DEC PDP-8/e H212 core mat repair

Joshua Rice rice43 at
Mon Jul 19 03:40:44 CDT 2021

------ Original Message ------
From: "Tom Hunter via cctalk" <cctalk at>
To: "General Discussion: On-Topic and Off-Topic Posts" 
<cctalk at>
Sent: Monday, 19 Jul, 2021 At 06:33
Subject: DEC PDP-8/e H212 core mat repair
I am curious if anyone has attempted to repair (replace) a broken wire 
in a
PDP-8/e H212 (MM8EJ) core mat (8 k word). The cores are not visible 
a microscope. I cannot imagine how these were even manufactured and 
if DEC service centers repaired core mat faults or if faulty boards were
simply discarded.
CDC 6600 cores were huge in comparison and I would not hesitate to 
a broken core wire on those.
Best regards
Tom Hunter

I believe much of the core manufacturing for DEC minicomputers was 
outsourced, but a lot of it had become much more automated by the late 
60's and early 70's. I believe it was done by machine, with a tray to 
hold the toroids in place, and a very fine needle-like "bobbin" that 
threaded the wires through the toroids. I believe threading the cores by 
hand had become largely obsolete by the time the PDP-8 came onto the 
Though i can't confirm it, i highly doubt that DEC engineers would 
repair core planes. These would more likely be sent back to the 
manufacturer for "recycling", with the cores being recovered and reused.
It's worth noting that most computer manufacturers appreciated the 
fragility of core memory planes at the time, with most of them being 
protected with either PCB's or perspex/plastic shields on top of the 
core planes. In this way, it was rather difficult for a clumsy ol' 
technician to put his thumb through the planes as he was servicing 

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