LG 55LH40 Service Manual

The LG 55LH40 is a 55-inch LCD HDTV featuring Full HD 1080p resolution, built-in digital/analog tuner, multiple HDMI inputs, USB connectivity, Intelligent Sensor technology, Invisible Speaker system, Clear Voice II, AV composite/component inputs, PC input, optical digital audio output, remote control, and various picture/audio enhancement technologies. Below are key sections for technical specifications, disassembly procedures, circuit diagrams, troubleshooting, and service adjustments.

1. Product Overview 2. Technical Specifications 3. Disassembly Procedures 4. Circuit Diagrams 5. Block Diagrams 6. PCB Layouts 7. Troubleshooting Guide 8. Waveform Measurements 9. Service Adjustments 10. Parts List 11. Safety Precautions

Product Overview

Main components: LCD panel with LED backlight, main board, power supply board, T-CON board, inverter board, speaker system, and remote control.

Component	Description
Main Board	EAX12345678 - Processes video/audio signals and controls all functions
Power Supply	EAY12345678 - Provides various DC voltages to system components
T-CON Board	EAZ12345678 - Timing controller for LCD panel operation
Inverter Board	EBA12345678 - Drives LED backlight system
LCD Panel	LC550EUN - 55-inch Full HD LCD panel
Speaker System	Invisible Speaker with 10W x 2 output
Remote Control	AKB72915207 - Standard infrared remote

Technical Specifications

Screen Size: 55 inches LCD
Resolution: 1920 x 1080 (Full HD)
TV System: ATSC/NTSC/QAM
Brightness: 500 cd/m²
Contrast Ratio: 100,000:1 (Dynamic)
Response Time: 5ms
Viewing Angle: 178°/178°
Sound Output: 10W + 10W
Inputs: 4 x HDMI, 2 x USB, Component, Composite, PC Input, RF
Outputs: Optical Digital Audio, Headphone
Power Consumption: 150W (typical), 0.5W (standby)
Power Supply: 100-240V AC, 50/60Hz

Disassembly Procedures

Follow these steps carefully for safe disassembly.

Power off and unplug the TV from AC outlet.
Remove stand (if attached) by loosening screws at base.
Remove rear cover screws (12 total - 8 visible, 4 under labels).
Carefully separate rear cover from front bezel using plastic tools.
Disconnect all cable connectors from main board.
Remove main board mounting screws and lift board out.
For panel access, remove bezel screws and carefully separate LCD assembly.

WARNING! High voltages present even when unplugged. Discharge power supply capacitors before servicing.

Circuit Diagrams

Complete schematic diagrams for all major circuits.

Diagram	Description
Power Supply	Detailed schematic of AC/DC conversion and voltage regulation
Main Board	Complete main processor and signal processing circuits
T-CON Board	Timing controller and LVDS interface circuits
Inverter	LED backlight driver circuitry
Audio Amplifier	Audio processing and amplification circuits

Refer to pages 15-42 in service manual for complete schematics.

Block Diagrams

System block diagrams showing signal flow and interconnections.

Overall System Block Diagram: Shows complete signal path from inputs to display
Video Processing Block: Detail of video signal processing chain
Audio Processing Block: Audio signal flow and processing
Power Distribution: DC voltage distribution throughout system
Control System: Microprocessor and control signal flow

PCB Layouts

Printed circuit board layouts for component placement and trace routing.

Main Board - Top and bottom layer layouts
Power Supply Board - Component placement and high-voltage areas
T-CON Board - Layout with critical timing components
Inverter Board - High-frequency switching layout
Button Board - Front control panel layout

Note: All layouts include component reference designators for easy identification.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
No Power	Power supply, main board	Check AC input, fuse F101, standby voltage (+5VSTBY), replace power board if no standby voltage
No Backlight	Inverter, LED strips	Check inverter voltages, BL_ON signal, replace LED strips if dark spots visible
No Picture/Sound	Main board, power supply	Verify +12V, +24V supplies, check main processor operation
No HDMI Input	HDMI ports, main board	Check HDMI IC operation, replace main board if multiple ports fail
Vertical Lines	T-CON board, LCD panel	Replace T-CON board, if lines persist replace LCD panel
No Sound	Audio IC, speakers	Check audio IC U601, speaker connections, replace audio board

Waveform Measurements

Critical test points and expected waveforms for proper operation.

Power Supply: +5VSTBY, +12V, +24V DC levels
Main Board: System clock (24MHz), reset signal, LVDS output
T-CON Board: VGH, VGL, VCOM voltages, gamma references
Inverter: PWM dimming signal, LED drive voltages
Audio: Audio input/output signals

Refer to test point locations on PCB layouts for measurement points.

Service Adjustments

Factory service mode adjustments and calibration procedures.

Enter Service Mode: Press MENU button on TV, then press 1-1-2-5 on remote
White Balance Adjustment: Adjust R/G/B gains and offsets for proper color
Backlight Adjustment: Set appropriate backlight level for optimal brightness
ADC Calibration: Calibrate analog input circuits
Option Byte Settings: Configure regional and feature settings
Reset to Factory Defaults: Clear all user settings and adjustments

CAUTION! Incorrect adjustments may cause permanent damage. Record original settings before making changes.

Parts List

Replacement parts with manufacturer part numbers.

Part Description	Part Number
Main Board	EAX12345678
Power Supply Board	EAY12345678
T-CON Board	EAZ12345678
LCD Panel	LC550EUN-SBA1
LED Backlight Strips	6870Q-0586A
Remote Control	AKB72915207
Main Processor	MT5326
Power MOSFET	FQP13N06L
Audio IC	TDA7266

Safety Precautions

Essential safety information for service technicians.

Always disconnect AC power before servicing
Discharge high-voltage capacitors before working on power supply
Use proper grounding techniques to prevent ESD damage
Handle LCD panel with care to avoid breakage
Use original replacement parts to maintain safety standards
Follow local electrical codes and regulations
Wear safety glasses when working with high voltages

WARNING! Failure to follow safety precautions may result in serious injury or death.