when a

very fast pulse of current is applied to a high Q component the tuned nature of the

component will produce a decaying AC voltage of several cycles or more. The

more rings the higher the Q. Little or no rings indicates low Q and a possible

problem.

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The Oscilloscope-

The oscilloscope or “scope” is a piece of electronic test equipment that is used to

view signal voltages and frequencies, usually as a two dimensional graph. It is

driven by an input signal that has the effect of producing a recognizable pattern

on the screen that describes certain aspects of the signal. Oscilloscopes are very

useful tools and can help to quickly locate problems within electronics devices.

I assume that many of the readers do not own an oscilloscope or have access to

one. Fortunately this book will be repairing LCD TVs with mostly a DMM ,

analogue meter, ESR meter, ring tester and other tools. I repair lots of TVs and

have for years just with mostly those meters I just mentioned, although sometimes

I will need a scope for those difficult TVs and in that case, except for a few

obvious critical waveforms, like the PWM output from the power and inverter ICs

and checking for ripple on the secondary outputs in the SMPS a schematic would

be needed to know where and what waveforms to look for in the particular model

of LCD TV you are testing.

To learn more about the oscilloscope and how to use one click on the link below.

The analogue oscilloscope

54

Digital Capacitance Meter-

A digital capacitance meter with a large range is a good tool for testing capacitors

capacitance value.

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Leak Seeker-

This is a great tool for hunting down difficult to find shorted components

especially circuits with lot's of SMD components. Simply touch LeakSeeker's

gold-plated probe on any solder pad along the suspicious trace and LeakSeeker

automatically calibrates itself to the resistance of the defect, within a 24 milli-ohm

"window". Because the test voltage is a current-limited eight volts, it will power

through good diodes to activate shorted components beyond. Touch a pad in one

direction or the other and LeakSeeker beeps higher or lower and lights the LED

distance scale to indicate that you're getting closer or further from the defect. The

24 milli-ohm window allows about 2 to 3 inches of PCB trace to be checked, then

automatically re-calibrates itself to a new window as you get closer to the short.

Because LeakSeeker has a range of zero to 150 ohms, it can locate not just

shorted, but leaky parts as well. And unlike your DVM, LeakSeeker's unique

"floating reference" circuitry doesn't give up resolution at these higher leakage

values. Once you're in the general area of the defect, switch from the AUTO to the

LOCK position, High Definition mode. This locks the "window" and increases

resolution to 0.1 milli-ohm, so you can pinpoint the location of the defect within a

quarter of an inch, even on multilayer boards with a power plane instead of

individual traces. The currently available LeakSeeker 82B HD has indicators for

"standard resolution" AUTO and "high definition" LOCK.

The solder pad where the beep is highest is the location of the defect.

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Smart Tweezers-

Smart Tweezers is an LCR(Inductance,Capacitance,Resistance) meter in a set of

tweezers. Smart Tweezers features a unique patented mechanical and electronic

design that integrates a highly accurate digital multimeter with a built-in high

precision SMD probes and a display. This light weight device can be easily held

by one hand. It is designed for component evaluation on a PCB or a production

line, component testing and sorting of SMD components. Smart Tweezers

dramatically reduces time necessary to troubleshoot or debug a complex PCB

significantly simplifying process of locating a faulty component.

Some Precautions-

Be careful with your test equipment, don't store meters in extreme conditions(e.g.

Extreme hot or cold storage). Don't slam or seriously vibrate the meter, especially

analogue meters which have delicate parts inside.

Read the manuals that come with your different meters.

Avoid making measurements that exceed the makers recommendation in any

particular setting.

Avoid accidentally attempting to make voltage or current measurements with the

meter switched to the resistance setting.

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Schematic Diagrams

Schematic diagrams can really make troubleshooting LCD TVs much easier and

can often be found online in service manuals available for download. Many times

though, the service manuals will not have that much info and have only partial or

no schematics at all. This is why it is so important to further your electronics

knowledge and to be constantly studying. The more you know about electronics

and common circuits, the easier it will be for you to recognize the circuits in the

TV you are working on and to quickly see how they work and have a good idea of

where to start troubleshooting the TV regardless of whether or not you have a

schematic diagram. If you have a strong understanding of electronics you will be

able to analyze, troubleshoot and repair any circuit. It still never hurts to see what

information you can find as the internet has made so much available to the

technician and hobbyist repairer that was not available to us in the past so easily.

Here is a link to the best source of great schematics for LCD TVs, much better

than what is found in most service manuals. SAMS Technical Publishing.

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Understanding and Testing Resistors

Resistors In An LCD TV Power Supply

A resistor opposes the flow of electrical current. Resistance is measured in ohms.

The two main characteristics of a resistor are it's resistance value in ohms and it's

power rating in watts. Never replace a resistor with a lower power rating than the

one removed from the circuit, it is OK to use one with a larger rating but never

lower. Resistor resistance values add normally when connected in series, but add

in reciprocal when connected in parallel.

Resistor Schematic Symbols

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Reading Resistors-

Resistors are marked with colored bands to indicate the resistance value. Reading

from left to right, the first band is the first digit of the resistance value the second

band is the second digit of the resistance value. The third band is the the multiplier

and determines how many zeros follow the first two digits, the fourth band is

another important value and that is the tolerance.

The tolerance is the percent plus or minus that the value of resistance can vary

from the value calculated from the color bands. Example, a resistor with bands

from left to right of red, brown, yellow and gold would be 210,000 ohms or 210k

ohms with a tolerance of 5%. Sometimes you will see resistors with five bands, in

this case the first three bands are digits, the fourth band is the multiplier and the

fifth band is the tolerance band. Some resistors like SMD ones for instance will

use numbers instead of bands but the idea is the same. For an SMD resistor with 3

numbers the first number is the first digit of the resistance value the second

number is the second digit and the third number is the multiplier. So an SMD

resistor marked 103, would have a value of 10k ohm. An R may be seen in the

value for instance 4R7, the R represents a decimal and in this case the value would

be 4.7 ohms.

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Testing Resistors-

Testing resistors can be done with your DMM or analogue meter. Determine the

value the resistor is supposed to be by using the color bands or numeric code. If

the resistor is burnt or discolored so that you can not read the bands you will need

the schematic diagram for the TV you are working on or you will have to use the

techniques described in the book, “Find Burnt Resistor Value” to determine the

resistors value through a systematic process.

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Once you know the value of the resistor you want to test is supposed to be you can

simply set your DMM or analogue meter to the proper resistance range and

measure the actual value of the resistor. You should get a resistance measurement

with in the tolerance of the given resistor, most bad resistors will have increased

in value or gone open reading O.L./ infinite resistance. Charred and burnt resistors

are obviously bad and need replacement. Always remove resistors from circuit

before testing, as surrounding components may cause erroneous readings. Wattage

is not always listed on resistors and is determined by the physical size.

Testing Resistor On LCD Main Board

Meter Reading 218.6 ohms the resistor stated 220 ohms on the side and at 5%

tolerance so this as a good resistor

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Understanding and Testing Capacitors

Capacitors In LCD Power Supply

A capacitor is a passive electronic component that stores electric charge. A

capacitor consists of two conductors separated by a dielectric(insulator). When a

voltage is present across the conductors an electric field is developed within the

dielectric which stores energy. The effect is greatest between wide, flat, parallel

conductors that are very slightly separated by dielectric.

Capacitor Schematic Symbols

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Capacitors are widely used in electronic circuits for coupling(blocking the flow of

DC while allowing AC to pass), decoupling(also called bypassing, passes DC

while bypassing AC), filtering interference, Smoothing the output from a bridge

rectifier(often called a reservoir capacitor), filtering ripple on the output of SMPS

power supplies, in timing and resonant circuits and many other purposes.

Capacitor capacitance values add normally when connected in parallel, but add in

reciprocal when connected in series. Some important characteristics are the

capacitance value(often in micro-farads) and the working voltage(the max voltage

that can be applied across the capacitor). Never replace a capacitor with a working

voltage lower than the working voltage of the capacitor being replaced. Discharge

capacitors and observe polarity when testing.

Testing Capacitors, method one-

Use your analogue meter set to the x1 ohm range and connect the test leads to the

capacitor. The meters needle should kick up and then return to infinity if it does

not flick or respond reverse the test leads, if it still does not flick try again with

your analogue meter set to x10 ohm, x100 ohm, x1k ohm and then x10k ohm

range until you get a response if the meter needle does not flick when the test leads

are applied to the capacitor in any setting than the capacitor is considered open. If

the needle flicks up and stays at zero ohms the capacitor is considered shorted,

also if it stays to any other value other than infinity after flicking it is leaky. This

method of testing is fairly old and not the best method. Just because a capacitor

can charges does not mean it is a good capacitor. You can also use your DMM set

to the resistance setting to do this test, it should show some reading and then

gradually increase until the value reaches O.L.(Over Limit), and if you reverse the

test leads it should do the same again. This test will not work on small capacitors

10nF or lower.

Method Two-

The second method for testing capacitors is to use a capacitance meter or the

capacitance setting on your DMM, if it has one. By placing the test leads of the

meter to the leads of the capacitor the meter will display the measured value.

The value of measured capacitance should be what is marked on the capacitor(plus

or minus an amount indicated by the tolerance value). Most electrolytic capacitors

in LCD TVs have a tolerance of 20%. Electrolytic capacitors will have the value

of capacitance marked on the side in micro-farads(uF) along with the working

voltage, tolerance and max operating temperature(typically 85 or 105 degrees

Celsius). Most non-electrolytic capacitors will have their value in a numerical

code marked on the side.

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Just like resistors, the first two numbers are the first two digits of the value and the

third is the multiplier. The value will be in pico-farads. These three numbers are

followed by a letter which denotes the tolerance value(the amount that the actual

value of capacitance may very from the value stated on the capacitor). Both

methods one and two are great for testing non-electrolytic capacitors like ceramic

disc, ceramic chip and polyester film type.

Method Three-

This method involves the use of an ESR meter.

ESR Meter

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Electrolytic capacitors are the most common capacitors to fail in electronic devices

and one of the most common components you will find failed in LCD TVs in

general.

