The Samsung S42AX-YD05 (W3) plasma screen is experiencing issues where the machine indicator light turns on, but the screen doesn't power up. This suggests a possible fault in the secondary power board or the protection circuitry. Based on the initial symptoms, the problem seems to be related to the power board. To isolate the issue, the power board was removed and tested separately. For single-board operation, it was necessary to ground PS-ON and connect VS-ON to D5V. However, the board still triggered a protection mode upon powering up, confirming that the power board is faulty. The indicator light being on indicates that the 5VSB circuit is functioning correctly. Therefore, the fault might be in the VAMPA12VGDSV voltage generation circuit, the PFC circuit, or the VS and VA voltage circuits. All shorting contacts were disconnected, and the 5VSB voltage was measured at 5.2V, which is within normal range. Then, PS-ON was grounded, and the PFC voltage was measured at around 360V before the system shut down due to protection. Re-powering the unit, the VAMP voltage was found to drop to about 10V, pointing to an issue in the VAMPA12VGD5V voltage generation circuit and its associated protection components. During power-up, the DSV voltage was observed to rise to 5.6V before triggering a protection event. This behavior suggests a problem in the D5V feedback detection loop. The loop consists of resistors R8321, R8322, and R8320. A circuit analysis indicated that an open circuit in R8321 or R8322, or a leakage in R8320, could cause this issue. After checking, R8321 and R8322 showed no open circuits. However, during inspection, rust was found under R8320, suggesting potential leakage. The resistor was removed and replaced, and after reinstallation, the board operated without triggering the protection mechanism. The issue was resolved successfully.
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