"Alien Zord" wrote in message
...
"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.


I've had my session with the problem PC and this is what I found:
When the PC shut itself down the Vsb remained normal and the PON pin was low
indicating that the PSU was not shut down by the power control flip-flop. In
fact holding the power button for 5 secs reset it to the OFF state and
pressing it again powered up the PC as normal. So the PSU shut itself down
via a different mechanism. Inserting a CD in the troublesome CDRW drive
revealed why. When it spun the disc it sounded like a jet engine even
vibrating the desk itself. The 12V line dropped to 9V indicating a large
current drain and the PSU shut down about half a second later. So an
overload on one of the supply lines can cause the PSU to switch off. Changed
the CDRW (Ionics - a Philips subsidiary) for a Samsung and all is now well.

If you have an access to a multimeter I suggest you check pin 9 (Vsb: +5V)
and pin 14 (PON should be close to 0 when the PC is ON and close to Vsb when
OFF).