Electrolytic capacitors may test fine with methods one and two but can have an

increased ESR which is causing a failure that would be missed by the first two

testing methods. In this case you need the ESR meter to find the bad capacitors. To

use the ESR meter simply place the test leads to the leads of an electrolytic

capacitor and compare the reading(in ohms) to the one on a chart of typical ESR

values for electrolytic capacitors that should come with your ESR meter. Most

ESR meters will have an ESR value chart right on the meter itself.

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You can usually check electrolytic capacitors in circuit (unless testing capacitors in

parallel), but I still recommend pulling them out of circuit or at least de-soldering

and lifting one lead of the capacitor before testing.

Using ESR meter to test SMD electrolytic capacitors on the main board

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Checking ESR Of Secondary Filter Capacitor On Power Supply Board

To sum things up the first two methods are great for testing non-electrolytic

capacitors like ceramic disc, ceramic chip and polyester film capacitors, or the first

method for testing electrolytic capacitors to see if they are open , leaky or shorted.

Also it doesn't hurt to make sure the capacitance of electrolytic capacitors is within

tolerance. The third method, using the ESR meter is the best way to test

electrolytic capacitors and these are the most common capacitors to fail in

electronic circuits, typically because of increased ESR. Rarely an electrolytic

capacitor will cause circuit failure because of lowered capacitance value and the

ESR will be within tolerance, but it does happen.

You can tell this electrolytic capacitor is bad just by looking at it, always

replace any puffy or vented electrolytic capacitors, no need to even test them

except for curiosity.

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More obviously bad electrolytic capacitors

Symbol For ESR

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Testing Ceramic Capacitors-

Ceramic Capacitors

SMD Ceramic Chip Capacitors

Use method one and two described earlier to test these types of capacitors.

Testing Ceramic Chip Capacitor On Main Board With Capacitance Setting

Of DMM

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Meter Reading in nF(nanofarads) 0.052 nanofarads or 52 picofarads

To test the high voltage capacitors like the resin coated ceramic capacitors found

on the secondary side of some inverters, you will have to use an insulation tester

as the low voltage output from your DMM or analogue meter is not enough to test

this type of capacitor for leakage or dielectric breakdown. These capacitors will

often have physical damage that you can see like a crack in the resin coating,

discoloration or burn.

5pF 3KV Ceramic Capacitors on Secondary Side Of An LCD TV Inverter

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An Insulation Tester

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Understanding and Testing Inductors

Inductors In Secondary Side Of PSU

An inductor also called a reactor, coil or choke is a component that exhibits

reactance resisting changes in current flow and can store energy in a magnetic

field when an electric current pulses through it. Since inductors resist changes in

current flow, they attenuate or “choke” high frequency AC signals making them

very useful in filters.

Schematic Symbols For Inductors

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Testing Inductors-

Testing inductors is fairly simple. Basically an inductor is a wire coil wrapped

around a core( some have no core called an air core) often made of ferrite.

Use your analogue meter set at x1 ohm or your DMM set to resistance setting and

place test leads onto the leads of the inductor. You should get a resistance reading,

often very very low ohms your DMM may even show 0 ohms. If you get an

infinite or O.L. resistance reading the inductor is considered open and should be

replaced.

Testing Inductor In LCD TV Power Supply

If the coil is not open you can also use the inductance setting of your DMM or an

inductance meter if you have one or the other and check that the inductance is

within tolerance of the value marked on the inductor.

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Inductor Color Code

75

For high Q(quality factor) low loss coils like the primary winding of the SMPS

switching transformer you should use a ring tester to check for shorts between

turns in the winding.

Place the test leads of the ring tester onto the leads of the inductor and check the

amount of LEDs that light up to indicate the “rings” for the coil you are testing,

the more LEDs the better. Most High Q coils will light up at least one green LED.

You should see what kind of reading you get from various known good inductors,

so you know what kinds of readings you should be looking for when testing

inductors in LCD TVs.

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Understanding and Testing Transistors

A transistor is a semiconductor electronic component used to amplify or switch

electronic signals. A transistor is made of doped semiconductor material junctions,

with three terminals for connection to an external circuit. A voltage or current

applied to one pair of the transistor's terminals changes the current flowing

through another pair of terminals(one terminal being common to both pairs).

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BJT-

Bipolar Junction Transistors(BJTs) are made from three sections of semiconductor

material, alternating p type and n type resulting in two pn junctions, one pn

junction existing between the emitter and the base the other existing between the

collector and the base. BJTs are classified as npn or pnp depending on the

arrangement of there n and p type material.

Schematic Symbol

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Testing BJTs-

First determine whether you will be testing an npn or pnp transistor and which

pins are the base, emitter and collector by looking the transistors part number up

online or looking in the LCD TV schematic diagram, if you can not find this

information, use the method found in this book “Testing Electronic Components”

to determine the information stated above.

Once you have determined the type (npn or pnp) of BJT you will be testing and

which leads are the base, emitter and collector you are ready to test. Set your

DMM to the diode test setting. For npn type BJT place the black test lead on the

base pin and the red test lead on the emitter pin and then with the black test lead

still on the base pin put the red test lead on the collector pin, both readings should

be O.L.(Over Limit). Next place the red test lead on the base pin and the black test

lead on the emitter pin and then with the red test lead still on the base pin place the

black test lead to the collector pin. You should get about a 0.4V-0.7V voltage drop

for both readings. Now switch the DMM to the resistance setting, place the black

test lead on the collector pin and the red test lead on the emitter pin, you should

get an O.L. reading and if you reverse the test leads so that the black test lead is on

the emitter pin and the red test lead is on the collector pin, you should again get an

O.L. reading. You can also just leave the meter in the diode test setting when

checking between collector and emitter, you should still get an O.L. reading in

both directions, getting a low voltage drop reading across the emitter and collector

in either direction indicates a short or leakage(note: some BJT have a damper

diode in parallel with the emitter and collector causing a normal 0.4V-0.7V voltage

drop in one direction between emitter and collector). Getting a small voltage drop

reading in both directions between base and emitter indicates a shorted junction,

also if you get a small voltage drop reading in both directions between base and

collector this is also considered a shorted junction. Getting any resistance or

voltage drop(depending on test method) reading between collector and emitter

indicates a short or leakage between collector and emitter. A transistor can have

one or both junctions shorted when they fail and typically will be shorted between

collector and emitter. An O.L. reading in both directions between base and emitter

indicates an open junction as does an O.L. Reading in both directions between

base and collector.

For pnp type BJTs you will perform the same test only it will be done with the

polarity of the test leads reversed for each step.

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Step 1 Testing an NPN BJT With DMM

Step 2 Testing NPN BJT With DMM (Simply Repeat 1 & 2 With Test Leads

In Reverse Polarity For Steps 3 And 4)

80

Step 1 Testing an PNP BJT With DMM

Step 2 Testing PNP BJT With DMM (Simply Repeat 1 & 2 With Test Leads

In Reverse Polarity For Steps 3 And 4)

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You can also use your analogue meter to test BJTs. Set your meter to the x1 ohm

range and perform the test in the same manner as with the DMM only instead of a

voltage drop reading you will be looking at a resistance reading(on the lower

portion of many analogue meters indicators you can also read the voltage drop of

the junctions) and the test leads in the resistance setting are reversed in comparison

to a DMM the black test lead is positive and the red test lead is negative, but the

idea is the same between the base pin and emitter pin you should have a low

resistance reading in one direction and an infinite resistance reading in the other

direction readings between the base pin and collector pin will be the same. You

should get infinite reading in both directions between collector and emitter in the

x1 ohm range.

Testing PNP BJT with Analogue meter

Testing PNP BJT with Analogue meter

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Some DMM have a transistor test mode for checking hFE(beta or gain). You can

use this setting to test if a BJTs hFE is within tolerance.

DMM hFE Setting

MOSFETs-

Power MOSFETS IN LCD PSU

83

The MOSFET or Metal Oxide Semiconductor Field Effect Transistor is a

component similar to the BJT in the fact that it can amplify or switch electronic

signals. The BJT relies on making a reverse biased junction conduct by applying

an electronic signal to the other junction. The MOSFET or FET(Field Effect

Transistor) is entirely different. In a MOSFET a strip of semiconductor material

either n or p doped between the source and drain is made either more or less

conductive by the presence of an electric charge between the gate and source.

The MOSFET has three terminals or leads, the gate, source and drain. Both n

channel and p channel devices are used in LCD TVs.

Special care must be taken(ESD Bracelet etc.) when handling small signal

MOSFETs, because the gate is completely insulated from the source and drain by

a very thin film of silicon dioxide. The insulation breaks down at roughly 20-100V

depending upon the thickness of the silicon dioxide film.

MOSFET ICs on the inverter board.

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MOSFET Schematic Symbol

Testing MOSFETs-

Once you have looked up the part number of the MOSFET you want to test online

and acquired a data-sheet, found the info in a parts reference manual or looked at

the schematic diagram for the TV you are working on and know the pin

configuration, set your analogue meter to the x10k range to check the MOSFET.

Let's say you are testing an n channel MOSFET, put the black test lead on the

drain pin then touch the gate pin with the red test lead. This will discharge the

MOSFETs internal capacitance. Next place the red test lead to the source pin while

still holding the black test lead to the drain pin. Now take a finger, while still

holding the test leads in place, red on source and black on drain and use that finger

to touch the gate and drain pin at the same time, the analogue meters needle should

move from infinity to around the center position of the meters indicator. Taking the

red test lead off the source pin and placing it back on the source pin the needle

should still go back to the middle of the meters indicator. To discharge the

MOSFET lift the red test lead from the source pin and touch it to the gate pin, this

will discharge the internal capacitance again and if you again place the red test

lead on the source pin and the black test lead to the drain pin, the needle on the

indicator should not move and give an infinite ohms reading. Testing a p channel

MOSFET is the same as for an n channel MOSFET only you will reverse the

polarity of the test leads for each test.

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Step 1 Testing N Channel MOSFET

Step 2 Testing N Channel MOSFET

86

Step 3 Testing N Channel MOSFET

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Special testing devices are available just for testing MOSFET transistors.

