Article 3: How to Repair Logic Board: Smart LED / LCD TV Screen Driver Board

      The logic board, also known as the screen driver board, main control board, or TCON board, plays a crucial role in LCD TVs. It processes the LVDS (Low Voltage Differential Signaling) image data signal from the digital board (which includes RGB data signals, clock signals, and control signals), converting them into LVDS signals that can drive the LCD screen. These signals are then sent directly to the LCD screen’s LVDS receiver chip. The logic board handles the conversion of image data and clock signals into control signals that the screen can recognize, which then controls the MOSFETs inside the screen to regulate the twisting of liquid crystals, thus displaying images. The logic board is a software-driven component with built-in shift registers (horizontal and vertical) and a specialized FLASH module, which even the manufacturer cannot alter. 

Power Supply for the Logic Board:

The power for the logic board doesn’t come directly from the main power supply; instead, it's provided by the voltage regulator circuit on the signal processing board.

Voltage in the Logic Board:

The working voltage of an LCD screen is generally divided into five groups: +3.3V, +5V, +15V, -15V, and +45V. The +3.3V and +5V can be obtained through a step-down regulator circuit, while the other three voltages are derived from the power management IC on the logic board. This IC boosts the +12V or +5V from the digital board to the voltage levels required by the screen.

1. VGH (VON): This is the high voltage level for the gate, used to turn on the gate-level circuit.

2. VGL (VOFF): This is the low voltage level for the gate, used to turn off the gate. It's effective during second-stage driving, but in third-stage driving, it's used to generate VgoffL.

3. VgoffL: The low voltage for the gate turn-off in third-stage driving, generated from VGL through a voltage conversion circuit.

4. VgoffH: The high voltage for gate turn-off in third-stage driving, used to counter voltage changes caused by the gate storage capacitor (CS ON GATE).

5. VCOM: The reference voltage for liquid crystal bias. VDDA on the PCB produces 10-14 groups of voltage, which serve as reference voltages for the IC’s internal DAC output (VGMA). This reference voltage reduces the need for complex voltage division within the IC.

Logic Board Input/Output Interfaces:

Currently, major logic board manufacturers in China, like Visionox, support various output interfaces such as USI-T, EPI, CMPI (P2P interface), MINI-LVDS, and RSDS TTL (traditional interface). The input interfaces include LVDS, V-BY-ONE, DP1.2a, and HDMI 2.0.

Supported Input Video Formats:

The logic board supports video formats such as FHD at 50Hz, FHD at 60Hz, FHD at 120Hz, UHD at 50Hz, and UHD at 60Hz.

Software on the Logic Board:

1. Visionox LCD mainboard test system

2. Visionox LCD splicing control system

3. Visionox LCD backlight control system

4. Visionox LCD driver software system

5. Visionox digital inverter algorithm software

6. Visionox FULL HD LCD driver software

7. Visionox 4K four-color HD imaging algorithm software

Typical Faults and Troubleshooting on Logic Boards

     Logic board faults in LCD TVs usually stem from power-related issues, such as voltage drops or component failures within the power conversion circuitry. Common faults include:

     1. White Screen: A common indicator of failure on the logic board, usually caused by the absence of the VGHP voltage, which is necessary to control the gate of the transistors inside the LCD panel. This prevents the TFT cells from properly modulating light, leading to a constant white display.

      2. Black Screen: This could be due to an open-circuited fuse, faulty capacitors, or failure of the DC-DC converter circuit responsible for providing the necessary voltages to the panel.

      3. Vertical or Horizontal Lines: This can often be traced back to faulty data or clock signals sent from the logic board. Bad connectors or IC issues might be the cause.

      4. Gray Screen: A gray screen can occur when there is a mismatch between the logic board’s LVDS transmission format and the panel's LVDS receiving format. Ensure that both the transmission and reception formats are aligned.

      5. Dim or Overly Bright Screen: This can result from issues in the backlight control system or failure of the brightness control signals within the logic board.

