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Fun with Core Store

D

DaveH

Experienced Member
Joined
Sep 1, 2012
Messages
173
Location
Scotland
Iv recently had some fun trying to get some more core stacks back to life, with some success but also some issues.
One stack had LOTS of steering diodes failed, now Iv "bodged" it using the good one 1N4148's but for reliability would like to change the actual DIL packages, DEC -2501-00, has anyone found a replacement for this or will I have to make something up ?.

I have also found 2 stacks with open sense/inhibit lines :( , anyone tried actual stack repairs ? , I still need to try and find the actual location of the OC, I intend to ground all the lines apart from the faulty one and then inject a 1k tone on it and see if I can do a non contact trace, then borrow a microscope and see what I can see.

Pic of the bodged stack below, the vertical red band is a flash artifact !

DaveH HPIM2003.jpg
 
I've been curious about core stack repairs, and I've recently obtained my first PDP-8/M machine with core memory. I'm still working on the front panel, and I don't know the condition of its 8k words of core yet.

I have a hard time believing that I have the dexterity to repair a core problem such as a broken core, but I wonder how do-able it is and whether it was routinely done to damaged stacks back in the day. Did they ever include extra rows and/or columns of cores in stacks so that a stack with a single damaged core could be repaired without needing to weave a new core in?
 
Hi All;
NF6X, To my Knowledge that was Never done.. I almost makes sense, But with Core that would be nearly impossible..
A company that I worked for, before I came had manufactured Core Memories, before they switched to Ic/RAM memories..
Another Company that I used to work for, did just that with Their Memory Ram Boards.. Each Board included, an Extra Ram Ic in a Socket on each Board, and of course all the Rams in the Board was Socketed..
And of Course it was one of their Selling points..
THANK YOU Marty
 
NF6X said:
I've been curious about core stack repairs, and I've recently obtained my first PDP-8/M machine with core memory. I'm still working on the front panel, and I don't know the condition of its 8k words of core yet.

I have a hard time believing that I have the dexterity to repair a core problem such as a broken core, but I wonder how do-able it is and whether it was routinely done to damaged stacks back in the day. Did they ever include extra rows and/or columns of cores in stacks so that a stack with a single damaged core could be repaired without needing to weave a new core in?
Click to expand...

I think you would be very unlucky if you had an actual faulty stack like the 2 I have, most memory problems are due to fault in the xy driver or the sense/inhibit, a faulty stack is rare unless its been mistreated. Most of my cards , including the stack boards were from the remains of a DEC field service engineers collection so many may well have been faulty, pre seleted dead as it were, also they were NOT stored well for 30 years or so , damp. and I dont think they liked that.

I believe that repairs were sometimes made during manufacture, this is why DEC has a transparent cover so they could tell if this had been done. As far as I know spare rows ect were not incorporated, certainly all the DEC ones I have seen have not had this.
BTW the cores and the wires are as you would expect VERY fragile, I dont try to clean or disturb the core itself, if theres dirt ect in there it just gets left there, wont do any harm , and trying to clean it could well damage it.
I expect that trying to repair my ones may well not be possible but Im at least going to see if i can find the location of the break, the cores them selves are I think ok, just a sense/inhibit, one leg in fact , open circuit.
Dave
 
I have read about companys that specialized in core memory repair, but I think they could only work on core memory that had cores over a certain size, and were not cheap, I think they rewove part of the core memory plane, I think core memory repair was only done on early expensive core memory with large cores and stacked core memory planes! I think DaveH's core may be to small and dense, but I think it will just be electronic problems stopping the core memory working, and you may have better luck using 1N914 diodes, but what ever diodes you use make shore you use the same manufacturer's batch for the all of the repair, as the same diodes from different manufactures could have different characteristics.
good luck with the core memory
 
rorypoole said:
but I think it will just be electronic problems stopping the core memory working,
Click to expand...

Hi Rory, I do wish that were the case but unfortunately its not, both planes have an open circuit sense/inhibit line on one bit, it should be 150 ohms or so and its open, you are correct that most memory problems , maybe 99% , are due to electronics , these two are the 1%. If the fault is in the core area itself then , no Im not going to be able to repair it, if its a break or bad connection else were then MAYBE.
Thanks for the suggestion re 1N914, it does look slightly better, had a slightly higher current rating, mind you its only conducting for 500nS so its the pulse rating rather than the normal average that counts . Basically I think that any general purpose small signal diode should be ok here as long as its good for 400-500mA pulse, but if I have to do the a whole plane I may use the1N914.
If I cant fix the 2 faulty then I may run them in parallel, making 1 good from 2 faulty, be a bit of a bodge but do able, 2 X/Y's, 2 cores and 1 sense/inhibit.
DaveH
 
To find the open, see if you can find an electrician, cable man or phone man
with one of the wire sounders.
It puts a tone on the wire and has a receiver to pick it the signal.
I have one but loaned it to a friend.
Don't think about buying one new. They go for about $200 or so.
Even on ebay, they are about $70 or so.
Best to find a friend that can bring his over and help you trace the open.
If you have a square wave generator and a scope, you can do about the
same thing.
Dwight
 
Dwight Elvey said:
To find the open, see if you can find an electrician, cable man or phone man
with one of the wire sounders.

Dwight
Click to expand...

Hi Dwight, Yes I did try that as I have cable tracing unit, however its just to course for this job, you get a tone anywhere near the core stack. Im going to have to make up my own detector, probably using some thin coax so we have the core as the detector and the outer as a screen, hopefully we will be able to resolve to 1 or 2 mm which is what we need. I will also need to ground all the other core wireing as there is a lot of coupling between them.
For the moment im not going to attempt any repairs as the chance of a repair is slim at best and the chance of doing more damage high , I intend to use the 2 with failed bit lines coupled together as a single 8k, and the one I have with a failed high field y line as just a 4k, that's easy , just use a G104 in place of the G111
DaveH
 
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