Alternative MOSFET Test Methods-

DE-MOSFET (Depletion/Enhancement Type,) Test Using an ohmmeter set to

the x 100 ohm scale, measure the resistance between the MOSFET drain and the

source, then reverse the ohmmeter leads and take another reading. The readings

should be equal, regardless of meter lead polarity. Connect the positive lead of the

ohmmeter to the gate. Using the negative lead, measure the resistance between the

gate and the drain and between the gate and the source. Both readings should show

infinity. Disconnect the positive lead from the gate and connect the negative lead

to the gate. Using the positive lead, measure the resistance between the gate and

the drain; then measure it between the gate and the source. Both readings should

show infinity. If the MOSFET has a substrate connection, Disconnect the negative

lead from the gate and connect it to the substrate. Using the positive lead, measure

the resistance between the substrate and the drain and between the substrate and

the source. Both of these readings should indicate infinity. Disconnect the negative

lead from the substrate and connect the positive lead to the substrate. Using the

negative lead, measure the resistance between the substrate and the drain and

between the substrate and the source. Both readings should indicate a low

resistance (about 1,000 ohms).

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E-MOSFET (Enhancement Type, The most common type you will encounter)

Test Using an ohmmeter set to the x 100 ohm scale, measure the resistance

between the drain and the source, then reverse the leads and take another reading

between the drain and the source. Both readings should show infinity, regardless

of meter lead polarity. Connect the positive lead of the ohmmeter to the gate.

Using the negative lead, measure the resistance between the gate and the drain and

then between the gate and the source. Both readings should indicate infinity.

Disconnect the positive lead from the gate and connect the negative lead to the

gate. Using the positive lead, measure the resistance between the gate and the

drain and then between the gate and the source. Both readings should indicate

infinity. If the MOSFET has a substrate connection, Disconnect the negative lead

from the gate and connect it to the substrate. Using the positive lead, measure the

resistance between the substrate and the drain and between the substrate and the

source. Both of these readings should indicate infinity. Disconnect the negative

lead from the substrate and connect the positive lead to the substrate. Using the

negative lead, measure the resistance between the substrate and the drain and

between the substrate and the source. Both readings should indicate a low

resistance (about 1,000 ohms). Most MOSFETs will fail by shorting from drain to

source and sometimes they will also have a short between gate and source or

between gate and drain or both.

You should always try and find a data sheet for the MOSFET you are testing

because you will find some MOSFETs will have different characteristics that will

make the test readings slightly different. For instance the P11NK50Z has a diode

between source and drain, so that when testing you will get a reading in one

direction between source and drain in x100 ohm setting and this is normal.

Always try to use exact replacements when replacing a bad BJT or MOSFET. If

you must use a substitute always replace with the same type(NPN, PNP, N

channel, enhancement mode etc..)BJT and with the same or higher IC(continuous

collector current) and the same or higher VCEO(max voltage as measured between

collector and emitter when base is open that the transistor can handle without

breaking down). Try to match the gain as close as possible. For MOSFETS replace

with the same or higher BVDSS (drain to source breakdown voltage) and the same

or higher ID(continuous drain current). Also if you substitute a transistor used in a

half bridge that uses two identical transistors replace both the transistors in the half

bridge so that both of the transistors are the same, don't just replace the bad one in

this case.

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Understanding and Testing Diodes

Diodes In LCD TV Standby Circuit

A diode is an electronic component that allows electric current to flow in only one

direction. The word diode is usually associated with the semiconductor diode

which is the most common diode in use at the time I am writing this. The

semiconductor diode is made up of a pn junction.

Rectifier Diode Shown Next To Schematic Symbol

90

Schematic Symbols For Different Types Of Diodes

91

Testing The Diode-

Using your DMM set to the diode test mode, place the black test lead on the

cathode(marked with a band) lead of the diode and the red test lead on the anode.

You should get a voltage drop reading of between 0.45V-0.7V. Reverse the test

leads so that the red test lead is on the cathode and the black test lead is on the

anode and you should get an O.L. Reading.

If you get a low reading in both directions the diode is considered shorted and if

you get an O.L. reading in both forward and reverse bias directions the diode is

considered open, in both cases the diode must be replaced. Most commonly diodes

will be shorted.

When replacing diodes try to use exact replacements. If an exact replacement is

not possible you can use a substitute with the same or higher PIV(peak inverse

voltage), this is the max amount of instantaneous reverse bias voltage that the

diode can withstand without breaking down(avalanche effect) listed on data sheets

as VRRM (maximum repetitive reverse voltage) and the same or higher average

forward current, this is the maximum amount of current the diode can withstand

when forward biased listed on data sheets as IF(AV).

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Step 1 Testing Diode With DMM

Step 2 Testing Diode With DMM

Testing Diodes With analogue Meter-

Using your analogue meter set to the x1 ohm range place the red test lead on the

cathode and the black test lead on the anode, you should get a low resistance

reading and reversing the test leads you should get a reading of infinity. On the

lower portion of the indicator of many analogue meters there is also a scale for

reading the diode junction voltage drop.

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Now set the meter to x10k ohm range and repeat the same test, you should get the

same results. When you have the red probe on the anode and the black probe on

the cathode in the x10k ohm range getting any reading means the diode is leaky

and must be replaced.

Step 1 Testing Diode With Analogue Meter Set To x1 ohm Range

Step 2 Testing Diode With Analogue Meter Set To x1 ohm Range

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Step 1 Testing Diode With Analogue Meter Set To x10k ohm Range

Step 2 Testing Diode With Analogue Meter Set To x10k ohm Range

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Schottky Diodes-

Schottky Diodes In LCD TV SMPS

Schottky diodes also known as hot carrier diodes are semiconductor diodes with a

lower forward voltage drop than a standard diode and a very fast switching action.

When a current flows through a diode there is a voltage drop which as stated

earlier is about 0.45V-0.7V for normal diodes, but a schottky diodes voltage drop

is between 0.15V and 0.45V, the lower voltage drop means higher circuit

efficiency. The most important feature of the schottky compared with the normal

pn diode is reverse recovery time, the time it take to switch from conducting to

nonconducting and nonconducting to conducting. Schottky diodes can look very

similar to normal diodes in design. Often they come in a dual package with the

two diodes cathodes being common.

Dual Package Schottky

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Testing Schottky Diodes-

Once you know the diode you are going to test is a schottky than you need to use

your analogue meter and set it to the x10k ohm range. Testing is similar to the

normal diode only you will get a reading in both directions. This is a normal

characteristic of a schottky diode. The reading should be almost full scale

deflection(low resistance)with the red test lead on the cathode and the black test

lead on the anode, then with the black test lead on the cathode and the red test lead

on the anode you will get a small leakage reading (large resistance). If you get two

almost full scale deflection readings(low resistance) the schottky diode is shorted

and needs to be replaced, if the resistance reading is infinite in both directions the

schottky diode is open and must be replaced. Testing the schottky in the x1 ohm

range will be just like testing a normal diode, also note that not all schottky diodes

will give a reading in both directions when set to x10k but just be aware that this

kind of diode when normal can have a reading in both directions when measuring

in the x10k setting unlike a normal diode.

Step 1 Testing Schottky Diode In x1 ohm Setting

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Step 2 Testing Schottky Diode In x1 ohm Setting

Step 3 Testing Schottky Diode In x1 ohm Setting

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Step 4 Testing Schottky Diode In x1 ohm Setting

You can also test schottky diodes with the diode test mode of your DMM just like

we did with the standard diode earlier. Expect lower junction voltage drops when

testing schottky diodes with the diode test mode of your DMM compared to those

of the standard diode.

Always find an exact replacement for the schottky and ultra fast recovery diodes

used for rectifying the secondary voltages in the PSU. When substitution is

necessary you must follow the voltage and current replacement guidelines

discussed earlier for standard diodes but also you must make sure the replacement

has the same or faster trr(maximum reverse recovery time), this only applies to

ultra fast recovery diodes as schottky diodes data sheets won't list trr but a schottky

diode is a high speed diode so always use a schottky diode to replace another

schottky diode.

Zener Diodes-

Zener Diodes In LCD TV SMPS Secondary side

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A zener diode is a diode that not only permits current flow in the forward bias

direction but also in the reverse bias direction when the voltage applied is greater

than the breakdown voltage called the zener voltage(also called the avalanche

voltage). A zener diode exhibits very similar properties to that of a normal diode

except it is specially designed to have a low reverse breakdown voltage or zener

voltage. This is done by heavily doping the pn junction of the diode. Doping is the

process of introducing specific amounts of impurities to the semiconductor

material for the purpose of changing it's conductivity. The breakdown voltage of

zener diodes can be controlled quite accurately through the process of doping.

Common breakdown voltages range from around 1.2V to 200V. Zener diodes are

normally used as a voltage reference or as shunt regulators for voltage regulation

in smaller circuits because of their ability to maintain a fairly constant voltage

drop with a varying current.

Testing Zener Diodes-

Testing zener diodes is best done with a zener diode tester.

100

Zener Diode Tester

Although another process that can be used is found in this book “Testing

Electronics Components”

Another method for testing zener diodes involves the use of a variable DC power

supply and an ammeter.

101

Connect the zener diode you want to test in series with a resistor to limit current

flow through the testing circuit(the value of the resistor will depend on the zener

diode and how much current it is rated for). Then connect the ammeter of proper

size or the current setting of your DMM set to the proper range in series with the

zener diode and resistor. Connect the cathode end of the diode to the positive

terminal of the variable DC power supply and the free anode to the negative

terminal of the DC power supply. Turn on the variable DC power supply and

slowly increase the voltage. No current should flow through the circuit as

indicated by the ammeter until the voltage is raised to the breakdown(actually a

reverse or avalanche breakdown) voltage of the zener diode under test.

Example Zener Diode Test Circuit

When replacing zener diodes always try to use an exact replacement but when

necessary use a substitute with the exact same VZ(zener voltage) and the same or

higher IZM(maximum zener current).

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Understanding and Testing Bridge Rectifiers

Bridge Rectifier In LCD TV PSU

The purpose of the bridge rectifier is to convert AC voltage into DC voltage.

Symbol For Bridge Rectifier

A bridge rectifier is an arrangement of four diodes in a bridge configuration and

commonly comes in one integrated package containing all four diodes. A diode

bridge or a bridge rectifier give full wave rectification.

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Bridge Rectifier IC

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Testing Bridge Rectifiers-

Using your analogue meter set to x10k ohms range place the test leads to the

negative and the first AC pin and then reverse the test leads, you should show a

low resistance in one direction and infinite reading in the other direction. Next do

the same test with the negative pin and the second AC pin the results should be the

same. Now do the same tests only with the positive pin and both the AC pins, the

results should again be the same. If you find low resistance readings in both

directions on any of these tests then the bridge rectifier must be replaced as one or

more of the internal diodes is shorted, also if you get an infinite reading in both

directions the bridge rectifier must be replaced as one or more of the diodes is

open. If instead of a bridge rectifier in integrated package you find four individual

diodes in a bridge configuration, test each diode individually and if even one diode

is faulty replace all four.

