I have an 800W ATX power supply given to me by a friend because it doesn't work any more. It's quite a beast and would set me back a considerable amount new, so I'd like to see if I can fix it.
Well, here's the story. It has standby power at 5.15V. The power indicator on the back lights up (on the mains switch – it's an LED not a neon apparently, because when you turn off the unit it fades slowly, making me think it's not directly connected to the mains.) Anyway, when you turn it on (by grounding the PS_ON pin) it makes a faint clicking sound but does nothing else. With my scope I have found three short pulses of ~11.8V on the 12V rail for about 500 ms to 1.5 seconds before the supply goes dead, which makes me think some kind of protection system is being activated, as the power supply does work – it is producing the required output.
If it doesn't work it is going in the trash, so no loss there.