Did you try the known good video board with the CPU board from the bad
computer? And while you’re swapping boards, there is a good chance you’ll break
a fragile connection where the 32-way cable connects to the boards, compounding
problems. Like Terry mentioned, there is a good chance this cable is the cause
of the original problem, before you started adding new problems by disturbing
this cable. It’s worth checking the signal on each wire in the cable, at each
end, at the component the signal is supposed to be getting to/from (not just at
the cable end).
It’s definitely worth checking the dc voltages if you don’t know the
history of the computer. If you know it once worked, there’s a very good chance
the problem is with the cable. If you don’t know whether it ever worked, then
all those modifications are a worry, and I wouldn’t know what to suggest other
than painstakingly trying to restore it to the original circuit as per the
schematics available on Terry’s website.
My knowledge of this is not as good as other members of this group, but in
my limited experience the garbage screen was never caused by a faulty CPU or
ROMs, and only once ever by a faulty logic chip (on the video board) so I tend
to regard the probability of a faulty logic chip as extremely low. Also if the
computer is known to have worked, the chance of a RAM suddenly failing without
being interfered with in my experience is extremely low (like, never). Hopefully
no-one has been pushing too hard on the RAMs in the hope of fixing connection
problems.
Don