> On Jul 7, 2025, at 7:45 PM, Marvin Johnston via cctalk
> <[email protected]> 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
