On 7/7/25 19:42, Paul Koning via cctalk wrote:
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.
I maintained an 11/45 (SN 343) so, maybe that was an early
one, but I SURE didn't consider it an RFI-tolerant
construction. There were no doors with RF gaskets, etc.
Ours did run quite well, except that the AMP Mate-n-Lok
power connectors on the regulator modules kept burning up.
Jon
Jon
