I have on too, and i did repair it 2 years ago. I did replace the faulty ?BU508? (can't really remember well) switching transistor, and came back to life.
Later i've accidentally shorted GND with either the 5v or 12v line near the Motherboard connector while measuring voltages,and *Puff* fuse blew up.
Cause? BU508 was shorted again. Replaced fuse and said transistor,and my poor old PCS 86 came back to life ..again..
I came to the conclusion that Olivetti Pcs86 power supplies are really 'sensitive', and have no efficient short ciruit protections, excluding that fuse.
Now, regarding your problem.
In your specific case, if you can find and solve your PSU problem,(there surely is a shorted component!) switch it on without anything attached.When the PSU works again, i'll recommend to check if something on the motherboard, or isa addon cards, is shorted.
I fear that a short circuit on Mobo or eventual cards can cause cascade damage to that PSU (like it happened to me)
Here's a pic of my Pc,taken minutes ago, still working perfectly. Reading your comments, i will surely check/replace those Rifa Caps, they are encased in a particular resin that cracks with age.
pcs861.jpg