Repairing core memories....

O. Sharp ohh at
Sun Oct 31 13:22:35 CDT 2010

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 

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 

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.  :)


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