'New' IO board arrives...

But gives exactly the same results: CPU A shows SYSTEM RESET, CPU B shows nothing. Hmm.

So what other circuit boards do we have in play that can be replaced?

Card Cabinet:

• 2x 5/300 CPU boards (known difference in behaviour, but both get warm)
• 2x 512MB boards (system knows when both are missing)
• backplane board
• already swapped-out IO board (2 boards, exactly the same behaviour - I suspect the boards are OK)
• Remote Ports board (serial, keyboard, etc. I missed this off the earlier list of components. However, I get the same OCP display regardless of whether this is connected or not.) 

Front cabinet:

• Fan Control Module board (fans spin OK and don't cause system shutdown)
• OCP board (Display works, Power & Halt buttons have LEDs which work. Reset button measured for switching behaviour) 
• StorageWorks bay (can likely be completely ignored until the SRM boots)

The visible rust damage is much more pronounced towards the rear of the machine. So is corrosion damage more likely to be on components towards the rear? The backplane board doesn't seem to do very much, other than provide IO and expansion slots, but there's at least one large chip on there. Each of the above components has been removed and had its connectors cleaned at least once. The exception being the backplane, which only has receiving slots that I can't work out how to clean... Perhaps it's time for a gentle compressed air dust blower for the backplane. Other than that, time for a replacement?

One other angle could be the cables. Ignoring the EISA cable to StorageWorks, and the IDE? cable to the floppy, the IO board has two main cables: the OCP connection, and the Remote Ports connection. We know the OCP cable is good enough to enable the OCP to display NO MEM INSTALLED and SYSTEM RESET, so current assumption is that it's probably OK.