In message , Computer Nerd Kev
writes:
[]
I'm pretty sure that I've encountered that with a faulty PC PSU
before. Something's not working proberly and causing a part inside
to overload as the various parts power-up, so the power supply's
control circuitry is detecting this and shutting itself down. Then
it forgets any of that ever happened and tries to power itself up
again.

Sounds very plausible.

Given the reference to 5VSB though, I gather that it is an ATX
supply, not an AT one where the front power button/switch switches

Oh yes, not _that_ old! (Though I think I still have an AT supply or two
in the pile. [I remember the original IBM PCs with the real heavy-duty
red lever switch on the side of the case - no doubt that _those_ were a
real switch!])

the mains directly. In such case it's certainly possible that the

No, little rocker switch on back of PSU did that.

motherboard has an issue that is preventing it from signalling the
PSU properly. The "signalling" is just shorting a pin on the
motherboard power connector to GND though, so probably less likely
to go wrong like this than the PSU is itself.

I reckon so, i. e. most likely PSU fault. I'll change the PSU - though
it might not be this year (-:
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

Linux is a car kit and Mac is a car with the hood welded shut - Mayayana in
alt.windows7.general, 2015-12-4