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

From: Marvin Johnston <marvin_at_rain.org>
Date: Thu Mar 27 20:24:00 2003

Charles wrote:
>
> 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.

I had a similar problem in the early 70's with a PDP-16; periodically
one of the chips would "blow its top" and I'd come in to find the
machine not working and the chip blown apart. Checking found nothing,
and replacing the chip solved the problem for another couple of months.
I finally had occassion to pull the computer and check out the back
plane for another reason, and found the same thing you did ... except it
was *major* intermittent :). The buss line was IIRC -15VDC or so and
periodically would touch one contact thus blowing the chip apart. Only
took a year to solve it.
Received on Thu Mar 27 2003 - 20:24:00 GMT