The TV, a TCL2975D model, was repaired using a color TV. When the camera was turned on, the image and sound were basically normal. However, as time passed, the display began to show issues: some colors gradually disappeared, then it turned into black and white. Eventually, the black and white image became completely white, with faint traces of the retrace lines visible at the edges. After about ten minutes, the image vanished entirely, leaving a white grid across the screen. At this point, the audio remained normal unless the channel was changed, which caused the TV to crash. Through prior experience, I knew that this model uses a Philips chassis, and after further investigation, it became clear that the issue stemmed from an output abnormality in the chassis, which eventually led to a complete failure. However, the image degradation happened gradually, so a full disassembly and inspection were necessary. After disassembling the unit and powering it on, even though the machine had been off for a few minutes, the screen still showed black and white images. I checked the cathode and acceleration voltages of the picture tube, along with the video supply voltage. When the image completely disappeared, the three cathode voltages slightly dropped, while the accelerating voltage and power supply remained relatively stable. These readings didn’t show any obvious faults, so I shifted my focus to the decoding circuit. I first measured the OM8838 power supply, specifically pin 8, which should provide 8V. It was stable, and the voltages related to brightness and color were within normal ranges. Although I suspected the OM8838 might be faulty, I didn’t have replacement parts available. Then I took a step back and thought: if the image was changing, there must be a component causing it, and its associated voltage should also be affected. After checking the voltages around the OM8838, I decided to temporarily remove the decoding circuit for testing. Looking again at the output from the view and decoding circuits, the three primary color voltages appeared normal. So why was the screen turning completely white with a retrace line? I ruled out the bright spot circuit and then remembered something—several times I had encountered failures due to faulty high-voltage discharge circuits, which could result in a white screen with retrace lines. Upon inspecting the brightness control board, I noticed that the bright spot clearing circuit was made up of components like A817 and its surrounding parts. The principle is that when the TV is turned off, the C terminal of A817 outputs a high voltage, which is sent through three diodes to the three video channels. This triggers the C4544 transistor to saturate and short the cathode electrons of the picture tube via two resistors (422 and 423), thus eliminating the bright spot. When I examined A817, I noticed a noticeable burn mark on one of its pins. After cleaning the area, the image returned to normal, and the colors were vibrant again. To be safe, I replaced the A817. The root cause of the problem was traced back to an uneven voltage output from A817, which led to inconsistent image output capabilities, resulting in the series of symptoms observed.
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