PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Mon Oct 12 12:40:26 CDT 2015

> Date: Sun, 11 Oct 2015 18:44:54 -0500
> From: Jay Jaeger <cube1 at 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.

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

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

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