Logic Board Repair Methods:

    1. Resistance Testing Method

     When the logic board is not powered on, test for continuity, focusing on whether the fuses on the logic board are blown, and check if the power pins of the related ICs on the logic board are short-circuited. Also, check for leakage or faults in the transistors on the logic board.

    2. Comparison Method

     Due to limited available circuit diagrams of logic boards, you can compare a faulty logic board with a working board of the same model to identify differences. This helps in diagnosing and finding faulty components, providing hands-on repair insights.

    3. Power-On Testing Method

    This method tests critical points while the logic board is powered:

    (1) Check if the voltage to the screen is normal (screen voltages can vary by model, commonly 5V or 12V).

    (2) Check whether the DC-DC converter circuit on the logic board generates the correct 3.3V, 2.5V, or 1.8V power supply (different manufacturers label these voltages differently; for example, on the AUT420HW04 screen logic board, 3.3V is labeled as "V3D3").

    (3) Check if the VDA voltage generated by the DC-DC converter on the logic board is around 15.8V.

Note: Different manufacturers have different voltage labels. For example, on the AUT420HW04 screen logic board, AVDD is labeled with a voltage of 15.81V.

Example of Fault Repair:

Logic Board Model: C07

TV Model: TLM40V68P

Mainboard: LCD-MST6M68FQ

Fault: White Screen

Repair Analysis:

     After powering on, the TV operates normally, but the screen takes longer to light up and shows a white screen, while sound and other functions are normal. This indicates a problem with the logic board.

First, the power supply lines on the logic board were tested. It was found that the VGHP test point had no voltage, which should normally be 19.5V. Other test points had normal voltages. After removing the logic board and testing the VGHP output terminal, a resistance of 0 ohms to ground was detected, indicating the absence of VGHP voltage.

      After removing the filter capacitor CP19, the resistance returned to normal. The logic board was reinstalled, but the fault persisted with no VGHP voltage. Further testing revealed that pin 10 of UP1 had normal 2.25V DC and 5V AC outputs, while the VAA test point had a normal 13V reading. This confirmed that the open-circuit DP5 was the cause of the VGHP voltage failure. Replacing it restored the VGHP voltage to 19.5V, and the TV returned to normal operation.

      The VGHP voltage is crucial for providing the high gate voltage required to activate the transistors in the LCD panel. When this voltage is absent, the TFT cells in the LCD screen fail to operate properly, leading to issues like the white screen.

The logic board is primarily composed of a format converter and DC-DC converter circuits. From the analysis and troubleshooting of these two examples, we can conclude that most logic board failures are related to the DC-DC conversion circuit.

Summary of Logic Board Repair:

     Visual Inspection: Check the main chip, power IC, filter capacitors, and other easily damaged components for signs of burning or discoloration.

Resistance Testing: Measure the resistance at various power supply test points for short circuits or low resistance values. Check if any fuses are blown.

Voltage Testing: Measure the voltage at key points, including:

(1) Power supply Vcc5V/12V

(2) Main chip: 3.3V, 2.5V, 1.8V, 1.2V

(3) VAA: 14V, VGH: 20V, VGL: -5.5V

(4) VREF: 13V, VCOM: 6V

IC Signal Line Resistance: Check the resistance to ground of the LVDS DDR signal lines and whether there are data exchange issues between the main chip and LVDS.

Cable and Connector Inspection: Ensure the screen cable, logic board, and connectors between the logic board and screen are free from poor connections.

Conclusion

     The logic board (or TCON board) is a critical component in an LCD TV, bridging the gap between the main signal processing board and the display panel itself. It plays a key role in signal conversion, power regulation, and communication with the LCD panel, ensuring accurate image display. While many issues with logic boards can be traced to power supply or signal mismatches, careful testing, proper diagnosis, and appropriate repair techniques can effectively address these problems. Having a good understanding of the logic board’s operation, input/output interfaces, and the role of various voltage levels will help technicians successfully repair and maintain LCD TVs. 

     By following standard testing and repair methodologies, common failures such as white or black screens, noise, vertical or horizontal lines, and brightness issues can be systematically addressed, ensuring the LCD TV returns to proper functionality.