I was repairing a Changhong LCD TV model LED42C3000, where the indicator light is located on the remote control button. The issue was that the TV couldn't be turned on twice, and after disassembling, I checked the motherboard's 12V power supply with a multimeter and found it normal. The 3.3V and 1.8V rails were also stable. The remote control was sending a boot signal to the motherboard, but the PS-ON and BL-ON signals weren’t rising properly. At this point, I started to suspect a problem with the motherboard. To be more thorough, I used a pointer meter instead of a digital one. When measuring the 12V power supply, the needle didn’t move, and the digital meter showed a slow response. The voltage fluctuated between 11V and 12.18V, which was clearly unstable. After disconnecting or jumping the 12V power supply, I concluded the fault was in the power supply section. Initially, I suspected a faulty filter capacitor. I tested the capacitance using a frequency meter, and it was within normal range. Then I carefully checked both the primary and secondary circuits but didn’t find any obvious issues. The initial pole was driven by a U-F2LD7577 chip. When I measured the 300V power supply at pin 8, the voltage was clearly unstable. The resistors R12 and R11 had normal resistance values. Eventually, I replaced the LD7577 chip, and the power supply was restored to normal. The TV booted up correctly with no issues. Unfortunately, I couldn’t find the exact schematic for the 7757 part. Below are some photos of the repair process and the components involved.
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