I tried a bunch of ODD/EVEN ROM pairs (even the original BIOS on a new pair of chips) but none worked. I checked the voltages of the ROM pins and they seem fine so the ROMs are probably OK. System chips get slightly warm so power seems ok, +5V is present on the correct input pins.
I noticed one strange thing: the power LED on the PSU only works when the mainboard is disconnected. When I connect the mainboard the LED won't work.
Maybe there is a short I didn't notice but why would it affect the LED and not the rest of the circuits?

I think I could be onto something:

"The DS1218 holds both pin 28 (Vcc) and pin 20 (/CE) of the DS2064 Sram IC at the battery
potential (around 3v) when the 5V power is off. This saves the data in the DS2064 Sram and
write protects it regardless of the voltages on /CE and /WE pins. In addition notice how pin 26
of the DS2064 IC is held at the battery voltage via a 20k resistor. One consequence of a flat
battery is that pin 26 of the internal DS2064 falls low, which causes the DS1225 module to
malfunction even when powered by the external 5 volt supply. So when the battery is low, the
DS1225 won’t work in the scope or a ram reader/writer for that matter even with an external
5V supply on pin 28 or VCC. "

http://worldphaco.com/uploads/TEKTRONIX_2465b … _THE_DS1225.pdf