Today, I received a LG 21-inch color TV. The customer mentioned that they smelled something burning last night, and then smoke came out of the TV. Upon inspection, I found two resistors in series on the ABL circuit had burned black. This is an over-voltage detection issue, and since the resistors are burnt, there must have been a local short circuit. Later, I discovered that Q301 was damaged. These two resistors were Samsung's 22k and 51k. After checking the board, I didn't find any other obvious faulty components. The screen was bright, but the AV signal image appeared slightly gray. Even when adjusting brightness and contrast to maximum, there was no layering effect, and the colors weren't very vibrant. The accelerating voltage brightness wasn't very noticeable either.
I wondered if there was an issue with the ABL circuit. Could the 8880 block be broken? The brightness wasn't too low, but there were visible retrace lines. Was the CRT aging? I reluctantly measured the tailgate voltage and found that the filament voltage was only around 2.8V AC. Looking at the surface of the current limiting resistor, it was yellowed. When I removed it for testing, the resistance had increased from 1 ohm (1 watt) to nearly 10 ohms. I switched back to the original 1-ohm resistor, and the screen became brighter. Adjusting the acceleration voltage to a moderate level, the image became clear and vivid. The repair was complete. It was just a current limiting resistor that was faulty, and it took me almost two hours of frustration and confusion to figure it out.
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