LCD TV logic board failure phenomenon and maintenance - android - Phones Developers

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Although the basic principle of LCD and CRT TVs is similar in many parts of the circuit, some technicians still lack understanding of the LCD TV logic board. Their maintenance knowledge hasn’t been updated, so when an LCD TV has issues, they often hesitate to repair it, leading to no profit. In reality, most faults on the logic board—except for LVDS signal issues—are still repairable. To address common questions from users on forums about logic board problems, this guide aims to simplify the content and share practical repair ideas with readers in a clear and easy-to-understand way.

The logic board, also known as the screen driver board or TCON board, is responsible for receiving LVDS signals from the mainboard. These signals include RGB data, clock, and control signals. After processing, the logic board converts the LVDS signal into a TTL-level parallel signal that drives the liquid crystal panel. If the LVDS signal is not properly converted, the screen won’t display correctly.

To check the logic board, start by testing the screen voltage (5V or 12V depending on the model). Then measure the voltage at the LVDS output interface. If there’s no change, the issue likely lies with the logic board. A pointer multimeter can be used to detect small fluctuations (usually around 1.2V). If possible, replace the logic board with a compatible one to confirm the fault.

The LVDS cable between the mainboard and the logic board typically follows a pattern: red for positive power, black for negative, and twisted pairs for the signal lines (4, 5, 8, or 10 pairs depending on the resolution). Some newer models use X+C integrated boards, while older ones are separate. Due to their complexity, these boards are generally not easy to repair. However, with proper knowledge and soldering skills, even complex logic board issues can be resolved.

Common signs of a faulty logic board include no image, incorrect colors, vertical lines, or flower-screen effects. The menu may still work, but the screen might show intermittent color lines or no characters. If the screen has poor contact, inserting and removing the connection cable can help identify the problem. Both the logic board and the screen can cause screen issues, but logic board-related problems usually result in regular patterns, unlike screen defects.

Here’s a quick method to diagnose logic board issues:

1. Check if the input video signal waveform on the logic board is normal. If yes, the digital processing part is working.
2. Verify the power supply voltage on the logic board. If it’s normal, the power circuit is functioning.
3. Test the output signal waveform from the logic board to the screen. If it’s abnormal, the logic board is likely faulty.

Common Faults and Solutions:

Symptom 1: No Image
Most often caused by a lack of power to the logic board. For screens larger than 26 inches, the screen may appear gray or black, while smaller screens may show white. Check the logic board’s power supply first. If the voltage is normal, test the fuse. If the fuse is blown, replace it after checking for shorts. If no voltage is present, unplug the upper screen connector and retest the signal board’s output.

Symptom 2: Abnormal Color or Contrast
Test the data signal on the logic board. Use an oscilloscope for accuracy. If unavailable, measure DC voltage to make a rough assessment. After powering on, touch the ICs to see if they heat up normally. Excessive heat indicates a fault. Common symptoms include blurred images, motion trails, or color spots.

Symptom 3: Vertical Bars on Screen
This can be full-screen or partial. Replace the logic board to determine if the issue is with the board or the screen itself. If the problem persists after replacement, the screen is likely faulty.

Repair Examples:

Model: TLM4236P
Fault: No sound, no black screen.
Diagnosis: The backlight turns on, indicating the motherboard is working. Focus on the logic board. Voltage tests revealed a low VGHP voltage. Replacing the UP1 chip fixed the issue.

Model: TLM40V68P
Fault: White screen.
Diagnosis: All functions were normal except the screen. No VGHP voltage was detected. Replacing DP5 restored the voltage and fixed the issue.

Flower Screen (General Case)
If the image appears blooming, the issue could be with the motherboard or scaler chip. Try pressing the chip to see if the screen changes. If it does, the problem is likely a cold solder joint. If the issue occurs after repeated power cycles, the driver board may be faulty.

Screen parameter errors can also cause display issues. Adjusting the bus settings, like on Changhong TVs, can resolve this. If software is outdated, consider updating via USB.

If you're unsure about the cause, it's best to consult a professional. Different brands and models may have unique issues, and without specialized tools, repairs can be challenging. Always proceed with caution to avoid further damage.