basic power problem

[ric]

To recap
I had a computer that suddenly died. (ATX 5 year old PentiumIII one that
comes on a 'daughter board' with 2 sticks of 128MB memory on long black DIMM
connectors)This machine had worked pretty much ok for 5 years then one day
the computer wouldn't switch on, there was no fan activity on the power
supply, no beeps nothing, just silence)
It was taken to a pc shop. They said they tried a new power supply with it
that worked for a while but was then destroyed!!!!

[...major snip]

Nothing, I repeat *nothing*, on the MB or drives should "destroy" the
PSU if it is a quality PSU. Find a good PC tech.

As for your problem:

First, disconnect everything from the power supply (including MB.) Plug
AC cord into the rear of the PS. IF the PS has its own rocker on/off
switch, turn it ON. There should be +5v on pin 9 of the 20 pin connector
(+5vsb, usually a violet wire.) There should be >0.8v on pin 14 (PS-ON,
usually a green wire.) This is from an internal pull-up to the +5vsb.

Connect PS power leads to 1 or 2 IDE HDs (for a load). Jumper pin 14
(PS-ON) to pin 13 (ground, black wire.) At this point, fan in PS should
start spinning, drives should spin, and + 5/12v, -5/12v, +3.3v, and
+5vsb should be present at 20 pin connector.

Disconnecting pin 14-13 jumper should turn supply back off.

When your system fails to start, what is the voltage on pin 9? If it is
less than 4.75v, either a) the logic on your motherboard is shorting
b) your +5vsb is under powered, or c) your PS is bad. It is
most likely a combination of b) and c).

If your +5vsb is OK, measure pin 14 of the PS/MB connector. If it is <0.8v
(and the +5vsb is OK), the power supply should be on. If pin 14 is <0.8v,
the +5vsb is OK, and the PS is OFF, the PS is bad. If pin 14 is >2.0v, the
power supply is being told to stay OFF by the MB. If depressing the front
panel ON/OFF switch does NOT lower pin 14 to <0.8v (and the +5vsb is OK),
then the problem is NOT the power supply, but rather MB related.

I sure hope you are *not* attempting to troubleshoot your system
without a digital voltmeter.