Very likely a bad power supply block. Not hard to replace. It's the same part as used in HP Laserjets of the period. My old notes say these are the suitable part numbers. Of course, you can recap/repair the one you have.
RG9-0205
RG9-0319
HP SX-ACPM-B
A little thread necro, but look at what I found in the teardown pile at Free Geek Twin Cities! Fastpath 5R - basically two FP5 boards in a rackmount case.
I never thought I'd see one in the wild.
Bummer is that it has only the 10Base-2 modules. Would love to find/trade for either the AUI...
Free Geek Twin Cities recently got a donation of a Franklin Ace 2200, but unfortunately without they keyboard.
I thought I would ask here in case anyone might have a spare or know of some other solutions to at least see what the functionality is of this guy. I see some older posts on VCF.org...