PDP 8/L repair log - hopefully the LAST installment!

From: Charles <charlesmorris_at_direcway.com>
Date: Thu Mar 27 20:16:01 2003

The 8/L is finally fixed! The final interesting failures were:

The Group 2 microinstructions (CLA, IAC, etc) would not work.
Changed the relevant 7400 - still only weak wiggles. Tracked it
down to a dead short on the AC CLEAR line. Turns out that one of
the ground bus strips that run the length of the backplane had
buckled just enough to touch the exposed wire at the bottom of the
pin it was supposed to make a circle around! That was really fun
to locate.

Lastly, the TTY receiver card M706 would only work when on an
extender card. When in the backplane it would receive only one
character and "hang" until power cycled. I spent a lot of time in
a fruitless search for a bad backplane contact. Finally I found on
the stop bit flipflops that their PRESET pins were floating (a
frequent bad habit of DEC in their early design was to leave
uncommitted TTL inputs to float, which works fine as long as it's
an electrically "quiet" environment. Which a card handling 20mA
TTY signals is NOT). A quick 1K pullup to +5 and it hasn't dropped
a bit since.

I've just finished constructing my "DF32x4" disk simulator (PC
boards in a 1U [1.75" high] rack chassis). I'm starting to debug
now - so far I had a bad IDC connector and (of course) no spare. I
will make Eagle .SCH and .BRD files available to anyone who wants
them IF it ever works ;)

-Charles

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Received on Thu Mar 27 2003 - 20:16:01 GMT