Repairing core memories....

Peter C. Wallace pcw at
Sun Oct 31 13:51:51 CDT 2010

On Sun, 31 Oct 2010, O. Sharp wrote:

> Date: Sun, 31 Oct 2010 14:22:35 -0400 (EDT)
> From: O. Sharp <ohh at>
> Reply-To: "General Discussion: On-Topic and Off-Topic Posts"
>     <cctalk at>
> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at>
> Subject: Re: Repairing core memories....
> On Sun, 31 Oct 2010, Jos Dreesen wrote:
>> So I have these 2 PDP-8/L core stacks I am trying to recover:
>> One would be perfect, if bit 3 @ adr 0 would be alive...
>> Although this is 99.99% OK, it is of course not good enough.
> Just to mke sure I'm hearing you right: Are you saying one individual core is 
> 100% dead, and everything else is 100% okay? If so - if it's one single, 
> individual core which is broken - I'd suspect this is pretty much 
> irrepairable. The options would be to A) try to form/mold/create a new 
> ferrite core around the wires, which would be insanely difficult assuming it 
> could be done at all, or B) to unwire that plane of the core stack, replace 
> the broken ferrite core with a new one, and then rethread/rewire it. That's 
> extremely nontrivial work, I have to believe. But they were originally 
> threaded by hand, albeit by people with microscopes and _very_ good eye-hand 
> coordination and sewing skills, so it's within the realm of possibility.
> If I've read you wrong, though, and the whole thing is working except address 
> 0 bit 3 is working _some_ of the time, that would be really good news.  :) 
> You could try scoping out the sense amps and inhibit drivers for bit 3 and 
> see if they were just borderline to specifications, something which might be 
> just tweaky enough to affect one bit.
>> The other stack seems to be a total loss :
>> 2 sense wires are  open circuit, more than 20 select diodes shorted. Of 
>> course the further quality of the cores is unknown.
> Oddly enough, this one might be easier to deal with. Have you tracked down 
> where the sense wires go open-circuit? If they're failing at a solder-joint 
> near an outer edge, you could potentially repair them without having to open 
> up the whole damn stack. The diode replacement would likely mean partially 
> disassembling the stack assembly to gain access to both sides of the diode 
> PCB, but at least you're only having to dig in one level.
> O'course making those repairs might simply reveal more problems at the core 
> level. But you've gotta start somewhere, I s'pose.  :)
>> Is there any realistic way of getting one fully functional stack out of 
>> these ?
>> Removing a single core from the really bad stack to the almost OK stack 
>> would seems almost feasible to me, since address 0 is bound to be on a edge 
>> of a core mat.
> I suspect I'm not the only one on the list who:
> 	-thinks opening up a core-stack and repairing it is
> 	 theoretically possible;
> 	-also thinks it would be a hell of a daunting project;
> 	-is somewhat amazed at the dexterity and patience of the people
> 	 who originally hand-wired them at manufacture; and
> 	-thinks pulling off a repair of a core-plane by rewiring it by
> 	 hand would give significant bragging rights.  :)
>              -O.-

If you look at core planes, many have repairs visable in even in "new" planes, 
probably ones that failed margin tests. I looks like the way cores in the 
middle of the mat are replaced is by snipping a bit of the drive and sense 
wires a couple of cores away from the faulty core, replacing the core and 
threading in a bit of new wire for the drive and sense lines, with enough 
slack to solder the splices above the plane. You can see the little splices in 
many core planes (usually covered with a tiny drop of glytol or some such)

Peter Wallace

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