ubiq wrote on 2024-02-27, 20:17:Anyway, the PSU in the Compaq Deskpro XE 450 I recently acquired is kaput. ☹️ I’ve cracked it open and poked around with my multimeter. Nothing appears shorted or blown up. There’s a Rifa in there, but it looks in one piece, no cracks. I can see 160V going in to the big step down transformer and then about 0.5V coming out. I put a load (duff HD) on the 12V rail and that didn’t help.
Start a new thread and post some pictures of the insides of the PSU there. If you have a multimeter and are willing to poke with it a little, that PSU shouldn't be too hard to revive. Most AT PSUs are either half-bridge or STF or flyback topology and fairly trivial. The Rifa needs to go regardless of its condition. If it's after the fuse, check the fuse and the NTC thermistor. Since you measure 160V, though, those are probably fine. Also, I imagine this measurement implies there is only one large primary 400V capacitor and that's what you measured the 160V across? If so, you probably have a flyback or STF topology PSU.
In any case, before starting any troubleshooting, do the hair dryer trick first: open the PSU (unplugged from the wall) and heat it up to about 40-50C (components should feel nice and very warm to almost hot to the touch.) Then plug it in and see if it starts. Make sure you put a good load on the 5V rail and not necessarily the 12V rail. AT PSUs expect most of their load on the 5V rail, and typically at least 1 Amp.