7 Jul
2025
7 Jul
'25
8:42 p.m.
On Jul 7, 2025, at 7:45 PM, Marvin Johnston via cctalk
<cctalk(a)classiccmp.org> wrote:
While not on a 780 back plane, about 50 years ago, I was basically in charge of some 26
PDP16M computers. Periodically, one of the chips would be found with the top blown off. At
some point, I pulled the computer and examined the backplane filled with wirewrap wires.
The solution turned out to be fairly simple... a -15V bus was about .001" near one
of the wirewrap posts. A slight moving of that bus away from the wirewrap post solved that
problem. I have to assume it was a temperature related problem since increasing that
spacing solved the problem.
Unrelated to the computer problem, another source of angst was intermittently one of
those computers would crash. That one took probably a year to find... one of the interface
M series pullup cards had a pulldown card installed in its place.
I was pretty involved (as a student staff member) with a college main timesharing system,
an 11/20 running RSTS/11, that would crash roughly once a day. DEC spent ridiculous
amounts of time on it, including bringing in assorted wizards from Maynard. Eventually
the simply replaced the system by an 11/45 running RSTS/E, calling that a
"replacement part" :-)
About 10 years ago I happened to run into one of those wizards on the NetBSD kernel
hacker's list, and reminded him of that situation and the fact that it was never
diagnosed. He replied "sure it was: we figured it was due to the FM transmitter down
the hall".
Oh. Yes, the college radio station, a 5 kW FM transmitter, was located in a closet about
100 feet from the computer center. And the 11/20 predated FCC EMI/EMC standards, so that
is indeed somewhat plausible. Curious that we weren't told about it, though.
paul