26LC51.pdf

website:http://biz.LGservice.com

LCD TV

SERVICE MANUAL

CHASSIS : LP78A

MODEL : 26LC51(C) 26LC51(C)-ZA

MODEL : 26LC7R

CAUTION

BEFORE SERVICING THE CHASSIS,

READ THE SAFETY PRECAUTIONS IN THIS MANUAL.

26LC7R-ZA

CONTENTS

CONTENTS .............................................................................................. 2

SAFETY PRECAUTIONS ..........................................................................3

SPECIFICATION ........................................................................................6

ADJUSTMENT INSTRUCTION .................................................................9

TROUBLE SHOOTING ............................................................................17

BLOCK DIAGRAM...................................................................................25

EXPLODED VIEW .................................................................................. 26

EXPLODED VIEW PARTS LIST ..............................................................27

REPLACEMENT PARTS LIST ............................................................... 28

SVC. SHEET ...............................................................................................

- 2 -

SAFETY PRECAUTIONS

IMPORTANT SAFETY NOTICE

Many electrical and mechanical parts in this chassis have special safety-related characteristics. These parts are identified by in the

Schematic Diagram and Replacement Parts List.

It is essential that these special safety parts should be replaced with the same components as recommended in this manual to prevent

Shock, Fire, or other Hazards.

Do not modify the original design without permission of manufacturer.

General Guidance

Leakage Current Hot Check (See below Figure)

Plug the AC cord directly into the AC outlet.

An isolation Transformer should always be used during the

servicing of a receiver whose chassis is not isolated from the AC

power line. Use a transformer of adequate power rating as this

protects the technician from accidents resulting in personal injury

from electrical shocks.

Do not use a line Isolation Transformer during this check.

Connect 1.5K/10watt resistor in parallel with a 0.15uF capacitor

between a known good earth ground (Water Pipe, Conduit, etc.)

and the exposed metallic parts.

Measure the AC voltage across the resistor using AC voltmeter

with 1000 ohms/volt or more sensitivity.

Reverse plug the AC cord into the AC outlet and repeat AC voltage

measurements for each exposed metallic part. Any voltage

measured must not exceed 0.75 volt RMS which is corresponds to

0.5mA.

In case any measurement is out of the limits specified, there is

possibility of shock hazard and the set must be checked and

repaired before it is returned to the customer.

It will also protect the receiver and it's components from being

damaged by accidental shorts of the circuitry that may be

inadvertently introduced during the service operation.

If any fuse (or Fusible Resistor) in this TV receiver is blown,

replace it with the specified.

When replacing a high wattage resistor (Oxide Metal Film Resistor,

over 1W), keep the resistor 10mm away from PCB.

Leakage Current Hot Check circuit

Keep wires away from high voltage or high temperature parts.

Before returning the receiver to the customer,

AC Volt-meter

always perform an AC leakage current check on the exposed

metallic parts of the cabinet, such as antennas, terminals, etc., to

be sure the set is safe to operate without damage of electrical

shock.

Good Earth Ground

such as WATER PIPE,

CONDUIT etc.

To Instrument’s

exposed

METALLIC PARTS

0.15uF

Leakage Current Cold Check(Antenna Cold Check)

With the instrument AC plug removed from AC source, connect an

electrical jumper across the two AC plug prongs. Place the AC

switch in the on position, connect one lead of ohm-meter to the AC

plug prongs tied together and touch other ohm-meter lead in turn to

each exposed metallic parts such as antenna terminals, phone

jacks, etc.

If the exposed metallic part has a return path to the chassis, the

measured resistance should be between 1M Ω and 5.2M Ω .

When the exposed metal has no return path to the chassis the

reading must be infinite.

An other abnormality exists that must be corrected before the

receiver is returned to the customer.

1.5 Kohm/10W

- 3 -

SERVICING PRECAUTIONS

CAUTION: Before servicing receivers covered by this service

manual and its supplements and addenda, read and follow the

SAFETY PRECAUTIONS on page 3 of this publication.

NOTE: If unforeseen circumstances create conflict between the

following servicing precautions and any of the safety precautions on

page 3 of this publication, always follow the safety precautions.

Remember: Safety First.

unit under test.

2. After removing an electrical assembly equipped with ES

devices, place the assembly on a conductive surface such as

aluminum foil, to prevent electrostatic charge buildup or

exposure of the assembly.

3. Use only a grounded-tip soldering iron to solder or unsolder ES

devices.