Step 1 Testing Bridge Rectifier IC

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Step 2 Testing Bridge Rectifier IC

Step 3 Testing Bridge Rectifier IC

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Step 4 Testing Bridge Rectifier IC

Step 5 Testing Bridge Rectifier IC

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Step 6 Testing Bridge Rectifier IC

Step 7 Testing Bridge Rectifier IC

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Step 8 Testing Bridge Rectifier IC

The bridge rectifier can also be tested with the diode test mode of the DMM of

course. Do the test in the same way testing each of the four diodes that are inside

the bridge rectifier IC only instead of resistance you will be looking at the diode

junction voltage drops.

Use exact replacements for bridge rectifiers when possible but if necessary to use a

substitute then find one with the same or higher VRRM (maximum repetitive

reverse voltage), the same or higher IF(AV)(average forward current) and the same

or higher VRMS(maximum RMS(root mean square) bridge input voltage).

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Understanding and Testing LEDs

LEDs On LCD TV Status Board

An LED or Light Emitting Diode is a semiconductor component source of light.

LEDs are used as indicator lights in LCD TVs. The LED is based on the

technology of the semiconductor diode, when an LED is forward biased (turned

on) electrons recombine with holes inside the component creating an effect called

electroluminescence.

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Up close Picture of SMD LEDs

Schematic Symbol In Comparison With Physical LED

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Testing LEDs-

Testing an LED is very simple, it will emit light when forward biased and will not

emit light when reverse biased. If an LED does not light when forward biased then

it has gone open circuit and must be replaced. To test, set your analogue meter to

x1 ohm range. Place the red test lead on the cathode pin this side is indicated by a

flat spot on the plastic package or a line on SMD packages, the anode side will not

have the flat spot. While the red test lead is on the cathode touch the black probe

to the anode pin and the LED should light, remove lead quickly after LED lights

so as not to damage it.

Testing SMD LED On LCD TV LED/Status Board

If you reverse the test leads the diode will not light. If the diode does not light in

either direction and also gives a low resistance reading in both directions than the

diode is considered shorted. You can also check LEDs with the diode test setting

of your DMM. You should get a voltage drop of around 1.75V in the forward bias

direction and O.L. in reverse bias direction. A reading of O.L. in both directions

means the LED is open and a low voltage drop in both directions(typically 0.00V)

means the LED is shorted.

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Understanding and Testing Switching

Transformers

Switching Transformer In LCD TV PSU

Basic Transformer Schematic Symbol

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Switching transformers also called SMPS or power transformers are found in the

SMPS of LCD TVs. A switching transformer does not actually switch anything

itself it is merely a transformer that is designed specifically for use in a switching

power converter or SMPS and designed for the high frequency switching(10s to

100s of kilohertz) in SMPS type power supplies. They are smaller and lighter than

the typical transformer found in a linear power supply and also use a ferrite core

instead of laminated iron. The function of the switching transformer is to convert a

voltage applied across it's primary winding into a lower or higher voltage across

it's secondary windings depending on the amount of turns in the primary and

secondary windings of the switching transformer(the turns ratio). Switching

transformers are robust components and rarely breakdown, and when they do it

will normally cause components in the primary side of the switching transformers

circuits that drive it's primary winding to blow/fail as well and most likely open

the main fuse. The most common failure is a shorted primary winding. The

switching transformer secondary windings seldom have issues in the LCD TV

SMPS due to the fact that voltage is stepped down so the secondary windings have

very few turns and a low voltage across them. The primary winding however has

many turns and a larger voltage across it. To test the primary winding of the

switching transformer use a ring tester. Locate the primary winding

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Place the test leads of the ring tester to the two pins of the primary winding. Most

switching transformers will light up four to eight LEDs, of course if you can,

check your reading against that of known good switching transformer that is the

same exactly as the suspected one under test. Often this is not possible, so if you

only light up one to two or no LEDs it is most likely that the switching transformer

primary has shorted windings and needs to be replaced. If the switching

transformer tests bad always remove it from the circuit board and test again to be

sure. If it tests good out of circuit suspect other failed components in the SMPS

perhaps a shorted secondary side schottky or ultra fast recovery diode. If a

switching transformer is found to be bad you may have trouble finding an exact

replacement and you may have to find a company or person online that will

rewind it for you or you can attempt to rewind it yourself with instructions found

online, but be warned this is not an easy task. You can also check switching

transformers for an open primary winding although it is not a common failure. Set

your DMM to the ohms setting or your analogue meter to x1 ohm range. Check for

a low ohms reading across the primary winding. If the reading is O.L. or infinity

than the winding is considered open and the transformer must be replaced or

rewound. You can also check the secondary windings to make sure they are not

open the same way you check the primary and if you have another of the same

switching transformer to compare with you can ring the secondary windings as

well although because they have less windings they will not light up as many

LEDs, even only one LED or none and this is normal. If you get no rings on the

secondary windings you can also check them with an ESR meter and compare

with another of the same transformer, because the windings have a certain

inductive reactance at the ESR meters test frequency(typically 100kHz) and a

resistance that together make up a complex impedance, the windings will give a

reading on the ESR meter, typically a high reading like 45 ohms or beyond the

range of the meter. If you read zero ohms on the ESR meter this would be a sign

of shorted secondary windings. You can also measure the resistance(many

secondary windings will read 0 ohms using the resistance setting of your meter use

your ESR meter to measure the impedance in this case) or inductance values of the

secondary windings and compare the results to a good transformer that is the same

as the one in question. Another test is to set your analogue meter to the x10k ohms

range and place one test lead on a pin of the primary side and the other lead touch

to the pins on the secondary side. You should get an infinite or O.L. reading

between the primary and secondary windings.

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Checking For Open Primary Winding In SMPS Switching Transformer

116

Understanding and Testing Opto-Isolators

Opto-isolators In LCD TV SMPS

An opto-isolator also called an opto-coupler or photo-coupler is a component that

allows a signal to pass from one circuit to another but allows the two circuits to

remain electrically isolated. The most common opto-isolator which comes in IC

package consists of an LED which shines onto the base of a photo-transistor

(usually an npn transistor) and allows current to flow from collector to emitter

until the LED is turned off. When a signal is applied to the LED it then shines light

that is varied in brightness with the same amplitude as the input signal, this light

lands upon the photo-transistor (the resistance of the collector/emitter junction

now changes with the varying light) which passes the signal onto the next circuit.

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Testing Opto-Isolators-

Using your analogue meter set to the x1 ohm range test the LED side of the opto-

isolator. Which will be found be looking up the part number online or referring to

the LCD TVs schematic. A common opto-isolator found in LCD TVs is the 817

type for instance the PC817.

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Multiple Package Types

Placing the test leads on both pins of the LED side of the opto-isolator and then

reversing them you should get a low resistance reading in one direction and

infinite reading in the other. If you get a low resistance reading in both directions

the LED side is shorted and the opto-isolator needs to be replaced. If you get an

infinite reading in both directions, the LED side is open and again the opto-isolator

must be replaced.

Next you must test the transistor side of the opto-isolator. Some opto-isolators will

have six pins in which case the three pins that correspond to the transistor side of

the IC will be base, emitter and collector and on the LED side the three pins will

be the anode, cathode and one pin with no connection. If you have a six pin opto-

isolator, testing will be the same as for a four pin, just remember to check the pin

configuration. If you have a four pin opto-isolator which I find is most common in

LCD TVs, then set your analogue meter to the x10k ohm range and place the test

leads on the two pins of the transistor side which will be the emitter and collector

and then reverse the test leads. You should get a high resistance (typically around

500k ohms) reading with the test leads in one direction and an infinite reading in

the other direction.

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Six Pin Opto-isolator

If you get a high resistance reading or low resistance reading in both directions the

opto-isolator must be replaced. Now set the meter to x1 ohm range and place the

test leads on the emitter and collector in both directions again, now you should get

only an infinite reading in both directions, otherwise the opto-isolator must be

replaced.

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Step 1 testing LED side

Step 2 testing LED side

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Step 3 testing transistor side

Step 4 testing transistor side

Here is a free download with more techniques for testing opto-isolators including

instructions and a parts list for an opto-isolator test circuit.

http://preher-tech.com/Documents/opto-isolators.pdf

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Understanding and Testing Voltage Regulators

Voltage Regulator ICs on LCD TV Main Board

A voltage regulator is an electronic circuit designed to automatically maintain a

constant output voltage regardless of fluctuations in input voltage or current draw

from the load(to an extent).

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Testing Voltage Regulator IC-

To test the voltage regulator IC you must check it in circuit with the TV plugged in

and the LCD TV turned on. Power the TV on and set your DMM or analogue

meter to the proper DC voltage setting. Place your black test lead to cold ground

and carefully place your red test lead to the output pin of the regulator, usually pin

three on many voltage regulators, like the LM7805. Remember to always look up

your part numbers for the part you are testing if needed so you can be sure to test it

properly.

Typical LM7805 Package

Now you should expect the output voltage to be within tolerance of the regulators

specified voltage. So for an LM7805 you should expect about 5.1V-5.9V DC, if

you measure a low voltage like say 1V you should turn the TV off and unplug it

then lift the output pin of the regulator from the circuit. Then power the TV on

again and retest checking the voltage right on the lifted pin, If the voltage is back

to around 5V again instead of 1V then the IC is most likely good and you should

suspect shorted components in the circuit after the regulator that are pulling down

the voltage. If the voltage is still low then the regulator is most likely bad and

needs to be replaced(granted the input voltage is good). If you measure OV or low

voltage on the output you should check the voltage on the input of the voltage

regulator and make sure it is at least the expected regulation voltage plus the

regulators “drop out” voltage. The drop out voltage is the voltage that the input

must be above the regulation voltage for the given regulator, to maintain a

regulated output. For instance if a LM7805 had a drop out voltage of 2V then it

would require at least a 7V input to maintain a 5V regulated output. If the input

voltage is low suspect bad components in the circuit supplying input voltage to

the regulator.

