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I have an old PC assembled from components. All of a sudden I cannot turn it on. Nothing except the motherboard strobe light turns on. No fan, no hard disk spin, no nothing. I have unplugged everything except the power cables (ATX24 and 4-pin EATX12V) to the motherboard and fans.

The PSU (Seasonic SS-500HT) seems to be fine because:

  • The motherboard strobe light stays on.
  • All pin out have proper voltages.
  • With pin 15-16 shorted, all fans turn on.

The power button is fine as I measured that it properly shorts the output.

Curiously, about 8 seconds after I turn on power, all fans spin quickly (< 1 second) and then stop. This always happens.

Could it be that something on the MB is shorting? Or the MB (Asus P5B) is toasted? Or could it still be that the PSU cannot provide enough wattage?

Thanks for your help!

UPDATE: It turns out that the components are fine. I did the checks mentioned in comments and answer, except that I don't have a spare PSU to isolate the problem. So I made a last ditch attempt to remove everything including the MB and blew compressed air on them, and that cleaned up whatever was causing the problem.

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can you just try another PSU and also check if anything is loose, like memory or graphics card – jay Feb 5 '13 at 6:21
Have you checked for blown capacitors? I wouldn't mess around with checking the PSU as that can be dangerous. Just check the motherboard. – Eric Roper Feb 5 '13 at 8:19
up vote 1 down vote accepted

In this case you should check both items separately.

  • Check the PSU alone by shorting the PS_ON (usually green wire) to any one of the black wires (COM). If you have a multimeter, you could check the output of the ATX24 and the EATX12.
  • Check the motherboard with another PSU (a working one if possible) to discard a voltage failure in the one you have mounted right now.

As for the fans behaviour, its normal in most of the modern motherboards. When you turn them on they usually check the temperature sensors with the fans at maximum speed and then adjust them accordingly.

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