I don’t think your problem could be classed as “Simasimac” – it refers to a very specific fault, but it’s good to hear that you go to the bottom of it. It’s amazing how simple these things are compared to modern PCs, which is very fortunate in our case!

Again, thanks for the information – hopefully it will help someone else in your situation.

I unscrewed the thing and started to analyse the motherboard, which had nasty corrosion from leaking electrolytes, especially around C7. Figured out that the 2 video memory chips (D41264C-12) are supplied with 2 times 8 bits of addressing information on pins 9-11 and 13-17 (not in that order), where RAS (row addr strobe) and CAS (col addr strobe) are used to tell the chips which part of the address is given (effectively a form of time division multiplexing). Found out that the four 74F253 chips are used to multiplex the 16 bit addr onto the 8 bit time divided channel (2 bits per 74F253). While trying to find the source of the 16 bit address on the board (seems to be the 2 TIT chips) realized that the most logical explanation would not be that two adjacent address lines went short with ground, but instead that one Select Input line of on of the 74F253 multiplexers was probably disconnected by corrosion. (pin 2 or pin 14)

Which, when measured, turned out to be correct! Since I stumbled upon this page when trying to find info on the SE/30, it seemed like the place to share my experience.