On 10/12/2015 12:40 PM, Michael Thompson wrote: >> >> Date: Sun, 11 Oct 2015 18:44:54 -0500 >> From: Jay Jaeger <cu...@charter.net> >> Subject: Re: PDP-12 Restoration at the RICM >> >> Don't forget about the other more remote possibilities: cables, >> backplane, bad wrap, supply voltages at the actual card(s) for the >> mis-behaving channel, etc. >> >> JRJ >> > > We used different control and data cables for the TU55 and the TU56 drives > and observed the same track 3 bad behavior. > > The backplane appears to be in good shape. > > My scope had a little trouble looking at 10-15mV signals in differential > mode using the math functions, but we looked at the head signals going into > the track 3 amplifier, and they looked reasonable. > > The power supply voltages at the cards are within spec. The track > amplifiers are supposed to be differential, so they should be fairly immune > to power supply noise. We plan to connect a lab supply to the backplane > near the track cards and adjust it slightly higher than the PDP-12 power > supply. That should clean up any 60Hz noise on the power. Maybe that will > help? >
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. > We have swapped everything else between the tracks, including the logic > analyzer probe, and the issue always stays with track 3. Maybe it is a > backplane wiring problem? > 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