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Checking Input Voltage to Regulator

Checking Output Voltage From Regulator

125

Understanding and Testing Switches

Typical Tactile Switch Found In LCD TVs

A switch is an electronic component which can break a circuit or divert current

from one part of a circuit to another part. The most common type of switch you

will see in LCD TVs is the tactile switch or “tact switch”. The common

configuration is single pole single throw, normally open or “push to make”

momentary contact.

Switch Schematic Symbols

Testing Tactile Switches-

126

Testing tactile switches and any switch in general is very simple. Set your DMM

to the resistance or continuity setting or your analogue meter to the x1 ohm range

and place the test leads on to pins on opposite sides of the switch(polarity does not

matter). You should get a reading of infinity. Now while still keeping the test leads

on the pins, depress the button and the meter indicator should give a low resistance

reading and when you release the button the indicator should return to O.L. or

infinity. Please note this is only for testing single pole single throw, normally open

or “push to make” momentary contact switches, but I think you can see how easy

it would be to test any switch with the resistance setting of your meter.

127

Understanding and Testing Fuses and Varistors

Fuse In LCD PSU

Pico/Nano Fuses On LCD TV Inverter Board

128

A fuse is an electronic component that is used as a sacrificial device for over

current protection. It contains a wire or strip that melts when too much current

flows though it, which interrupts the circuit that it is connected to. A fuse is

destroyed by excessive current so that further damage or injury is prevented from

overheating or fire. Always replace a fuse with the exact same value, type and

rating as the one removed from the circuit.

Schematic Symbol For Fuse

Testing Fuses-

Set your DMM to the resistance or continuity setting or your analogue meter to x1

ohm range. Place the test leads the terminal caps or leads of the fuse(polarity does

not matter), you should get a very, very low resistance reading of zero or very

close to zero ohms. This is the characteristic of a good fuse. A bad fuse will give a

high, O.L. or infinite resistance reading. You can visually inspect glass fuses and

see when they have blown(blackened glass), or the metal strip has just slightly

opened but if a fuse looks good you must still test it because you can see when a

fuse is bad but you cannot see that it is good, meaning just because it looks good

does not mean that it will not test bad.

Testing A Fuse

129

Testing A Pico/Nano Fuse

Varistors

The name Varistor is a portmanteau(blend of two or more words) of variable resistor(this is misleading

as varistors are quite different from a variable resistor). Varistors have non-ohmic current-voltage

characteristics, at low currents and voltages a varistor exhibits a high resistance but at higher voltages

and currents the resistance drops dramatically. Varistors are often used to protect circuits against

transient voltages by placing them into a circuit in such a way that when triggered, they will shunt the

current created by the high voltage away from the rest of the circuit preventing damage. In an LCD TV

power supply you will find them right after the main fuse connected across the hot and neutral. The

most common type of varistor is the metal oxide varistor or MOV. A shorted varistor will cause the

main fuse to open.

Testing Varistors-

1.inspect varistor for obvious external damage(burns and cracks etc..)

2.Set multimeter to resistance setting and measure the resistance of the varistor by placing the red test

lead to one terminal of the varistor and the black to the other terminal(polarity does not matter).

3.Reverse multimeter test leads.

4.Resistance readings should be infinite ohms or O.L.(over limit) in both directions.

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Some Testing Tips

-Always test components with a meter that is known to be good, with fresh

batteries installed.

-I suggest removing components from the circuit before testing. Often surrounding

components in the circuit cause erroneous readings to occur. At least lift/remove

one lead from the circuit for most components.

-Learn how to test all electronic components and practice testing them so you can

feel confident in yourself when testing components. Again I highly recommend the

book “Testing Electronic Components” , as I was not able to completely go over

all the components found in electronic circuits and how to test them.

-Learn how to use your test equipment. Read the manuals and understand all the

functions and settings. Your test equipment is your best friend when repairing

LCD TVs. The better you know how to use your test equipment the faster and

easier it will be to apply them to testing circuits and you may even develop some

new testing methods yourself.

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Useful Formulas

These are just examples of the basic and most commonly used formulas in

electronics. You should know These by heart. There are many more that you

should study and learn as well if you plan to further your understanding of

electronics. These formulas are to be used for DC circuits or resonant AC(Meaning

voltage and current are in phase and that the circuit Impedance is equal to the

circuit resistance) circuit calculations.

132

How To Disassemble An LCD TV

First start by laying a thick soft blanket out on a table large enough to lay the face

of the LCD TV on.

Lay The LCD TV On Its Face

Next remove all the screws from the stand and remove the stand. Next remove all

the screws from the back of the TV, it help to have a cup to keep all the screws in.

133

Remove the back and set it aside. Now you will have full access to the

PCBs(Printed Circuit Board). To remove a PCB for component testing or for

replacement, simply disconnect any cables going to the board from their

connectors. Label them if necessary.

Exposed PCBs

134

Next remove all the screws that hold the PCB to the TV. The board should now

come loose. You may now test or replace components on the board you

removed(remember if you remove the PSU the first thing you need to do is to

discharge the reservoir or primary side filter capacitor).

135

Voltage Test Points

Warning!!!

Be careful when taking voltage measurements. Remember when taking

measurements on the primary/hot side of the SMPS use the corresponding primary

side ground(hot ground) and when taking measurements on the secondary/cold

side use the secondary(cold ground).

Bridge Rectifier-

136

Plug the LCD TV in and locate the AC pins of the bridge rectifier marked by a

wavy line(this is the AC symbol). Set your DMM or analogue meter to the proper

AC voltage setting. Now place your test leads against the AC pins of the bridge

rectifier. Hold them tightly and carefully in position be sure not to slip and short

anything with your test leads.

Placing Test Leads on AC Pins Of Bridge Rectifier

You should get a reading of around 110-120 volts AC(USA, in some other

countries the voltage may be different find out the mains in your country and this

is what you should expect) If you get zero volts then check in the circuits before

the bridge rectifier, could be bad components in the EMI circuit like an open

common mode choke, open thermistor, blown main fuse, bad AC cord, electrical

receptacle has no power, bad components in passive PFC circuit if the TV uses this

type of PFC, bad solder connection in circuits before or at bridge rectifier etc.

137

Reservoir Capacitor/Primary Filter Capacitor-

Once we have confirmed that the bridge rectifier is receiving an AC input at the

proper voltage we can now expect to find a DC voltage at the pins of the

reservoir/primary filter capacitor. Make sure the TV is plugged in. Now set your

DMM or analogue meter to the proper DC voltage setting place your black test

lead to the negative pin of the reservoir capacitor, next place the red test lead on

the positive pin of the reservoir capacitor. Be extremely careful not to accidentally

slip and short the pins of the capacitor together.

Testing Voltage At The Reservoir/Primary Filter Capacitor

You should get a reading of around 150-160V DC(USA, If active PFC is used

expect around 350-400V).

If you get the right reading then move on to the next voltage test. If you get zero or

very low voltage check the corresponding circuit components, look for bad solder

connections, cracks in the copper traces, check the components in the active PFC

circuit(MOSFET, diode, PFC controller IC) remove and test the bridge rectifier

etc.

138

Power IC-

First you must look up the part number of the power IC you will be testing the

positive supply voltage pin of. Once you locate the positive supply voltage pin of

your power IC, set your DMM or analogue meter to the proper DC voltage range.

Place the red test lead to the positive voltage supply(Vcc) pin and the black test

lead to hot ground(reservoir capacitor negative pin). Remember the TV needs to

be plugged in. You should get a reading that corresponds to the proper voltage for

the power IC normally between 16-20V DC. If you get the correct reading you can

assume all components up to this point must be good(bridge rectifier, fuse, EMI

Filter circuitry, start up circuit, etc.). If you can't find the necessary information for

you IC you can always skip to the next voltage test. If the reading is low or zero

volts the most common reason is that the start up circuit resistor/s have changed

value or opened. To find the start up circuit resistor/s, trace back from the positive

voltage supply pin to the large value resistor/s that bring the

SMD Type Power IC/Half Bridge Driver

139

Power IC And MOSFET In A Single Package

Testing Positive Supply Voltage Pin Of Power IC

voltage from the positive pin of reservoir capacitor to the Vcc pin of the power IC

so that it can get power before the SMPS is on. Once the TV is on a secondary

winding on the SMPS switching transformer in the primary side of the PSU

supplies voltage to the power IC, this winding and a few other components like a

resistor, a diode that rectifies the output from the secondary winding and a filter

capacitor make up the “run DC” circuit. Some start up circuits will grab voltage

directly from the AC line and not the reservoir capacitors positive pin in which

case you will also find a rectifier diode and filter capacitor in the start up circuit

that should also be checked.

140

Testing Secondary Output Voltages-

This is a very simple test for sure. Place your red test lead from you DMM or

analogue meter on the cathode side of the secondary output diode you want to

check and place your black test lead cold ground. Make sure your meter is set to

the proper DC voltage setting( most LCD TVs have secondary voltages of 5V,

12V and 24V but their can be more or different ones, if possible refer to the

service manual of the TV you are testing). Turn power on to TV. You should

receive a DC voltage that corresponds to the voltage you expect at that diode. You

may have to look at a schematic. If you get a good reading on all secondary

diodes, then you can assume that all the components on the primary side are

working properly and checking components in the primary side of the SMPS

would just waste your time most

Checking Secondary Voltage

likely. It obvious if the bridge rectifier, power IC, power MOSFET and etc. were

bad we would not get any of our secondary voltages. You can also check

secondary voltages on the output cable connectors of the PSU which are often

labeled.

141

Main Board Voltages-

If all of your voltage were good in the previous test next you should check that

voltage is coming to the main board. If it is low or not present you know it must be

something between the main board and the SMPS after the secondary diode,

perhaps a bad filter capacitor, shorted components or maybe even a bad ribbon

cable or bad solder connections etc. If the voltage is present to the main board then

you can next check the voltage at the output of the voltage regulators that will be

found on the main board typically 5V and 3.3V(and others, it depends on the TV)

linear regulators. If the outputs are correct you can them move on to test the

Positive supply voltage at each of the ICs on the main board. You may need a

schematic or service manual which can usually be found online, to get the pin

layout and Vcc voltage for each IC.