4. Use only an anti-static type solder removal device. Some solder

removal devices not classified as "anti-static" can generate

electrical charges sufficient to damage ES devices.

5. Do not use freon-propelled chemicals. These can generate

electrical charges sufficient to damage ES devices.

6. Do not remove a replacement ES device from its protective

package until immediately before you are ready to install it.

(Most replacement ES devices are packaged with leads

electrically shorted together by conductive foam, aluminum foil

or comparable conductive material).

7. Immediately before removing the protective material from the

leads of a replacement ES device, touch the protective material

to the chassis or circuit assembly into which the device will be

installed.

CAUTION: Be sure no power is applied to the chassis or circuit,

and observe all other safety precautions.

8. Minimize bodily motions when handling unpackaged

replacement ES devices. (Otherwise harmless motion such as

the brushing together of your clothes fabric or the lifting of your

foot from a carpeted floor can generate static electricity

sufficient to damage an ES device.)

General Servicing Precautions

1. Always unplug the receiver AC power cord from the AC power

source before;

a. Removing or reinstalling any component, circuit board

module or any other receiver assembly.

b. Disconnecting or reconnecting any receiver electrical plug or

other electrical connection.

c. Connecting a test substitute in parallel with an electrolytic

capacitor in the receiver.

CAUTION: A wrong part substitution or incorrect polarity

installation of electrolytic capacitors may result in an

explosion hazard.

2. Test high voltage only by measuring it with an appropriate high

voltage meter or other voltage measuring device (DVM,

FETVOM, etc) equipped with a suitable high voltage probe.

Do not test high voltage by "drawing an arc".

3. Do not spray chemicals on or near this receiver or any of its

assemblies.

4. Unless specified otherwise in this service manual, clean

electrical contacts only by applying the following mixture to the

contacts with a pipe cleaner, cotton-tipped stick or comparable

non-abrasive applicator; 10% (by volume) Acetone and 90% (by

volume) isopropyl alcohol (90%-99% strength)

CAUTION: This is a flammable mixture.

Unless specified otherwise in this service manual, lubrication of

contacts in not required.

5. Do not defeat any plug/socket B+ voltage interlocks with which

receivers covered by this service manual might be equipped.

6. Do not apply AC power to this instrument and/or any of its

electrical assemblies unless all solid-state device heat sinks are

correctly installed.

7. Always connect the test receiver ground lead to the receiver

chassis ground before connecting the test receiver positive

lead.

Always remove the test receiver ground lead last.

8. Use with this receiver only the test fixtures specified in this

service manual.

CAUTION: Do not connect the test fixture ground strap to any

heat sink in this receiver.

General Soldering Guidelines

1. Use a grounded-tip, low-wattage soldering iron and appropriate

tip size and shape that will maintain tip temperature within the

range or 500°F to 600°F.

2. Use an appropriate gauge of RMA resin-core solder composed

of 60 parts tin/40 parts lead.

3. Keep the soldering iron tip clean and well tinned.

4. Thoroughly clean the surfaces to be soldered. Use a mall wire-

bristle (0.5 inch, or 1.25cm) brush with a metal handle.

Do not use freon-propelled spray-on cleaners.

5. Use the following unsoldering technique

a. Allow the soldering iron tip to reach normal temperature.

(500°F to 600°F)

b. Heat the component lead until the solder melts.

c. Quickly draw the melted solder with an anti-static, suction-

type solder removal device or with solder braid.

CAUTION: Work quickly to avoid overheating the circuit

board printed foil.

6. Use the following soldering technique.

a. Allow the soldering iron tip to reach a normal temperature

(500°F to 600°F)

b. First, hold the soldering iron tip and solder the strand against

the component lead until the solder melts.

c. Quickly move the soldering iron tip to the junction of the

component lead and the printed circuit foil, and hold it there

only until the solder flows onto and around both the

component lead and the foil.

CAUTION: Work quickly to avoid overheating the circuit

board printed foil.

d. Closely inspect the solder area and remove any excess or

splashed solder with a small wire-bristle brush.

Electrostatically Sensitive (ES) Devices

Some semiconductor (solid-state) devices can be damaged easily

by static electricity. Such components commonly are called

Electrostatically Sensitive (ES) Devices. Examples of typical ES

devices are integrated circuits and some field-effect transistors and

semiconductor "chip" components. The following techniques

should be used to help reduce the incidence of component

damage caused by static by static electricity.

