Re: PDP-8 core memory problems.

[Mattis Lind]

2016-09-07 17:33 GMT+02:00 Doug Ingraham :

> The most likely cause of what you are seeing is a broken wire when the
> plane was originally assembled.  The wire was pulled back a few cores and
> the end stripped.  New wire was soldered to old, insulated and then they
> continued threading in that wire.  Over the years the solder joint has
> degraded or the wire broke at the stress riser found at one end of the
> solder joint and now you have an open circuit. I've not heard of this kind
> of problem on the Straight 8 but that may be due to the rarity of the
> processors.  It is apparently a fairly common failure on the 8I core
> planes.
>
> As was stated you have nothing to lose in attempting a repair as the core
> is useless as is.  A steady hand, good desoldering tools, lots of photos
> and you should be able to take it apart, effect the repair and
> re-assemble.  Keep in mind that the core beads themselves are extremely
> fragile so take precautions that nothing gets dropped on it.  Broken core
> beads are pretty much a death sentence to the memory.  Replacements are
> unobtanium and if you decided to make the beads you would have trouble
> matching the originals well enough to tune the core to work with both new
> and old.  You would end up making a whole new core assembly consisting of
> 49152 beads.  You would need to be really determined to attempt that.
>
> I did come up with an idea that is simply too dangerous to try.  Connect a
> power supply to the ends of the wire and ramp up the voltage until it just
> starts to conduct.  This could be several hundred to several thousand
> volts.  As soon as it starts to conduct the broken ends of the wire will
> start to heat and the moment the current starts to shoot up (the resistance
> drops) you need to cut power.  You will have welded the broken ends of the
> wire together.  The problem is that if anything goes wrong you are in worse
> shape than now and you really only get once shot at it.  And the assumption
> is that the broken ends are in close proximity.
>
> Here is wishing you a steady hand and lots of luck!
>
>
Thanks Doug and Anders for the encouragement. I have decided to put the
core in a safe place while I am gathering tools (stereoscopic microscope
and better tweezers). The core repair project will be on hold until some
vacation day when I have plenty of time and there are no disturbances
around. Possibly in the mean time I will look into the instruction decoding
problem that affects the front panel operation sometimes.

I prepared a small video on the non-operating machine:
https://youtu.be/8WYiUz4SNi8

/Mattis Lind
PDP-8 s/n 351 (non-operating)



> --
> Doug Ingraham
> PDP-8 SN 1175
>

Re: PDP-8 core memory problems.

[Doug Ingraham]

The most likely cause of what you are seeing is a broken wire when the
plane was originally assembled.  The wire was pulled back a few cores and
the end stripped.  New wire was soldered to old, insulated and then they
continued threading in that wire.  Over the years the solder joint has
degraded or the wire broke at the stress riser found at one end of the
solder joint and now you have an open circuit. I've not heard of this kind
of problem on the Straight 8 but that may be due to the rarity of the
processors.  It is apparently a fairly common failure on the 8I core planes.

As was stated you have nothing to lose in attempting a repair as the core
is useless as is.  A steady hand, good desoldering tools, lots of photos
and you should be able to take it apart, effect the repair and
re-assemble.  Keep in mind that the core beads themselves are extremely
fragile so take precautions that nothing gets dropped on it.  Broken core
beads are pretty much a death sentence to the memory.  Replacements are
unobtanium and if you decided to make the beads you would have trouble
matching the originals well enough to tune the core to work with both new
and old.  You would end up making a whole new core assembly consisting of
49152 beads.  You would need to be really determined to attempt that.

I did come up with an idea that is simply too dangerous to try.  Connect a
power supply to the ends of the wire and ramp up the voltage until it just
starts to conduct.  This could be several hundred to several thousand
volts.  As soon as it starts to conduct the broken ends of the wire will
start to heat and the moment the current starts to shoot up (the resistance
drops) you need to cut power.  You will have welded the broken ends of the
wire together.  The problem is that if anything goes wrong you are in worse
shape than now and you really only get once shot at it.  And the assumption
is that the broken ends are in close proximity.

Here is wishing you a steady hand and lots of luck!

-- 
Doug Ingraham
PDP-8 SN 1175

Re: PDP-8 core memory problems.

[Anders Sandahl]

So what are the other options?
* Trying to repair the unit. Every plane is soldered together with the ones
nearby to convey the X/Y signals. This can probably be undone with a
patience and soldering braid. But what are the chance that the X/Y wires
gets lose then? Are those soldered or welded into place?