Checking For Voltage To Main Board

142

Checking Vcc Voltage OF LVDS IC

(Note this board is removed from the TV, but obviously when doing voltage tests

boards must be in the TV and completely connected properly)

Checking Positive Supply Voltage Pin Of A NAND Memory IC

(Please use a very fine test lead when checking Vcc/Vdd of SMD ICs as the pins are very close

together)

T-con Board Voltage-

With your DMM set to the proper voltage setting, place your black test lead to

cold ground and you red test lead to the the lead of the pico/nano fuse closest to

the cable that brings the voltage to the board, then check with the red test lead to

the pico/nano fuses other lead(terminal caps if SMD). You should get about 12V

DC for both reading. If you only get one reading(on the side closest to the

connector) The fuse has most likely gone open, try replacing it. If you get no

reading on either lead, trace back and check all the components in the circuit that

supplies the voltage to the T-con board.

143

The Tap Test

This is an old test that is crude and simple, but it does work. Use this test to isolate

connection problems or intermittent failures caused by bad solder connections etc.

Using the back of your screwdriver or some slightly heavy non conductive tool,

lightly tap around on the circuits careful to not damage any components or bend

leads and cause shorts. The idea is to find the most sensitive part of the circuit that

responds to your tapping causing the TV to go in and out of failure. When you find

this sensitive area or maybe the only area where you get a reaction, inspect all the

components for any damage and then look over all the solder connections

carefully with a magnifier and light or a lit opti-visor. Look for cold solder joints,

ring cracks or any solder joints that are questionable. Mark every bad or

questionable solder connection you see with a marker. After you are done marking

every solder connection go back with your soldering iron and some quality solder

and re-solder all the marked connections.

144

Ring Crack

Cold Solder Joints And Some Ring Cracks

You may also find that the connections are all good, but when you tap right on a

component the TV fails. You should directly replace the component if possible.

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Freeze Spray and Hair Dryers

This is another test which will help you isolate a problem to a single area of the

LCD TV. You may run into TVs that will not run properly until sometime after the

TV has been turned on and warmed up. You may also come across LCD TVs that

runs fine until some time after being turned on and then after warming up has a

failure, maybe shutting down, distorted video, distorted audio etc. In the case that

you have a TV that has a failure from start up which goes away after warming up

you can try the hair dryer method. Let the TV set set for awhile so it is cool. Then

turn the TV on and verify it is having the known failure. Then take the hair dryer

and heat up the circuits with a smooth even action and carefully and slowly

panning across them until you find a location that gives you a reaction. If you heat

that area the TV starts to function properly. Once you have isolated the problem to

any area recheck(by starting the test from cold again) to make sure, once verified

check all the components in the isolated area.

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If you can not find any bad components or connections you may want to directly

replace components in this area if you have the parts. Now if you have a TV that

falls into the other category. Working fine at start up and then failing after

warming up then you will use the freeze spray. Turn

Using Hair Dryer

on the TV and let it warm up and stop operating normally, once this happens

power off and unplug the TV use the freeze spay to cool down a section of one of

the PCBs and components in the TV carefully going over all that section(but do it

quick), after thoroughly cooling quickly plug in and turn on the set and see if

failure is gone, meaning that an area you cooled down caused the TV to start

functioning properly again if not, try again with another section, remember you

have to be quick enough to make sure the TV circuits in general don't all cool

down causing the TV to work normally as well. You need to make sure it is when

you spay the freeze spay on that area that the TV is made to start working meaning

you have isolated the problem. The most efficient way is to use freeze spray that is

safe for use on live circuits so that you can spray components while the TV is

running and look for normal operation after a component or area has been sprayed.

The small nozzle of the cool spray allows you to more precisely cool a small

location unlike the hair dryer that's more wide spread when it heats, this will allow

you to isolate the problem to a smaller area of a circuit and maybe even a single

component. Once the problem is isolated carry on as you did previously in the hair

dryer example.

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Using Cool Spray

(Note the board is removed from the TV in this photo but this is actually done as

described earlier with the boards in the TV and completely connected)

148

Connection Problems

Solder Cracks

Loose Cables

Connection problems, we discussed some of them earlier in the book, can cause a

lot of problems and you will find they are the reason for a good number of failed

TVs. Most commonly you will find ring cracks and cold solder connection and

even totally burnt up connections when inspecting PCBs with a magnifier or an

opti-visor. Sometimes you will also find cables that have slightly crept out of their

corresponding connectors causing a connection problem.

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Connection problems will often cause intermittent issues but not always when they

are really bad effectively causing a part to be removed from circuit or causing an

open circuit in which case they will cause constant issues. Connection problems

are so common in LCD TVs the first thing you may want to do is open the TV,

inspect all solder connections on each PCB and re solder all questionable ones and

bad ones. Re-seat all the cables into their corresponding connectors, careful not to

bend or break any pins on the cable connector.

Inspecting PCB For Bad Solder Connections

Re seating Cable To Its Connector 1

150

Re seating Cable To Its Connector 2

Re seating Cable To Its Connector 3

Once finished double check and make sure everything is properly reassembled and

turn the TV on to see if the problem still persists.

151

PSU (Power Supply Unit) Failures

152

Now we will go over some common LCD TV PSU failures. PSU failure is quite

common in LCD TVs, so I suggest you study as much as you can about SMPS and

SMPS troubleshooting, as these are the types of PSUs used in LCD TVs. We can't

cover everything in this book but we will cover the most common failures.

-TV is dead(no power and standby LED not lit) and main fuse is blown.

Check for shorted bridge rectifier, shorted diodes in primary side(especially

snubber diodes), reservoir capacitor(check for short and leakage), varistor(check

for visible damage and low resistance reading)and check for shorted power

MOSFETS. If you find lot's of shorted components in the primary side, replace the

power IC along with replacing all the bad components as it will often have been

destroyed as well.

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-No power, standby LED not lit and Main fuse is not blown.

Check the standby power circuit check the voltage at the secondary output

diode(typically 5V DC) cathode if not present or within tolerance check

corresponding components. Check the secondary diode, filter capacitor and choke

of the standby circuit. I have repaired dead LCD TVs with a shorted standby

circuit secondary diode and LCD TVs that would not power on and the standby

LED would flicker caused by bad secondary output filter capacitors in the standby

circuit causing the failure. Note that a blinking LED can indicate a failure code

from the MCU, and the failure may not even be related to the power supply. If you

get a pulsing LED that seems rhythmic or in a pattern( like 3 flashes pause two

flashes repeat, etc.) refer to the service manual to see if the TV has LED blink

codes or not and if so what they are as they can lead you to the right circuit for

repair.

-TV has no power, Power/standby LED is lit and fuse is not blown.

Check for Vcc voltage to the power IC. If not present check start up resistors in the

start up circuit.

-Start up circuit

The start up circuit usually consists of one or more large value resistors that drop

the voltage from the 150-160V source at the positive pin of the reservoir capacitor

to a voltage that is used to power the power IC when the SMPS is not on. The start

up circuit may also grab voltage from the AC line in which case it will typically

also have a rectifier diode and a filter capacitor that should be checked.

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Some more common failures.

-One or more secondary outputs with voltage out of tolerance or with ripple at

mains frequency(50/60 Hz) or twice the mains frequency(100/120 Hz). Check the

reservoir or main filter capacitor for high ESR. Also check surrounding

components in the primary side.

-One or multiple secondary outputs with voltage out of tolerance and or ripple at

the SMPS switching frequency(usually 10s to 100s of kHz). Check the secondary

filter capacitors and chokes. Also if secondary voltage is low, check for shorted

components in the circuit that could be dragging down the voltage.

-Whining noise coming from PSU with low voltage on one or more secondary

outputs. Check for shorted semiconductors( diodes, BJT etc.) in the corresponding

circuit.

-TV power supply making cycling/pulsing or chirping sound. This is almost

always caused by shorted/failed components in the secondary side of the SMPS.

Check secondary output diodes, secondary output filter capacitors, also check the

feedback circuitry, meaning check the opto-isolators,directly replace the adjustable

shunt regulator, check the resistors in the sampling circuitry etc.

155

A bad power IC can also sometimes cause the power cycling symptom. If you

have tried and checked everything with no success, you might try directly

replacing the power IC.

-Intermittent power failures. This can be a frustrating thing to troubleshoot. You

will want to use the tap method discussed earlier in the book because most

intermittent failures in general are caused by loose/dry, burnt or oxidized solder

connections. If checking and re soldering all the connections does not work the

next think I would do is check all the electrolytic capacitors in the PSU as they can

also cause intermittent failures.

-Replacing a PSU over repairing it. Sometimes you will have a PSU board with

such extensive damage, say from a power surge that you will really need to

consider looking up the price for a new PCB online as it may not be worth your

time to try and save one that needs so much work done, but if you are like me you

are going to try to fix it anyways to gain skills and satisfaction in a repair.

156

Please note:

PSU board connection problems and especially filter capacitors with raised ESR

are so common and cause so many different problems in LCD TVs that may not

even appear to be PSU related that the first thing I do on all repairs is inspect the

PSU and check every electrolytic capacitor on the board, If all is good I then do

the same on the rest of the PCBs.

157

Inverter Board Failures

Most inverter board failures will cause one of these two symptoms. Either the

TV will turn on but as soon as the back light turns on the TV shuts down(this

can be very quick, the TV may just quickly flash and you must watch closely)

or the TV will turn on and operate normally with audio but no picture. If you

look carefully you can see that the picture is their but it has no back light to

illuminate it. If you have one of these symptoms you will want to start by

checking the inverter board(after checking the electrolytic capacitors in the

PSU, they can also cause these same symptoms even a dim flickering display

as well when the secondary filter capacitors on the voltage supply line to the

inverter board go bad).

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You may also have TVs with dim display or flickering display which is

usually the inverter board also. Please note that these symptoms can also be

caused by a bad CCFL

-Checking the inverter board

Check the pico/nano fuse/s usually found close to the cable connector that

connects the cable coming from the PSU to the inverter board.

-If the pico/nano fuse or fuses are open, you might first try replacing them

and see if this fixes the problem(normally it won't), if it does not move on to

checking the components in the inverter circuits(if only one of multiple

pico/nano fuses is open check components in the circuit that correspond to

that fuse). Inverter ICs are robust and don't fail very often. Suspect shorted

MOSFET Ics(very common), HVTs(High Voltage Transformers) shorted

between primary and secondary windings, shorted secondary or primary

windings in HVTs, compare results with known good HV transformer of

same type. Fortunately there are usually more than one on LCD TV inverter

boards so you can compare them against each other as most likely not all are

bad(remember to pull them off the board to test and to identify the primary

and secondary pins, they may not be the pins you assumed). Check that the

resin coated ceramic capacitors on secondary side of inverter circuit are not

shorted or leaky.

