The specs of the PC in question (it should be about 5 years old):
MOBO: ASRock H55M-LE
CPU: Intel Core i3
A friend brought the PC to me, saying it won't boot into Windows. He also told me that he's turning it on with a screwdriver since the power button doesn't work.
He bought it to me yesterday and today I tested it. I immediatelly put the power switch cable back on the mobo and turned it on by pressing the power button on the front panel. It worked normally, booted but didn't go into Windows (I suspect a HDD failure as it displayed a warning about a possible failure of the HDD). But it booted and I was able to go into BIOS etc.
I left it running and went somewhere and when I returned after half an hour it was off (by itself).
I tried turning it on again, nothing.
I swap the power sw and reset sw, didn't help.
I remove both cable from the mobo and try with a screwdriver, nothing.
I did that a couple of times, then I pulled everyting out, RAM too, so that only the CPU and it's heatsink fan were left. That didn't help either, it didn't power on.
Interesting though, if I left the keyboard in the PS/2 port the lights turned on for a second when I enabled the power on the PSU, but that's all.
Then I tried another PSU (an old but working one, sadly it's only 280W, but that should be enought for the CPU and MOBO onyl?), anyway, nothing happens.
Then I shorted the original one with a wire and it did normally turn on, I was able to open the CD-ROM tray door etc.
Friend tells me that the PC was working fine except for the screwdriver-power-on-switching and the latest HDD thingy. Except for a few freezes in the last times (probably to the failing HDD?).
All the capacitors on the MOBO look OK. I don't know what else to try. If I had a compatible spare MOBO I'd try it but I don't.
What else can I try befeore I can declare that it's a MOBO's fault? I will, of coruse, borrow and try another PSU before anything else.
I would really like to solve this mystery. Thank you for reading all the text!