PDP-8 core memory problems.

Mattis Lind mattislind at gmail.com
Mon Sep 5 13:59:50 CDT 2016

For some time I have slowly been working on restoring our PDP-8 to
operating condition.

Here are some notes on the progress:

It is sort of working now after reforming capacitors in the PSU, adjusting
memory currents and replacing a 2N3639 in a MA, MB, PC (R211) board.

But bit seven is still showing a 0 when trying to set it. I found an old
note that I wrote somewhere in 1990 that indicated the same problem was
present then as well so it hasn't happen lately.

I have now concluded that the fault is in the core memory module itself.
The sense winding is broken on bit plane 7.

So what are my options?

What are the chances that a repair is successful? It looks really complex
to take it appart and get to to layer seven. Then to find the broken wire,
somehow replace it and splice it together with the rest of the sense
winding... Not an everyday job.

Does anyone have a known good spare module? I have some MM15 modules for
PDP-15 to trade with!

Could a MM15 module be adapted to be used instead? It is completely
different foot print and it is a three wire stack.

The Russian stacks sold on Ebay? But what guarantees are that those work at
all and can be adapted... Probably other memory currents and cycle time?

Some other technology that can be used to make up for the missing bit
plane. But what technology and how. With as little intrusion in the
machine. I don't really want to replace the entire memory system with a
battery backed SRAM...

Any ideas, thoughts or a  working PDP-8 core memory module is highly sought


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