The root of the TCL color TV 2975D without image problems is unexpected! - appliance repair - Huaqiang Electronic Network
The door is repaired with a TCL2975D color TV. The image sound is basically normal when the camera is turned on. As time goes by, see the picture below: Some colors are slowly lost, then black and white, then the black and white image is white and the sparse retrace of the edge is faintly visible. The line, after ten minutes, the image disappears completely, presenting a white grating across the retrace line. At this time, if the TV function is not operated, the sound is always normal, and once the station is changed, the crash state occurs.
TCL2975D repairs more, know that it is using Philips movement, the TV finally crashes well understand, this movement will be dead after the operation of the output abnormality. However, the image becomes impossible to step by step, and only the disassemble is examined.
Disassemble--turn on the power. Since the machine has been turned off for a few minutes, the screen still has black and white images. Check the cathode and accelerometers of the picture tube and the voltage of the video supply. When the image is completely out of time, the three cathode voltages are slightly reduced, and the accelerating voltage and the apparent power supply are basically unchanged. Through these data, I can't see any obvious flaws, and I turned my attention to the decoding circuit.
First measure the OM8838 power supply - 12 feet 8V wire does not move, in the measurement and brightness and color of the relevant foot voltage are in the normal range, suspected 8838 bad, but there are no components to change.
Calm thinking: the image change should be caused by a certain component, then the voltage associated with it should be changed. From the measurement of the 8838 related pin voltage, the decoding circuit can be temporarily removed.
Returning the gaze back to the output of the view and output, the three primary color voltage outputs of the decoding circuit are normal, so what is the reason for the all white and the retrace grating? Eliminate the bright point circuit ---- I suddenly think of it in my mind, I have repaired several high-voltage package leads to the fire-and-discharge circuit, causing the all-white grating with the retrace line. The result is that the triode of the bright-light circuit is broken. .
View the viewing board, after the physical comparison, the brightness clearing circuit is composed of several components of A817 and peripheral. The principle is: the C pole of A817 outputs a high level when the power is turned off, and the voltage is sent to 3 video channels through 3 diodes. The output of the C4544 base makes the C4544 saturate and conducts the cathode electrons of the picture tube through two 422 and 423 in a short time, thereby achieving the purpose of eliminating the bright spot. When I looked at the A817, I found that there was a noticeable blackening of the pin parts. After removing the dirt, the image of the boot was always normal, and the color of the image was also bright. For the sake of safety, the A817 was replaced.
The final source of the problem is that the voltage of the A817 is different, and the voltage of the error output is different, which is caused by the difference in the image output capability of the interference image, which causes this series of fault changes.
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