The original owner has replied to my question about whether the box has successfully been used with the CPUs in place, and he said yes. The machine was left in a garage for a long time, and must have deteriorated.
So. A failed component somewhere. The IO board? Both CPUs failing seems less likely, somehow. Do I just need to clean things more? It's very positive to know that it was running OK at last use.
Another interesting discovery: the CPU which causes the SYSTEM RESET message also shows the OCP message: NO MEM INSTALLED when the memory is removed. The other CPU causes no OCP output at all with memory present or removed.
Hoff on #vms suggested checking the OCP board's reset switch. It seems OK to a quick multimeter test, the pins have 5v across them when the button is pressed, otherwise they don't.
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