What Is a Logic Board?
Logic board is also called screen driver board, central control board, TCON board. The function of the logic board is to input the LVDS image data input signals sent by the digital board (the input signals include three types of signals: RGB data signals, clock signals and control signals). Directly sent to the LVDS receiver chip of the LCD screen. By processing the shift register storage, the image data signal, the clock signal is converted into a control signal that the screen can recognize, and the row and column signal RSDS controls the operation of the MOSFET tube in the screen to control the distortion of the liquid crystal molecules. Drive the LCD screen to display images. The logic board is a component with software and inherent programs, and a special module FLASH with built-in shift registers (horizontal and vertical shift) cannot be changed even by the manufacturer. [1]
Logic board
- Logic board function diagram
- Schematic diagram of logic board architecture
- It is not directly provided by the switching power supply, but generally provided by the voltage stabilization circuit on the signal processing board.
- 1. LCD display motherboard test system, 2. LCD display splicing control system, 3. LCD display backlight control system, 4. LCD display driver system, 5. Digital inverter algorithm software, 6, FULL HD LCD screen driver software, 7. 4K four-color HD imaging algorithm software.
- Black screen, white screen, gray screen, negative image, noise, vertical band, image is too bright or too dark.
- Simply looking at the circuit, you cannot see which data signals are contained in the output signals TXOUT- and TXOUT + from the LVDS transmitter chip, and how these data are arranged, because the LVDS transmitter chips produced by different manufacturers have an arrangement of output data It is different, so the output format of the LVDS sending chip on the logic board must be the same as the receiving format of the LCD panel, otherwise it is not equipped, so we must consider this issue when clicking the screen, which is also an important information that must be established.
1 Logic board Resistance test
- The test is performed when the logic board is not powered, mainly to check whether the fuse on the logic board is open, whether the power pin of the relevant integrated circuit on the logic board is grounded, and whether the transistor on the logic board is leaking or defective.
2 Logic board 2. Contrast method
- Because upstream manufacturers control the component data on the screen, there are not many logic board circuit diagrams available at present. During practice, a good logic board of the same model and a bad logic board can be compared for testing. This provides both first-hand repair information and helps to locate faulty components.
3 Logic board 3. Power-on test method
- The power-on test mainly detects the following key points:
- (1) Check whether the voltage on the screen is normal (the voltage on the screen of different models is different, and the voltage on the screen is mainly 5V or 12V).
- (2) Check whether the 3.3V, 2.5V, or 1.8V power supply generated by the DC / DC conversion circuit on the logic board is normal (labels are different for different screen manufacturers, such as AUT420HW04 screen logic board with "V3D3").
- (3) Check whether the VDA voltage generated by the DC / DC conversion circuit on the logic board is normal. The voltage is usually around 15.8V.
- Note: The labels of different screen manufacturers are different, and the voltage is also different. For example, AUT420HW04 screen logic board is marked with AVDD, and the voltage is 15.81V.
Logic board failure repair example
- C07 logic board
- Model: TLM40V68P
- Movement: LCD LCD-MST6M68FQ
- Symptom: White screen
- Analysis and overhaul: Turn on the machine and find that the whole machine starts up normally, but the screen is on for a long time, and after the light is on, a white screen appears. The sound and other functions of the whole machine are normal, so the fault is determined on the logic board.
- First check the power supply of the logic board and find that there is no voltage at the VGHP test point. When normal, this test point should have 19.5V voltage. Then test the other test points. The voltage is normal. Remove the logic board and measure the resistance of VGHP voltage output to ground. It is 0 ohms. Obviously, this problem is caused by no VGHP voltage. When the filter capacitor CP19 is removed, the resistance of the VGHP output terminal to ground is restored to normal, and the logic board is installed back to the original machine. VGHP voltage, the UP1 pin 10 has a normal DC 2.25V and AC 5V output, and the VAA detection point also has a normal 13V voltage. It is determined that no VGHP voltage is caused by the DP5 open circuit. After the direct replacement, start the test. The VGHP voltage has a normal 19.5V voltage output, and the whole machine returns to normal.
- It is analyzed that the fault is formed because the VGHP voltage is a high potential provided for the gate level, that is, the voltage at which the gate level is turned on. When the LCD LCD panel loses this voltage, the internal TFT of the LCD panel cannot work normally and such a fault occurs.
- The logic board is mainly composed of two parts of the format converter and DC / DC converter. From the analysis and repair process of the above two failure examples, we can see that most of the failures of the logic board are in the DC / DC conversion circuit.
Logic board logic board maintenance summary (overhaul ideas)
- [1] . Visual inspection to see if the fragile components such as filter capacitors of the main chip power supply have obvious burnout or abnormal color, etc .;
- Second, the resistance measurement measures whether there is a short-circuit resistance at each power supply test point, which decreases, and whether there is an open circuit in the insurance;
- Third, the voltage measurement measures whether the power supply is normal ( power supply Vcc5V / 12V, main chip 3.3V; 2.5V; 1.8V, 1.2V, etc., VAA14V, VGH20V, VGL-5.5V, VREF13V, VCOM 6V) ;
- Fourth, test the IC signal line to ground resistance LVDS DDR and the main chip data exchange is bad;
- Fifth, check each connection line to see if there is poor contact between the line and the logic board, and between the logic board and the screen.
- Maintenance Idea Map
- Logic board repair ideas
- Maintenance ideas Figure 2