Then it would be quite tricky to just identify where it is actually broken.
Any ideas for how to do this? A microscope of course. Any other ideas?
Applying an electrical field between the wire and something else and try to
detect it?

A stereoscopic microscope and a lot of patience is a good staring point.


Repair. If the wire is broken in the mat it is probably not to difficult to
pull out the broken parts. But then the new wire has to be spliced in. What
is t he best technique to do that?
How to push in the new wire in the matrix? I now that Anders was able to do
this with a broken X-wire in a PDP-8/L stack.
I used a pair of tweezers to push the new thread in, one core at a time. 
You should try to change as small part as possible. I just changed the 
thread thru the first 64 cores, then I joined the two threads together 
in the middle of the core plan. I then used nail polish as an isolatur.


I didn't thought I had something to lose to try to repair the existing 
core stack. Without it the computer is really not working at all. I'll 
think you should try to repair it. Start to bring out the core stack 
and  then turn it around for a while until you are brave enough.


I found some pictures on stacks to the 8/L in pieces before I started on 
the internet, that helped me to understand what I had to master.


/Anders

Re: PDP-8 core memory problems.

[Vincent Slyngstad]

From: "Mattis Lind: Tuesday, September 06, 2016 2:11 AM

* Use a PDP-15 MM15 stack and sense/inhibit boards.

I have several off these. Adding a small backplane, put the X/Y drivers,
sense amp/inhibit drivers and level converters there and then adapt to the
existing slots for the memory module. It would be a horrible mixture of TTL
and transistors. But it would still be core memory.


There's a guy on eBay who periodically offers stacks for 8/I or 8/L:
http://www.ebay.com/itm/172295154098

Those might (or might not) be more straight-forward to interface (though 
DEC's proof of concept is a TTL interface).



* Use solid state technology. Possibly inside the memory box so it looks
real but emulates the actual core memory module.
Any ideas how this could be done in the best way?


I did some (theoretical) work on a core replacement for the 8/S 
a few years ago based on some ideas from John Price.  That's 
embedded in the "memory" drawing in 
http://svn.so-much-stuff.com/svn/trunk/Eagle/projects/DEC/PDP8S/.

(See sheet 2; most of the schematic is just the schematic of an 8/S.)

Somewhere I also explored a notion of pulling the inhibit driver 
and sense amp stuff, replacing the stack itself with a memory board, 
then replacing the inhibit drivers and sense amps with interface 
boards that knew how to talk to the new memory board.  If I recall, 
that was in the 8/I or 8/L context, though.


I never built any of it, it was just a thought exercise.  There  seemed 
to be plenty of board real estate to work with, though.


   Vince 

Re: PDP-8 core memory problems.

[Mattis Lind]

>
> I'd tend to be more pessimistic about this working.
>
> There are different requirements in winding a wire for purposes of inhibit
> and sense.
> In the 3-wire arrangement the winding of the combined wire has to meet both
> sets of requirements.
>
> Specifically, for this case, in a 4-wire mem, where the inhibit wire was
> woven just for
> the purposes of inhibit, I don't anticipate it's going to have the noise
> cancellation topology
> needed to function as the sense wire.
>
> Sense wires were woven as a floating loop feeding a differential
> amplifier. The loop is kept
> quite closed or otherwise woven in a very balanced manner so that the
> magnetic fields from the
> large select currents (and other influences) will cancel out or be
> rejected as common-mode influence at the
> differential amp inputs.
>
> The select current magnetic fields trigger the magnetic field reversal of
> the core, you want to sense the latter distinct from the former.
> Unless you have circuitry with the wherewithal to distinguish those
> induced currents in the sense wire in time
> (delay of the core field reversal from the select field), you otherwise
> have to minimise the influence of the select current fields on the sense
> wire.
>
> If you look at the diagrams in my article you can see the sorts of
> differences in weaving topologies between 3 & 4 wire arrangements,
> as well as examples of the tortured topologies resorted to to balance the
> sense loop.
> In the 3-wire example there you can see how the S/I wire was split in half
> with a special resistor network at one end to allow inhibit current flow
> while at the same time configuring it as a balanced loop for the sense
> function.
>
> My article certainly isn't the last word on the variety of
> implementations, I believe there were 4-wire designs with sense wires
> parallel to
> select wires as in the 3-wire designs for example, so you never know until
> you examine the specifics at hand,
> but I think it unlikely you'd have much success getting the inhibit wire
> to function for sense, not without going to as much trouble
> messing with the stack as if you tried to fix the sense wire.
>


Thanks. I think that this information will rule out the idea of using the
inhibit wire instead. It is likely to be not viable.

