MSV11-D debugging question

Noel Chiappa jnc at
Sat Jan 24 13:59:32 CST 2015

OK, so I'm back to debugging busted memory cards. Again, I have a sitation
where a bit is stuck on, and I'm curious to see if anyone has any insight into
which chip (of 3) might be causing it (in an attempt to not have to try
replacing them at random :-).

Looking at the line from the memory chips (two chips - E26 and E27, if anyone
cares - have outputs wire-or'd together) to the input of an octal tri-state
latch (E40, an S373), that line goes to about +2V when the machine is powered
on (which is similar to what other bits do); then, when I run a
two-instruction read loop, (after writing 0 into that location), the line goes
to +5V and stays there (which is distinctly unlike how other bits behave).

Any guesses as to whether the culprit is the S373 or one of the memory chips?


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