I am troubleshooting a problem with a power supply for an HP EX490 Mediasmart server. The power supply no longer powers on when the power button is pressed on the back of the server. I have disconnected the power supply from the motherboard and found that it will turn on when I short the green power-on pin to ground. I have also verified that when the motherboard is powered, the power-on pin voltage on the motherboard drops low when the server power button is pressed. With the power supply connected to the motherboard I have tried shorting the green wire to ground at the connector, but the supply doesn't turn on. So, it seems odd that the power supply will turn on when disconnected from the motherboard, but not when connected.

I'm not sure if I can conclude that the power supply is defective or if there is possibly some other problem that might be causing this behavior. Any suggestions would be welcomed.


  • \$\begingroup\$ Is it worth trying to fix an ATX power supply? \$\endgroup\$ – Robert Endl Mar 17 '19 at 17:48
  • \$\begingroup\$ That sounds as likely to be a short-like fault on the motherboard as a power supply issue. Product repair questions are generally off-topic here unless a thorough understanding of the circuit being repaired has narrowed them to a very specific technical question. \$\endgroup\$ – Chris Stratton Mar 17 '19 at 18:23

There could be to only two outcomes: either your PSU is faulty, or mainboard, part of it.

To test PSU you need not only to power it up with the green-black short, but also to apply some reasonable load. Full-sized testing of ATX supplies is likely out of reach for you, but you can use power-size wire-wound resistors to load individual rails.

First rail to check is the +5VSB standby voltage, it should hold at least 2 A of load (use a resistor of about 2.5 - 3 Ohms, like this one).

enter image description here

Similar way you need to check all other rails, +12V, +5V, and +3.3V.

If each of these rails holds at least 3 - 4 A of load, your PSU is likely good. The ultimate test would be to test the PSU with the same kind known-good mainboard, loaded with some stress tests.

The other problem can be with mainboard itself. There are cases when mainboard chipset ICs die gradually, due to development of some internal faults. The faults can develop either if the IC was run in overheated state for long time (undersized heat management), or some ESD or "brown-out" events caused "electrical fatigue" and the chips started to consume more and more current until they die completely.

In both cases the repairs are un-economical, so be best course of action is to determine which part is at fault and replace it. Or buy a brand-new computer.


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