1. Immediately before handling any semiconductor component or

semiconductor-equipped assembly, drain off any electrostatic

charge on your body by touching a known earth ground.

Alternatively, obtain and wear a commercially available

discharging wrist strap device, which should be removed to

prevent potential shock reasons prior to applying power to the

- 4 -

IC Remove/Replacement

Some chassis circuit boards have slotted holes (oblong) through

which the IC leads are inserted and then bent flat against the

circuit foil. When holes are the slotted type, the following technique

should be used to remove and replace the IC. When working with

boards using the familiar round hole, use the standard technique

as outlined in paragraphs 5 and 6 above.

Circuit Board Foil Repair

Excessive heat applied to the copper foil of any printed circuit

board will weaken the adhesive that bonds the foil to the circuit

board causing the foil to separate from or "lift-off" the board. The

following guidelines and procedures should be followed whenever

this condition is encountered.

Removal

1. Desolder and straighten each IC lead in one operation by gently

prying up on the lead with the soldering iron tip as the solder

melts.

2. Draw away the melted solder with an anti-static suction-type

solder removal device (or with solder braid) before removing the

IC.

Replacement

1. Carefully insert the replacement IC in the circuit board.

2. Carefully bend each IC lead against the circuit foil pad and

solder it.

3. Clean the soldered areas with a small wire-bristle brush.

(It is not necessary to reapply acrylic coating to the areas).

At IC Connections

To repair a defective copper pattern at IC connections use the

following procedure to install a jumper wire on the copper pattern

side of the circuit board. (Use this technique only on IC

connections).

1. Carefully remove the damaged copper pattern with a sharp

knife. (Remove only as much copper as absolutely necessary).

2. carefully scratch away the solder resist and acrylic coating (if

used) from the end of the remaining copper pattern.

3. Bend a small "U" in one end of a small gauge jumper wire and

carefully crimp it around the IC pin. Solder the IC connection.

4. Route the jumper wire along the path of the out-away copper

pattern and let it overlap the previously scraped end of the good

copper pattern. Solder the overlapped area and clip off any

excess jumper wire.

"Small-Signal" Discrete Transistor

Removal/Replacement

1. Remove the defective transistor by clipping its leads as close as

possible to the component body.

2. Bend into a "U" shape the end of each of three leads remaining

on the circuit board.

3. Bend into a "U" shape the replacement transistor leads.

4. Connect the replacement transistor leads to the corresponding

leads extending from the circuit board and crimp the "U" with

long nose pliers to insure metal to metal contact then solder

each connection.

At Other Connections

Use the following technique to repair the defective copper pattern

at connections other than IC Pins. This technique involves the

installation of a jumper wire on the component side of the circuit

board.

1. Remove the defective copper pattern with a sharp knife.

Remove at least 1/4 inch of copper, to ensure that a hazardous

condition will not exist if the jumper wire opens.

2. Trace along the copper pattern from both sides of the pattern

break and locate the nearest component that is directly

connected to the affected copper pattern.

3. Connect insulated 20-gauge jumper wire from the lead of the

nearest component on one side of the pattern break to the lead

of the nearest component on the other side.

Carefully crimp and solder the connections.

CAUTION: Be sure the insulated jumper wire is dressed so the

it does not touch components or sharp edges.

Power Output, Transistor Device

Removal/Replacement

1. Heat and remove all solder from around the transistor leads.

2. Remove the heat sink mounting screw (if so equipped).

3. Carefully remove the transistor from the heat sink of the circuit

board.

4. Insert new transistor in the circuit board.

5. Solder each transistor lead, and clip off excess lead.

6. Replace heat sink.

Diode Removal/Replacement

1. Remove defective diode by clipping its leads as close as

possible to diode body.

2. Bend the two remaining leads perpendicular y to the circuit

board.

3. Observing diode polarity, wrap each lead of the new diode

around the corresponding lead on the circuit board.

4. Securely crimp each connection and solder it.

5. Inspect (on the circuit board copper side) the solder joints of

the two "original" leads. If they are not shiny, reheat them and if

necessary, apply additional solder.

Fuse and Conventional Resistor

Removal/Replacement

1. Clip each fuse or resistor lead at top of the circuit board hollow

stake.

2. Securely crimp the leads of replacement component around

notch at stake top.

3. Solder the connections.

CAUTION: Maintain original spacing between the replaced

component and adjacent components and the circuit board to

prevent excessive component temperatures.

- 5 -

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