T-CON Introduction and Maintenance Issues
T-CON board is also called logic board or control board. T-CON is the English abbreviation for Timming Controller, which means timing control circuit. There are not many T-CON boards in China now, CRT TV has decades of history. Traditional CRT TV works by scanning and the signals are serial. The current TV standard is based on CRT TV, so the TV signal passes through the TV mainboard. The output after decoding is also a serial signal. We know that the liquid crystal display operates in matrix display mode and processes parallel signals. That is, if the TV signal decoded by the motherboard is sent directly to the LCD screen, the LCD screen cannot be displayed. In order for the LCD to display TV signals, there must be a device that converts serial signals into parallel signals, so there is a T-CON board, which is a device that converts serial signals into parallel signals. The TV signal output from the main board is disassembled and reassembled on the T-CON board to ultimately form the control signal and data signal required for the row and column drive of the liquid crystal display. The overall T-CON board circuit can be roughly divided into three parts: signal processing circuit (main IC and peripheral circuits), grayscale rendering circuit (GMA rendering circuit) and power supply circuit (DC/DC circuit). The signal processing circuit mainly converts the TV signal transmitted from the motherboard into the control signal, pixel signal, auxiliary signal, etc. required for the LCD screen. transforms; The grayscale rendering circuit essentially creates the grayscale signal required for the LCD screen (grayscale can make the LCD screen’s image more layered, making the screen richer and more vibrant). The power supply circuit mainly includes the signal processing IC, the power supply voltage of the row driver and column driver, and the switching voltage of the TFT (TFT turn-on voltage VGH, TFT turn-off voltage VGL), etc. creates.
Common errors and analysis of the T-CON board:
- The color of the screen is abnormal
Generally speaking, the abnormal color of the LCD screen has a lot to do with the signal processing IC. The internal structure of signal processing is complex and integration is high, and it is easy to be damaged by static electricity, sudden strong voltage and high current. Most picture abnormalities are caused by damage to the signal processing IC. The IC outputs the control signals and pixel signals required for the LCD display. After the IC is damaged, the control signals cannot complete the sequential arrangement of pixel signals, so the picture is abnormal. Additionally, damage to the FFC (flexible cable) or incorrect connection will also cause display abnormalities.
2: No image on screen
There is no image on the screen, which means the IC has no control signal and no pixel signal output. There are several reasons why there is no image on the screen: 1. The IC is seriously damaged, causing the control signal and pixel signal not to be output, and the screen cannot be displayed. Image input is not displayed; 2. Due to the damage of the voltage regulator chip to the IC, the IC does not work, so there is no image; 3. Failure of the power supply circuit (DC/DC circuit) causes the IC to fail to provide the required voltage to operate, so there is no image.
3: The grayscale image on the screen is abnormal
The abnormal grayscale image has a lot to do with the GMA rendering circuit. The GMA voltage consists of a series of VS voltage signals collected by the resistor divider array after being amplified by the integrated operational amplifier and then outputting a series of GMA voltage signals. In this process, GMA voltage signal is used as the reference standard, and abnormal GMA voltage will cause the grayscale image to be abnormal.
Repair method of control panel
1. No diagrams and circuit diagrams with power supply parts caused by malfunctions in the power supply circuit of the control board:
The power supply part mainly provides operating voltage for the signal processing IC, row driver circuit and column driver circuit. Almost all voltages except GMA voltage are produced by this circuit. Failure of the power supply part will inevitably result in other circuits not working and no image on the LCD screen due to abnormal power supply. When overhauling, first measure whether the fuse is blown or not, and when it is determined that the fuse is blown, replace the fuse and re-energize it to check the display. There is no problem with insurance. First, use a multimeter to measure whether the VON (VGH) voltage is about 31.5V and the VOFF (VGL) voltage is about -5V. If VON and VOFF voltages are abnormal, measure whether VD10 is faulty. If there is no fault, the BD8161EFV chip may be damaged. If VON and VOFF are normal, measure whether the LVDS interface is 12V. If 12V voltage cannot be measured, there is likely a short circuit in the control board circuit. If 12V is normal, measure whether VDD is 3.1V. If it is not 3.1V, it proves that the output of BD8161EFV chip is abnormal. If VDD is 3.1V, it’s probably not caused by the power supply circuit. If there is a short circuit around the BD8161EFV chip, first use a hot air gun to blow the chip down and use a multimeter to measure the disappearance of the short circuit. It has been proven that the short circuit is caused by the chip and the chip can be replaced. If you are using a multimeter to measure whether a short circuit persists, you will need to eliminate the shorted components one by one.
2. Picture abnormality caused by signal processing IC failure:
Most image abnormalities are caused by signal processing IC failures. The T-CON chip (signal processing IC) generates control signals to control the regular arrangement of pixel data on the row electrodes and column electrodes. When the signal processing IC fails, the normal control signal cannot be generated to arrange the pixel data in an orderly manner, resulting in an abnormal picture; Signal processing IC failure will also cause the LVDS signal sent to the T-CON chip to fail to be converted into an RSDS control signal, resulting in a white or black screen on the screen. Picture abnormalities caused by the signal processing IC may cause all screen pictures to be irregular, TN models to display only black pictures, and MAV models to display only white pictures, blurry screens, etc. Includes viewing. An oscilloscope is required during the repair process to identify IC failure. First use an oscilloscope to measure whether the waveform, amplitude and duty cycle of the STH signal are normal. If it is not normal, it can be judged that there is a problem with the signal processing IC. Replace the IC and the general fault can be eliminated. If the STH signal is normal, measure the waveform, amplitude and duty cycle of STV, OE, CKH, STB, CPV, CKV, POL and other signals respectively. Until the signal processing IC is determined to be faulty. After the repair time is long, according to which display is abnormal, it can be understood whether it is the fault of the signal processing IC.
The role of each signal generated by the signal processing IC is added.
1. Starting signal of driver IC.
2. The POL inversion signal determines the inversion mode.
3. STB gate input allows signals on the same horizontal line to enter simultaneously.
4. The CPV/CKV line drives the IC clock. The CKH column drives the clock of the IC.
5. STH column
6. Start signal of STV line driver IC.
7. OE prevents the same signal from being transmitted on two adjacent horizontal lines at the same time.
Repair of abnormal grayscale image: Circuit diagram of the gamma voltage generating circuit:
If the LCD screen has a sense of hierarchy, it is necessary to add grayscale when viewing. The grayscale is generated in the column driver IC and the GMA voltage is used as a reference during grayscale generation. When the GMA voltage is abnormal, the display will be abnormal on the grayscale screen. When repairing, measure the GMA voltage test points to see if there is a short circuit between the GMA voltage test points. When a short circuit is found, first suspect a short circuit inside the integrated operational amplifier. Replace the integrated operational amplifier and remeasure each GMA voltage test point. If the voltage is normal, it proves to be the problem of the integrated op amp. If still abnormal, analyze the components between two GMA voltage test points to find the component causing the short circuit. Replace the circuit and it will return to normal. Measure the GMA voltage test point, but no short circuit was found, then measure the GMA voltage value at each point to see whether it is normal, find the point where the GMA voltage value is abnormal, and measure the voltage drop on each component. integrated operational amplifier to find the voltage Replace the abnormal component to see if the voltage at the GMA voltage test point returns to normal. If it is not normal, there is likely a problem with the integrated op amp.
