PDP-12 at the RICM

[Michael Thompson]

Two transistors on the front panel that turn on PC and MA lights failed.
They were painful to replace. Hopefully this won't be a weekly ritual.

We ran more of the LINC mode processor diagnostics. All that we could
figure out with no documentation worked OK. Hopefully someone has the
missing documentation that we need.

We worked on the TC12 LINCtape controller problem where it had a "LGP
GP=GPC PRESET" fault during diagnostics. We are not exactly sure what this
part of the TC12 does, something to do with tape groups, a LINC thing. It
looks like the diagnostic turns the GP=GPC flip-flop on, then presets
(clears) it, then tests to see that it was preset. Most of the time it
works OK, and always works OK when run in single step. We replaced the M216
module that contains the GP=GPC flip-flop, but that didn't change the
behavior. One of the M222 modules in the TC12 controller routes the GP=GPC
signal to the status register. The IC on the M222 that accepts this signal
is a SN7453. We have seen lots of failures of this unusual IC on other
restoration projects. We swapped the M222 that routes the GP=GPC signal and
the diag behavior changed. Next week we will replace the SN7453 and see if
that fixes this issue.

It will take quite a bit of work to fix the VR14, so we will try to use an
oscilloscope in X-Y mode as a display. It might even work!

-- 
Michael Thompson