PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sat Sep 5 08:16:26 CDT 2015

The Maintenance Manual-II describes the procedures for checking and
adjusting the TC12 timing. It needs a lot more notes about using the Auto
Restart speed settings. This facility allows you to put the processor in
Single Step and have the Continue button automatically pressed ad a
controllable rate. If the rate is set too high you will see the LTD ACIP
Delay activate twice and measure a delay that is more than twice real value.

Most of the delays are controlled by M307 One Shot flip-chips. These boards
have Fairchild 9601 ICs in them. We didn't have any spares, so we bought
some on eBay just in case...

The TC12 has extensive maintenance capabilities and will let the processor
simulate just about any condition in the TC12. With a short toggle-in
program we were able to check all of the basic timing signals LTT TP0 L,
LTT TP2 L, LTT TP3 L, and LTT TP4 L signals. We adjusted LTD XTLK H from
9.15 us to 9.5 us according to the written notes in the maintenance manual,
where the spec was 9 us. We adjusted LTD TTOK, LTD TAPE FAIL Delay, LTD
ACIP Delay, and the Mark Clock.

Unfortunately none of these adjustments made any difference, and the
LINCtapes still misbehave.

Michael Thompson

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