Today, I was working on repairing a Changhong LCD TV model LED42C3000. The issue was that the indicator light, which is located on the remote control button, would not turn on twice. After disassembling the TV, I checked the motherboard and found that the 12V power supply was normal (measured with a multimeter), as were the 3.3V and 1.8V outputs. The remote control was sending a boot signal to the motherboard, but the PS-ON and BL-ON signals weren’t rising properly, which made me think the motherboard might be faulty.
I decided to use a pointer-type meter for more accurate readings. When measuring the 12V power supply on the motherboard, the needle didn’t move much, while the digital multimeter showed a slow response. The voltage fluctuated between 11V and 12.18V. After disconnecting or jumping the 12V power supply, I concluded that the problem was in the power supply section.
Initially, I suspected a faulty filter capacitor. I used a capacitance meter to check it, and it appeared to be fine. Then I carefully examined the secondary circuit but didn’t find any obvious issues. I also checked the primary circuit components and found that the resistance values of R12 and R11 were normal. The main driver chip was U1, an LD7577. When I measured the 300V input at pin 8 of the IC, the voltage was clearly unstable.
After replacing the LD7577 chip, the power supply started working correctly, and the TV booted up normally with a clear image. Unfortunately, I couldn't find the exact schematic for the LD7577 chip, so I had to rely on my experience and testing. It was a bit challenging without the full diagram, but the repair worked out in the end.
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