Progress (PDP-8/L repair)

From: Charles <charlesmorris_at_direcway.com>
Date: Tue Jan 14 17:31:18 2003

I'm still looking for an M220 board since one of the six won't
carry from its LSB to MSB. Probably just needs a 7482 adder; I
have one on order.

In the meantime I've been looking at the core memory and MB
registers which appeared to be totally dead (no bits ever changed
on the front panel). (M+) - (M-) = 22.3 volts.

This weekend, after quite a bit of signal chasing, I found that I
had put the variable delay line in the wrong slot some time ago
while cleaning connectors/mice nests! When I replaced it in the
correct slot there was still no STROBE output. I then found that
the 7400 output buffer was internally shorted to Vcc (probably
from the mis-slotting). Replaced the 7400. Now it cycles.

The MB now lights up with the contents of the SR on a DEP, but I
can't tell if anything is getting stored, or whether it isn't
getting read, since EXAM always shows all-0's to the MB. I can't
run any diagnostics yet since I can't store any instructions! Any
help/hints greatly appreciated.

-Charles
Received on Tue Jan 14 2003 - 17:31:18 GMT