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

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Although the basic principle of LCD TVs and CRT TVs is similar in most parts of the circuit, some technicians are still unfamiliar with the logic board of LCD TVs. Their maintenance knowledge hasn't been updated, so they avoid repairing LCD TVs when issues arise, which limits their ability to profit. In reality, except for LVDS program failures on the logic board, other problems are often fixable. To address user concerns about the logic board on forums, this guide simplifies complex information and shares practical repair ideas in an easy-to-understand way.

The logic board, also known as the screen driver board, center control board, or TCON board, receives LVDS signals from the digital board. These signals include RGB data, clock signals, and control signals. After processing, the logic board converts them into TTL-level signals and sends them to the LVDS receiving chip on the panel side, which drives the display. When checking for logic board issues, test the screen voltage (either 5V or 12V depending on the model) and the LVDS output interface voltage. If there's no change, it likely points to a logic board fault. Using a pointer multimeter, you can observe slight fluctuations around 1.2V. If possible, replacing the logic board with a compatible one is the most reliable solution.

The LVDS cable from the mainboard to the logic board follows a specific pattern: red is the positive power line, black is the negative, and the signal lines are twisted (4 pairs for 6-bit, 5 pairs for 8-bit, etc.). Some newer models use X+C integrated boards, while larger screens typically use separate ones due to cost considerations. Due to high precision and technical complexity, these boards are not easily repaired. However, with thorough understanding and good soldering skills, even complex logic board repairs are possible.

A faulty logic board may cause the screen to show no image or only partial display, with no menu visible but functional buttons and remote controls. Vertical color lines or missing characters are common signs. Intermittent contact between the logic board and the screen connector can lead to small color spots. Both the logic board and the screen can cause a "flower screen," but the pattern differs—logic board-related flower screens are regular and can be checked by changing the input source or switching to 4:3 mode.

Here’s a quick method to identify logic board faults:

1. Check if the video signal waveform from the digital board is normal on the logic board. A normal waveform indicates the digital board is working.
2. Verify the power supply voltage on the logic board. Normal voltage means the power supply circuit is functioning.
3. Test the LCD drive signal output at the screen interface. No proper waveform suggests a logic board issue.

Common Fault Symptoms and Solutions:

Symptom 1: No Image
Most often caused by a lack of power to the logic board. Larger screens (26 inches and above) may appear gray or black, while smaller screens might show a white screen. First, check the logic board's power supply voltage. If it's normal, inspect the fuse. If the fuse is blown, replace it after confirming no short circuits. If the voltage is low, unplug the upper screen cable and recheck. A normal voltage here suggests a logic board short, otherwise the problem is in the mainboard.

Symptom 2: Abnormal Color or Contrast
Check the data signal at the logic board. Use an oscilloscope for accurate readings, or measure DC voltage if unavailable. If the IC gets too hot quickly, there’s likely a circuit issue. Common symptoms include loss of detail, negative images, motion trails, or color spots. These usually indicate a faulty logic board.

Symptom 3: Vertical Bars on Screen
This can be full-screen or partial. It often mimics a screen damage issue. Replace the logic board with a similar one to confirm. If the problem remains, the screen itself is likely faulty.

Maintenance Examples:

Model: TLM4236P
The backlight turns on, but the screen is black. After testing, the VGHP voltage was only 10.5V instead of the normal 19.5V. Replacing the damaged pin on UP1 resolved the issue.

Model: TLM40V68P
The screen showed a white image. The VGHP voltage was missing. After removing the filter capacitor CP19, the voltage returned to normal, and the board worked properly after replacement.

Flower Screen Issue:
Usually linked to the motherboard or scaler chip. Pressing the chip or using a heat gun to re-solder can help. Poor LVDS cable contact is another common cause, so cleaning the connection is recommended. If the screen changes after multiple power cycles, the driver board is likely faulty. Otherwise, the screen itself may be damaged.

Screen Parameter Errors:
Different screens require different settings. For example, in Changhong TVs, you can access the factory menu via a specific key sequence and adjust parameters. If the software version is outdated, upgrading via USB may resolve the issue.

If you're unsure about the cause of the screen issue, it's best to consult a professional. Each brand and model has unique characteristics, and some screens may have inherent defects. Without specialized tools and experience, it's difficult to match the quality of professional repairs. Always proceed carefully to avoid further damage.