ynotssor wrote:

1) Is this an AT or an ATX system?

I don't know the answer or even understand the difference. or whether the
question pertains to the computer or just the power supply, nor does a
careful scrutiny of anything on the machine or power supply indicate whether
it's AT or ATX. Does the power supply model number originally posted shed
any clue?

A) An ATX case has only 7 (max) expansion card slots.
B) An ATX case has a ~2" x ~6" slot above the 7th expansion slot.
C) An ATX case has a low current, momentary SPST on/off switch on the
front panel, which connects to the motherboard (not to the PS.)
D) An ATX power supply will have one 20-pin (two rows of ten) connector
to the motherboard.
E) An ATX case will have its keyboard and mouse connectors as part of
the above described ~2" x ~6" slot in the case. No separate holes.

2) If ATX, is the momentary front panel on/off switch being depressed,
or just the AC power on/off switch?

There is just a single front panel switch (depress-button type), and a
115/230V slide selector switch on the power supply itself.

I'm assuming that this is a momentary type switch, and that this is
an ATX system.

3) Do you have a DVM?

I can certainly borrow or rent one.

When you do:

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.

To test the ATX PSU:

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.

Good luck.