PDP-12 Restoration at the RICM

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? 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? -- Michael Thompson