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.