"Nick G" ignore @ nospam.com wrote in message
...
The following problem has confounded every IT friend and support person i
have presented it to and appears to defy logic so i am throwing it to a
broader audience of experts to see if it elicits some ideas. Thanks in
advance for considering it and apologies for cross-group posting!

Problem: Spontaneous power-offs that take place between 15 seconds and 30
minutes after booting. As if the power cord was yanked out of the back.
Green M/B LED remains on, power cord has to be disconnected for several
seconds before re-boot can take place.


One of the PCs under my supervision exhibits a similar problem. It very
often shuts down when a CD is inserted into its CDRW drive. It then cannot
be switched ON by pressing the power button, the PSU's rocker switch has to
be flicked to OFF position for a few seconds then back to ON. Only then will
the power button work as normal.
ATX mobos have a flip-flop powered by the Vsb line (pin 9) that is under
software and hardware control and sinks the PON line (pin 14) in order to
turn ON the PSU. I suspect that something on the mobo overloads the Vsb line
causing the FF to release line 14 and go into a latched state.
I shall be looking at this PC in a few hours time so will post my findings.