Issue: +5v is missing/low at +0,9v, therefore no CPU boot, nothing. The +12 and -12v are fine.
Solution: According to the schematics (link), the components responsible for +5v generation are IC1 (an MC1723CP) and TR5 (2N6057, the transistor on the huge cooling block). Changed both with known good parts, but the +5v were still at 0,9v. As these boards are all 20+ years old, looking for leaked or bulged caps is always a good idea - but visually they all looked good. I decided to desolder the caps one by one and measure them with a capacitance meter.
Started with C2 which is right after the bridge rectifier which should be a 100uf 25v cap. Once desoldered, you could see a tiny bit of leakage on the bottom side of the cap - this is unusual, because caps usually leak on the top. Sure enough, the cap measured between 6uf and 10uf. Replaced with a 100uf 35v (higher voltage is okay), and +5v are right back. Issue resolved.