After replacing the RAM, the display is now back to normal (there's also a
test switch on the motherboard (S6) that switches the display from blank
spaces to all zeroes.
The one that failed was a National Semi 2102, whereas the others are all
from another manufacturer. No sign of previous replacement. Interesting.
(I once fixed up a PDP-8/L, chased down several bad chips. Nearly all were
Signetics 7440's).
I may have made a tactical error though - turns out that wet/damp PVA is
electrically conductive! Enough to overcome the 5k pullup resistors. An
unpleasant surprise. So now the keyboard thinks multiple keys are being
pressed and won't work at all. Removing the keyboard to clean underneath it
would be very tedious since every key has two soldered pins...
There was a significant buildup of goop underneath the key scan mux and
demux chips where the pins are close together (0.1"). I gave the bottom half
of the board another good
rinse and will let it dry overnight, maybe a hair dryer too. Or get some
more 91% alcohol to it and a longer low-temp bake. It all worked before my
"cleanup" so I expect it will resume normal operation once the moisture is
out of any remaining goop. I hope ;)
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