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LCD TV HVT

-If no pico/nano fuses are open on the inverter board, check the HVTs, check

for open secondary windings(common failure) and shorted turns in both the

secondary and primary windings with ring tester. You can also check for

shorted secondary windings by checking the resistance of the HV

transformers secondary windings and comparing them to each other. A

secondary winding that has shorted turns will have a resistance that is

significantly lower than the rest of the secondary windings. Remember that

bad secondary filter capacitors in the PSU output that supplies voltage to the

inverter board can cause symptoms that are the same as a failed inverter

board. Also check the components in the feed back/Over Load/Open lamp

Protection circuit of the inverter board. When the inverter board shuts down

you should also first check to see if the voltage to the inverter board coming

from the PSU is still present and within tolerance as the problem may be with

the power supply and not the inverter.

Click on the link below to see a repair I did on a TV with a shorted high voltage

transformer(HVT):

http://preher-tech.com/Documents/LCD%20TV%20with%20a%20bad%20HVT.pdf

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Inverter Board Feedback and Over Load Protection Circuit

If you can not find anything wrong with the inverter board or PSU it is likely

you have one or more bad CCFL. You can use a CCFL tester for LCD TVs to

find and then replace the bad CCFLs or you can directly replace all the

CCFLs which wouldn't hurt since even ones that are not bad will likely be

right behind the bad ones. Check the CCFLs for physical damage like

blackened ends and for bad connections.

LCD TV CCFL tester

161

Using an LCD TV CCFL tester

Always check for loose or dry solder joints on the HVTs, MOSFET ICs and

inverter IC pins. These are a very common cause of inverter failure.

Sometimes you will not be able to find any good info for the parts on an

inverter board or parts may not be available in which case you may want to

just look into replacing the whole inverter board, usually they are not to

expensive especially compared to replacing the TV. Also always save old

inverter boards and all LCD TV boards for that matter so you can salvage

parts from them that you may not have been able to acquire any other way.

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Main Board Failures

Common main board failures can include no video, no OSD(On Screen

Display/Menu), no audio, tuning problems, video and audio distortions, no color,

white screen etc. Main board failures can also cause no power and in this case will

most likely have a symptom like blinking power/standby LED(often a code

indicating the section/PCB of the TV that is failing), amber colored standby LED

or some color other than normal standby or power etc and LED not lit. The PSU

must get a start up or “on” signal from the MCU on the main board in order for

the TV to turn on and in some designs the inverter also gets an “on” signal from

the main board as well. If you are suspecting main board problem, first check for

proper voltages to main board and ICs, you might need to get a schematic or

service manual or study the board and look up the ICs online to get an idea of what

the voltages you are looking for are, but typically you will have voltages like 5V,

3.3V, 1.8V, 12V and 2.8V DC. Check the outputs from the voltage regulator ICs

and make sure they are within tolerance. Remember to use cold ground when

checking voltages on the main board. If all voltages are present and within

tolerance remove the main board and inspect for bad solder connections and then

check all the electrolytic capacitors like the SMD decoupling capacitors and also

check the ceramic chip or disc bypass capacitors. Pay special attention to the

electrolytic filter capacitors in the voltage regulation section of the main board.

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Check components surrounding ICs including crystals and SMD capacitors.

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SMD Ceramic Chip Capacitors

SMD Electrolytic Capacitor

Crystal-

A crystal oscillator is an electronic circuit that uses the mechanical resonance of a

vibrating crystal of piezoelectric material to create an electrical signal with a very

precise frequency. This frequency is commonly used to keep track of time (as in

quartz wristwatches), to provide a stable clock signal for digital integrated circuits,

and to stabilize frequencies for radio transmitters and receivers. The most common

type of piezoelectric resonator used is the quartz crystal, so oscillator circuits

designed around them were called "crystal oscillators".

165

Quartz crystals are produced for frequencies from a few tens of kilohertz to tens of

megahertz. Over two billion crystals are manufactured annually. Most are small

devices for devices such as wristwatches, clocks, radios, computers, and

cellphones. Quartz crystals are also found inside test and measurement

equipment, such as counters, signal generators, and oscilloscopes.

Testing Crystals can be simple. Set your DMM to the frequency setting(if it has

one, if not you can use a frequency counter) and place the red test lead on one of

the crystals pins(TV Must be powered on)and black test lead to cold ground, The

frequency should be within tolerance of the frequency labeled on the crystal. You

will not be able to test all crystals as some will be out of the range of your meter.

Also take into account your meters accuracy. If you have a scope you can also use

it to test crystals again if it's range is withing the frequency of the crystal you want

to test.

Testing SMD crystal with frequency setting of DMM

If you get no frequency reading this could indicate a bad crystal, SMD capacitor in

the oscillator circuitry or a bad IC, check the surrounding components in other

words SMD capacitors etc.. Try directly replacing the crystal. If the surrounding

components test good and/or replacing the crystal does not work most likely the

corresponding IC is bad, in this case the main board will most likely have to be

replaced as replacement ICs can be hard to find and even if you could get them

many are in BGA(Ball Grid Array) or other SMD package which require special

equipment and or skill to replace.

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Testing SMD capacitor on main board

Firmware upgrades-

Sometimes main board type failures can be solved by uploading the flash

memories firmware, or reprogramming a corrupted flash with the original

firmware. Firmware can normally be downloaded and put on a flash drive or

memory stick to be put in the USB port or memory card reader of the LCD TV by

visiting the manufacturers website or calling them and ordering it.

167

Sometimes you may need to purchase the firmware and in this case the company

usually sends a thumb drive with the firmware on it.

Flash/Thumb Drive

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No Video

If you have a TV that works fine but has no video(blue screen, white screen etc.),

first check the obvious and make sure it is not a problem with the video

source(DVD player cable box etc.) or a connection problem like a loose cable

between the video source and the TV. After verifying that the source and

connections to the TV are good we can move on. No video can be caused by an

inverter board failure, but this is not common in LCD TVs, as usually the feedback

circuitry for the TV will shut the TV down in the event of inverter board failure.

Look closely at the TV if it is black but you can kind of see video when looking

closely than this verifies that it most likely is the inverter board, but remember it

could always be bad electrolytic capacitors on the PSU board. If you can tell that

the back lights are lit and you have OSD(On Screen Display) but no video than

you most likely have a tuning issue(like a failed digital tuner) with the main board

or maybe something simple like your on the wrong input or a setting in menu for

instance like needing to switch from antenna to cable. It could also be a bad VPU

or other another IC on the main board, check all the main board and main board IC

voltages and make sure they are within tolerance.. If you are using the co-axial

connection on the tuner you may also have a bad tuner, verify by fist running

through a different tuning source for instance a cable box or VCR first, then to the

TV if this fixes the problem you know the tuner is bad and needs to be replaced. If

you have checked the obvious like the menu setting then you should follow the

procedure for checking the main board. Try a firmware upgrade, sometimes no

video can also be caused by a bad controller board(not when OSD is visible).

Check to see if the pico/nano fuse on the controller board is open or has a high

resistance value if so try to replace it(unlike most fuses, pico/nano fuses have a

tenancy to sometimes open for no reason) if replacing open fuse only causes fuse

to blow again you can try fixing the controller board by tracing down shorted

components with a tool like the leak seeker or checking for shorted components

with your multimeter. In most cases you will need to replace the controller board.

Don't forget to look for solder connection problems on the main board or jack pack

when troubleshooting video and audio problems, especially right on the

connectors that the video/audio source cables plug into. Often cracks develop in

the solder connection because of cables being pushed in and pulled out of the

video/cable cable connectors.

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Checking controller board pico/nano fuse

If the pico/nano fuse is good and you have scope verify output signal from main

board, if present and testing main board checked good check all components

possible on controller board or directly replace, if controller board is found to be

good, then the most likely case is the main board will need to be replaced if you

can not find the bad components on the main board . Remember to check all the

electrolytic capacitors on the main board, main board and IC voltages and IC

surrounding components.

Electrolytic capacitors on LCD TV main board

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No Audio

As usual check the obvious, like the connections from the audio source to the TV

and check in the user settings within the menu to make sure the speakers are not

turned off. Sometimes issues such as low audio while watching DTV but normal

Audio during analogue programming etc. can be fixed with a firmware upgrade.

Remember bad filter capacitors on the PSU as well as the main board can cause all

kinds of problems like no audio or intermittent audio. Check the Vcc at the audio

processor and the audio amplifier ICs, if no or low voltage is present lift the Vcc

pin from circuit, if the voltage returns after removing the ICs pin suspect the IC is

bad and replace, if voltage stays low or is not present after lifting pin suspect

components in the voltage supply line to the IC.(remember to always check

surrounding corresponding circuit components). Again don't forget to look for

solder connection problems on the main board and or jack pack for video and

audio problems, especially right on the connectors that the video/audio source

cables plug into. Often cracks develop on the solder connections because of cables

being repeatedly pushed in and pulled out of the video/cable connectors or because

the cables get tugged on, tripped over or moved around to often.

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OSD/Menu Failure

If you have no on screen display or no menu or erratic menu functions, you may

also have menu functions but the user setting can not be saved, or reset every time

the TV power is cycled. This indicates an MCU or VPU failure and or EEPROM

failure and in most cases unless you have another main board to swap ICs from,

and in the case of the VPU very specialized expensive equipment for testing

removal and replacement, you will have to replace the main board. Remember to

check all components surrounding the MCU and VPU like SMD capacitors,

resistors, crystals etc. and also always check all the electrolytic capacitors on the

main board.

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White Screen

If the LCD TV powers on with an all white screen(can also be all black screen

with back lights lit depending on panel design), first as usual inspect the PSU for

bad solder connection and check all the electrolytic capacitors on the PSU. Next

check the pico/nano fuse on the controller/T-con board. Remember the controller

board is ESD sensitive, so take the proper precautions when testing the board.

Check the LVDS cable from the main board to the controller board and make sure

it is not loose. Also check the solder connections on the cable connectors. Make

sure voltage is present at the t-con board.

