In microsoft.public.win98.gen_discussion "J. P. Gilliver (John)" wrote:
About a month ago, I turned on my Windows 98/XP desktop machine, for the
first time in ages (possibly a couple of years).

The mouse lit up, as did the power and other (disc access?) LED, and
fans started - then they went out again. Then they came on again - then
they went off again. They do this on about a two second cycle. Nothing
is coming down the monitor lead - I turned the monitor off and on again,
to fool it into not being in standby, and nothing appeared on screen.

Cycling the mains power switch on the back (which had been on all the
time) didn't seem to change things - well, on switching back on, it
stayed in standby, until the power button was pressed, when the
motorboating started again.

I've discovered that the 5VSB seems OK - at least, some Christmas lights
I plugged into it (front panel USB socket) stayed on; I've just
discovered that having those plugged in when switching on at the back
(they come on) prevents _anything_ happening when I press the power
button. (No, I didn't have them plugged in when I first encountered this
behaviour.)

I'm guessing a power supply fault, but thought I'd ask here before
changing it (I have spare supplies), in case anyone'd encountered such
symptoms before and knows it isn't that.

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.

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

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