FWIW, a buddy of mine brought over his "modular" PSU to get me to replace the fan on it.

After a successful transplant, we attached it to one of the many known-good ATX boxes here for a quick smoke test, and the darned thing wouldn't power up.

Well, it would briefly, while we held the softswitch in, but not for more than a couple of seconds at best.

After much panic and reexamination of the fan wiring etc.. we tried it with a different motherboard. No problemo.

Just fussy, I guess.