PDP-8/e Timing Repair

The second timing card presented a basic fault. When trying to start any program, the timing chain never started. Looking at the logic analyser, the fault was particularly obvious. The system timing simply didn't start.

Diagnosing The Fault

Run flip-flop is latched High by the PRE input (pin 4) when the timing chain is intended to start. CLR is ment to reset the flip-flop to LOW.

However when looking at the circuit I noticed that CLR (pin 1) was constantly being forced low. Working my way back was through the circuit I found that E27 had failed and was malfunctioning.

The Repair

E27 was a DEC384 which is a relabelled Signetics SP384. Now, the SP384 is not the easiest or most affordable chip to obtain. Fortunately people have previously worked out that a 7432 is suitable replacement if you use an adaptor.

I spent a little bit of time building up a couple pcbs and getting them ordered. You can find them here: https://github.com/ashlin4010/DEC-Chip-Adaptors

On the arrival of the PCB I put it all together and tested it and the timing card is working once again.