On 10/12/2015 12:40 PM, Michael Thompson wrote: I had in mind the power to the cards in the PDP-12 itself, not the tape drive(s). No local sense in those days, with considerable voltage drop in the wiring. A bad wrap might add to that drop on the backplane itself. It is a possibility not to ignore. The reason I asked is that at Wis. DOT we had an Amdahl 580 that was acting flaky - odd crashes without any apparent connection. The machine was new, so Wis. DOT threatened to send it back. Amdahl flew in something like 3 engineers from the Sunnyvale plant, and they along with the 2 local FE's and one CE went over the machine with magnifying glasses (literally), looking at everything. They finally found one amazingly tiny coax wire (imagine a coax cable about the diameter of a normal wire wrap wire) that had been nicked/kinked by being pinched against a frame at some point - took them almost all night to find it. They were happy but pretty "ripe" when I came into work the next morning. Based on that story, it occurred to me that corrosion on a wrap on the PDP-12 backplane could be an issue. The other possibility that occurs to me is a timing glitch - where two signals change state at the same time and then are sampled too close to that time, resulting in a signal being clocked into a latch the wrong state. Those can be really hard to track down. JRJ