Hi !
I'm tryin' to service an IBM Server 325 PSU, suddenly dead.
I checked all capacitors, diodes and other stuff for shorts.
I put a paper clip connecting the power on pin with gnd.
The rectified voltage (generated by a separate board) reaches a primary
stage, with discrete components and three ICs
TOP200YAI - Three-terminal Off-line PWM Switch
LM393P - as usual
UC3843AN - Current Mode PWM Controller
CQY80NG - Optocoupler
and two transformers.

Well, from this stage exits nothing, with or w/o the paper clip.
My experience on switching PSUs is very limited.
The PowerOn signal doesn't reach directly the first stage but a custom made
riser card, so the fault could be elsewhere.
Any hints on where to begin lookin'?

As a last resort, since the output voltages are the same of a standard ATX,
only with different connectors, I could swap them and use a "normal" ATX.

But I'd give a try, it could be a useful experience...