Cable from main board to controller board

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Check and re seat the FPC or the flat flexible cable from the controller board to the

driver board.

Flat flexible cable connecting controller board to driver board

If all the previous checks out good then check the main board components like

electrolytic capacitors etc.

If the pico/nano fuse on the controller board is open you can try replacing the fuse

to see if it will fix the TV, If the fuse opens again you can replace the controller

board or use a tool like the leak seeker to find the shorted components on the

controller board or use your meter to check for shorted compoents. Usually with

controller boards it is best to just replace the PCB. Most of the time white display

is caused by a bad controller board or bad electrolytic capacitors in the

PSU(secondary filter capacitors on the voltage line to the controller board) or in

the voltage regulation section of the mainboard.

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Rainbow Screen

Steps for troubleshooting the rainbow screen or colored lines screen. First check

the LVDS cable from main board to controller board is not loose. I check that the

cable itself is actually good and re seat the LVDS cable, replace LVDS cable if

necessary. Re seat FPC from the controller board to the driver board and also

check the solder connections on the cable connectors to the boards. Check solder

connections and components on the controller PCB. If everything checks good,

although this still could be a main board issue it is most likely a bad controller

board IC or the panel. More likely the LCD panel has a bad driver board/driver

ICs, but use the procedure for checking the main board before you spend the

money to replace a panel if you actually decide to or if the cost is economical.

Note that you can almost never find the LCD panel alone, but only a full display

panel which has the back light assembly and all. Most people refer to this whole

assembly as the LCD panel, but actually you can disassemble and remove the

actual LCD panel like the photo on page 35. Together the LCD panel, and the back

lights make the LCD display. Usually when you order a full display(listed as panel

with part distributors) it will also have the controller PCB mounted to the back

already but not always so ask, this can be good if you are not quite sure if the LCD

panel or controller PCB are causing the issue.

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Screen Flashes Then TV Shuts Down

If you turn on the LCD TV and it flashes, meaning that the back light comes on for

just a moment and then the TV shuts down this almost always indicates an inverter

board failure. Go over the methods for testing the inverter board. Remember check

the electrolytic capacitors in the PSU, especially the secondary side filter

capacitors that are filtering the ripple from the PSU voltage output that goes to

inverter board.

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No Back light

This is when the LCD TV turns on and works normally has audio and if you look

closely it has OSD, video etc., but the display is not illuminated. This is almost

always an inverter board failure. Go over the procedure for troubleshooting the

inverter board. As I have said many times now check the electrolytic capacitors in

the PSU, especially the secondary side filter capacitors that are filtering the ripple

from the voltage output that goes to the inverter board.

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Lines In Picture

Vertical and horizontal lines as well on the display are almost always caused by

bad connections from the driver board to the panel. In this case the display or

panel must be replaced unless you have the specialized equipment necessary or

can come up with a device to safely redo all the connections on the FPC that

connects the driver board to the panel and even if you could repair the connection

there is still a chance you could have a bad driver IC on the FPC.

FPC(Flexible Printed Circuit) that connects the driver board to the LCD

display

178

Cracked panel

If the panel becomes cracked the only method of repair is to replace the display or

remove the LCD panel from the display and replace it, but most likely you will

have to buy an entire display, back light assembly and all.

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LCD TV Repair Case Histories

Vizio L37HDTV-

Symptom:

Amber light , no power

Repair:

Removed U26 (this is near the tactile switch) and added a 10Kohm resistor

between pins 2 and 3.

Vizio VO47LF-

Symptom:

Set has no back light, has audio.

Repair:

Replaced 2 680uF, 35V Electrolytic capacitors on the inverter PCB, and one in

PSU.

Samsung LNT4061FX/XAA-

Symptom:

TV had vertical colored lines no OSD and no remote function.

Repair:

Found 4 1000uF, 16V electrolytic capacitors in power supply puffed and vented.

ESR to high for meter to display reading. Part location numbers for capacitors,

CM11 CM12 CM17 CM6. Replacing Capacitors fixed TV.

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Samsung LNT4661F-

Symptom:

Takes a while to come on, when it is on, there are red dots over the video.

Repair:

Replaced three 1000uF, 16V electrolytic capacitors in the PSU. Location CS804,

CV812, CV813

Samsung LNT4671FX-

Symptom:

Unit cycles on/off for minutes before coming on.

Repair:

Replace electrolytic capacitors with locations, CM853 and CM852, both 2200uF,

10V in PSU.

Westinghouse LTV32W1-

Symptom:

No power, power LED turns blue to amber then off , might be intermittent.

Repair:

Found bad solder connection on PSU transformer T1 pin 4, soldering connection

fixed TV.

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Sylvania LC320SL8-

Symptom:

Dead set, main fuse blown, and blows violently if replaced.

Repair:

1 1N5406 diode D502 shorted, Bridge rectifier shorted. Replacing shorted

components and fuse repaired the TV.

Sharp LC37D43U-

Symptom:

No back light.

Repair:

Replaced fuse (F7502) rated 500ma on the inverter board. TV worked fine after

replacing fuse.

Polaroid FLM323B-

Symptom:

No Audio.

Repair:

Replace open 10 ohm SMD resistor (R385) near the

top end of the main board located between two audio supply filter caps.

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Polaroid FLM3732-

Symptom:

No power, no standby LED

Repair:

Replaced 1000uF, 10V electrolytic capacitors, locations C112 and C113on the

PSU.

Magnavox 19MF337B-

Symptom:

Flickering picture, loud audible noise, maybe be intermittent start.

Repair:

Replace electrolytic capacitors in PSU. Location C12, C117, C118(1000uF, 16V)

and C175(470uF, 16v)

Magnavox 37MF321D37-

Symptom:

No power.

Repair:

Replace electrolytic capacitors in PSU (likely vented) two

2200uF, 50V and two 1000uF, 50V.

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Magnavox 50MF231D/37-

Symptom:

Power LED lights but set does not come on.

Repair:

Replaced bad 3300uF, 10V electrolytic capacitors, locations C8059 and C8060 in

the PSU.

LG RM-30LZ50-

Symptom:

Irregular glittering speckles on screen.

Repair:

Remove C857, 220uF, 16V electrolytic capacitor on main board

and replace with 470UF, 16V electrolytic Capacitor.

LG RM-32LZ50-

Symptom:

No picture but has sound sound.

Repair:

Remove R60 SMD resistor on

Inverter PCB.

LG 26LX1D-

Symptom:

Wavy lines in picture.

184

Repair:

Replaced C708 and C704 on main board. 100UF, 16V SMD

electrolytic capacitors.

Emerson LC320EM8AN-

Symptom:

No power,Dead.

Repair:

Replaced shorted diode,type FR202, in PSU. Location,

D633.

185

Safety

A few important tips to remember when working on live circuits.

-Only attempt to work on electrical or electronics devices when well rested.

Working on electrical or electronic equipment when you are very tired is not only

a safety hazard but also it is very unlikely that you will be able to troubleshoot

properly and make successful repairs if your not thinking properly.

-Hot and cold ground. Always be aware of the difference between hot and cold

ground especially when taking PSU voltage and waveform measurements.

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The hot and cold side are usually clearly identified.

-Soldering can be hazardous to your health, remember to be in a well ventilated

open area when soldering as smoke inhalation is the main hazard when soldering,

besides giving yourself a really good burn. The solder we use on electronics

circuits contains rosin, which inhalation of the smoke from can cause asthma.

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-Discharging Reservoir/Filter capacitor/s

When you remove the PSU board to check components and etc. always discharge

the reservoir/primary filter capacitor to avoid getting shocked. I use a 1k ohm 2-5

Watt resistor to discharge the capacitor. Check the DC voltage on the capacitor

after discharge just to be safe.

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Discharging Capacitor

Checking voltage after discharge

-Working on live circuits. Try to never work on live circuits, of course this is not

always possible as tests like the voltage testing must be done with the TV on, but

always use the most care when working on live circuits as to not hurt or kill

yourself and or damage your test equipment and the TV under test

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Conclusion

In conclusion I hope you have learned a lot about LCD TVs, how they work and

how to troubleshoot and repair them. Obviously I could not even come close to

covering or mentioning all of the electronics circuits involved in the workings of

an LCD TV as it is way beyond the scope of this book which is based in common

repairs and not LCD TV design. The best you could do to further your knowledge

is to study the different designs and of course learn as much about electronics as

possible from the basics to the advanced from the past to the current. The more

you understand and recognize different electronic circuits the easier it will be for

you to troubleshoot and repair LCD TVs and any other electronic devices as well.

If you have any further questions or need troubleshooting help email me

john@preher-tech.com.

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Further Reading

Some good books to further your electronics knowledge.

Electronic and Electrical Servicing Consumer and Commercial Electronics 2nd

edition, By Ian Sinclair and John Dunton

Electronic and Electrical Servicing Consumer and Commercial Electronics Level

3" 2nd edition, By Ian Sinclair and John Dunton

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How To Find Burnt Resistor Value, By Jestine Yong

LCD Monitor Repair, By Jestine Yong

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Testing Electronic Components, By Jestine Yong

Troubleshooting & Repairing Switch Mode Power Supplies, By Jestine Yong

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Websites and Free Download

Some good websites for electronics information.

LCD TV repair membership site

Plasma TV repair membership site

Projection TV repair membership site

electronicrepairguide.com

Jestine Yong's Blog

Preher-Tech Blog

preher-tech.com

fastrepairguide.com

tvrepairinfo.com

Preher-Tech's newsletter

Free Download

LCD TV Display Failure Symptoms and Possible Causes

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Part, Tool and Test Equipment Distributors

Here are the distributors I most commonly use to get all the parts, tools and test

equipment I need for fixing LCD TVs.

Suburban

Encompass

Mouser

MCM

And I also always like looking on ebay, I have bought many TV parts and

electronic components on that site in general with great deals and I have only had

great transactions so far. Only buy from reputable dealers.

Places to purchase ESR meters, ring testers and more-

Preher-Tech(Tools and test equipment sales page)

Electronic Repair Guide(also sells Ring Tester)

Anatek(also sells Ring Tester)

EVB

Radio Devices(the ESR micro, which also checks capacitance)

CCFL Testers-

HR-TL1040

LST01

Good place to buy decent and affordable digital multimeters , analogue

meters and more-

Multimeter Warehouse

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