The Samsung S42AX-YD05 (W3) plasma screen failed to power on, and the machine's indicator light was not working properly. This led to a secondary open-source board protection issue. Based on the initial symptoms, the fault appeared to be related to the power board. To isolate the problem, the power board was removed and tested separately. In single operation mode, grounding PS-ON and connecting VS-ON to D5V allowed the board to function momentarily before triggering a protective shutdown. This confirmed that the power board was faulty.
Since the indicator light was on, it indicated that the 5VSB circuit was functioning correctly. The possible fault areas were narrowed down to the VAMPA12VGDSV voltage generation circuit, the PFC circuit, and the VS and VA voltage circuits. All shorting contacts were disconnected, and the 5VSB voltage was measured at 5.2V, which was normal. Then, the PS-ON pin was grounded, and after powering on, the PFC voltage was around 360V, but the system immediately entered protection mode. Re-powering the unit showed that the VAMP voltage dropped to about 10V, indicating an issue in the VAMPA12VGD5V voltage generation circuit and its associated protection system.
During power-on, the VAMPA12VGD5V voltage was monitored, and it was observed that the DSV voltage rose to 5.6V before the system shut down. This suggested a problem with the D5V feedback detection loop, which controls the D5V output. The DSV feedback loop includes resistors R8321, R8322, and R8320. Circuit analysis indicated that an open circuit in either R8321 or R8322, or a leakage in R8320, could cause this behavior. Upon testing, R8321 and R8322 were found to be intact. However, when inspecting R8320, rust was discovered underneath it, leading to the suspicion of internal leakage.
After removing R8320 and measuring its resistance, it was confirmed that the component was faulty. Once replaced, the board was reassembled and powered on successfully, with no further protection issues. This repair resolved the problem, restoring normal operation to the plasma screen.
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