During the repair of a Skyworth 32E60HR LCD TV at the user's home, the TV showed a red power light but had a black screen. The touch power button didn't respond, and the remote control was also ineffective. According to the user, the red light usually turns on normally, but when it turns off, the TV automatically powers on again. This issue occurred several times before, but the TV would work the next day. After opening the back cover, I first checked the power supply voltages. The auxiliary power supply provided a normal +5V output, while the main power supply failed to deliver +12V, +24V, and +100V. No signs of leakage or burning were observed on the capacitors. [attach]448922[/attach]
I then tested the voltage regulator chips on the motherboard, which appeared to be functioning properly. My initial analysis suggested that the problem might be related to the DDR not communicating with the main chip or the main chip being soldered or damaged. Since the main chip wasn’t working, there was no power-on signal sent to the main power supply, resulting in no output voltages. The user agreed to bring the motherboard, power board, and remote control for further inspection.
While testing the motherboard, I connected the power supply and motherboard and heard a soft "click" — could this be the relay engaging? Looking at the power supply board, there was a small relay near the 220V input, which I assumed was controlled by the motherboard. I disconnected all connections between the power board and the motherboard, turned it on again, and the relay did not engage. When I reconnected everything, the PW-ON terminal showed 2.9V. That must be the boot signal from the motherboard! How could I have overlooked this? [attach]448986[/attach]
Since the motherboard was sending a boot signal, it was clearly working. However, the main power supply still wasn’t producing any output, so the fault had to be in the power board. But why was the motherboard working if the power board was faulty? I started checking the power supply components. The PCF voltage was normal, and the switching transformer showed no signs of open or short circuits. The optocoupler also looked fine, and the 12V and 24V rectification sections were okay. However, during the 100V rectification check, I found that one side of the dual diode YG902C2 was broken. This is a 200V, 10A, 35ns Schottky diode. I replaced it with an MBRF20U200CT with similar specifications. After powering on, the red light turned off, and the 100V was restored. Other voltages were also present. But shortly after, the voltage disappeared again. I double-checked the diode and found that the other two were still good. Could the MBRF20U200CT not be a suitable replacement for YG902C2?
I searched online for genuine YG902C2 diodes, installed them, and tested the surrounding circuit. Everything seemed fine, so I powered on the TV again. The red light came on, but then went out. Oh no! It was broken again. Was the quality of the YG902C2 poor? I had bought expensive ones, so that didn’t make sense. Why was it failing repeatedly, while the other two were working? I took a closer look at the peripheral circuit. After the 100V voltage goes to the backlight constant current board, it goes through the optocoupler and also connects to the protection circuit. A diode rated for 200V must handle very high voltage or sharp pulses. I hadn’t connected the constant current board, so the other circuits wouldn’t generate enough voltage to damage the diode. Or was there something else causing the issue?
[attach]449000[/attach]
Using a magnifying glass, I carefully examined the pins of the switch transformer and noticed slight cracks. Pressing on the legs caused some looseness. Ah, it was a high-voltage pulse caused by poor contact. I re-soldered the connections, replaced the YG902C2, and powered on the TV. The red light turned off, and the voltages were stable. I repeated the test, and everything worked perfectly. I returned to the user’s home, installed the repaired TV, and it was fixed!
This was my repair process. I'm not very good at writing, but I hope this helps. If you have any suggestions, feel free to share!
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