So, after reseating all IC's on the logic cards (Some of them had pretty tarnished legs - a few missed some!), the previous fault resurfaced:
Low voltage on the 5V rail. Either some other problem in the PSU is killing the power transistor for that rail, or the logic boards are shorted.
They read about 130 ohm, but I'm not sure if that's low enough to consider it a short. It still would only draw about 0.2A at 5V...
Would it be worthwhile to just go ahead and change all electrolytes on the psu board (and tantalum's on the logic boards) ? Or is the fault probably elsewhere?
Also: The +-12V supply is fine, and that drives the buzzer/siren thing, so whenever the 5V rail is off: it howls like crazy, far too loud to concentrate. Would it damage anything to disconnect the 12V rail, even though some logic /proms might need it?
Update: Removing the logic boards, the 5V rail reads normal- so it's probably a short or atleast heavy load on the digital boards. Looking at the tantalums now, not sure wether regular ceramic decoupling caps fail like that too. - on the 5V rail, 16V tantalums are used, could one replace those safely with 10V's ? They're only decoupling so one might start to try the board without them.