The TV, a TCL2975D model, was repaired using a color picture tube. When the TV was first turned on, the image and sound were normal. However, as time passed, the issue began to develop. The colors gradually faded, then the screen turned to black and white. Soon after, the black and white image became completely white, with only faint traces of the retrace lines visible at the edges. After about ten minutes, the image disappeared entirely, leaving a white grid pattern across the screen. At this point, the audio remained normal if no channel changes were made. But as soon as a channel was changed, the TV would crash.
During the repair process, it was discovered that the TV used a Philips board, which eventually failed due to output abnormalities. However, the image degradation occurred progressively, making it difficult to pinpoint the exact cause without further inspection.
After disassembling the unit and powering it back on, the screen still showed a black and white image even after being off for a few minutes. I checked the cathode and acceleration voltages of the picture tube, as well as the video power supply. When the image fully disappeared, the three cathode voltages had slightly dropped, while the acceleration voltage and power supply remained stable. These readings didn’t reveal any obvious issues, so I shifted my focus to the decoding circuit.
I started by measuring the power supply for the OM8838 chip. The 12V line (pin 8) was stable, and the voltages related to brightness and color were within normal ranges. Although I suspected the OM8838 was faulty, there were no spare components available for testing.
Then I took a step back and considered that the image change must be caused by a specific component. If the voltage associated with that component had changed, it could lead to such a fault. I measured the relevant pins on the OM8838 and temporarily removed the decoding circuit to test further.
The primary color voltages from the decoding circuit appeared normal, so why was the screen showing all white with a retrace grid? I ruled out the bright spot circuit. Then it hit me—earlier repairs involved high-voltage discharge circuits, which could cause similar symptoms. The problem might be due to a damaged transistor in the brightness control circuit.
After inspecting the viewing board, I compared the components and identified the brightness clearing circuit, which consisted of A817 and its surrounding parts. The circuit works by sending a high-level signal from the C pole of A817 through three diodes to the three video channels. This signal activates the C4544, causing it to saturate and short the cathode electrons of the picture tube via two resistors (422 and 423), thus eliminating the bright spot.
Upon examining A817, I noticed significant discoloration around the pins. After cleaning, the image returned to normal, and the colors were vibrant again. To be safe, I replaced the A817.
The root cause was traced back to an unstable voltage from A817, leading to inconsistent image output and triggering the entire sequence of faults.
Tinned Copper Clad Copper TCCC
Corrosion-Resistant Copper-Clad Tinned Wire,Copper-Clad Copper Tinned Wire Production,Copper-Clad Copper Tinned Wire Processing,Copper-Clad Tinned Wire
changzhou yuzisenhan electronic co.,ltd , https://www.ccs-yzsh.com