>
> If the sense wire is open I'd guess there's a good chance it's at one of
> the end points where it's soldered to a terminal or at an existing splice
> from manufacture time and might be repairable if access could be had.
>

Access is of course the problem. I tried to check at the terminals and it
looked like the wire was ok there, but it is in an extremely tight place. I
tried to apply some small amount of solder in the hop that it was just bad
contact at one of the terminals but no difference unfortunately.


>
> Might the stack have a parity bit array that could be redirected to
> replace the faulty bit array?,
> perhaps foregoing the parity checking on that board-set if the parity
> logic is present.
>

I think I have checked the existence of a parity plane. It appears that
there is none present. The writing on the core module show no parity plane
and there are only twelve sense and inhibit terminals.

So what are the other options?

* Trying to repair the unit. Every plane is soldered together with the ones
nearby to convey the X/Y signals. This can probably be undone with a
patience and soldering braid. But what are the chance that the X/Y wires
gets lose then? Are those soldered or welded into place?

Then it would be quite tricky to just identify where it is actually broken.
Any ideas for how to do this? A microscope of course. Any other ideas?
Applying an electrical field between the wire and something else and try to
detect it?

Repair. If the wire is broken in the mat it is probably not to difficult to
pull out the broken parts. But then the new wire has to be spliced in. What
is t he best technique to do that?

How to push in the new wire in the matrix? I now that Anders was able to do
this with a broken X-wire in a PDP-8/L stack.

* Use a PDP-15 MM15 stack and sense/inhibit boards.

I have several off these. Adding a small backplane, put the X/Y drivers,
sense amp/inhibit drivers and level converters there and then adapt to the
existing slots for the memory module. It would be a horrible mixture of TTL
and transistors. But it would still be core memory.

* Use solid state technology. Possibly inside the memory box so it looks
real but emulates the actual core memory module.
Any ideas how this could be done in the best way?

/Mattis

Re: PDP-8 core memory problems.

[Jon Elson]

On 09/05/2016 09:28 PM, Brent Hilpert wrote:


I'd tend to be more pessimistic about this working.


In the 3-wire example there you can see how the S/I wire was split in half with 
a special resistor network at one end to allow inhibit current flow
while at the same time configuring it as a balanced loop for the sense function.

My article certainly isn't the last word on the variety of implementations, I 
believe there were 4-wire designs with sense wires parallel to
select wires as in the 3-wire designs for example, so you never know until you 
examine the specifics at hand,
but I think it unlikely you'd have much success getting the inhibit wire to 
function for sense, not without going to as much trouble
messing with the stack as if you tried to fix the sense wire.
Well, older core memories had larger cores, which slowed 
everything down as well as gave much bigger pulses when a 
core flipped.  That might make this exercise a bit easier.

If the sense wire is open I'd guess there's a good chance it's at one of the 
end points where it's soldered to a terminal or at an existing splice
from manufacture time and might be repairable if access could be had.
But, he has to split the whole stack to at least access the 
bad plane from one side.  Already a pretty daunting 
exercise.  Still, fixing the bad wire, if the break is at an 
exposed point, would be preferable than re-engineeering the 
whole sense-inhibit circuitry. If the wire can be fixed, 
there would be no engineering required.


I saw our old IBM 7094 memory, which had a combination of 
tube and transistor circuitry, and one inhibit (or maybe 
sense) wire had been burned up by a failed circuit.  The 
cores kept the wire cool, so the only places it opened was 
where the wire looped around the plane. There were dozens of 
bits of wire soldered to fix the places where it opened.  
Sheesh, I sure wouldn't have wanted to be the CE who had to 
do that repair!


Jon

Re: PDP-8 core memory problems.

[Brent Hilpert]

On 2016-Sep-05, at 4:36 PM, Jon Elson wrote:
> On 09/05/2016 05:46 PM, Mattis Lind wrote:
>> måndag 5 september 2016 skrev Jon Elson :
>> 
>>> On 09/05/2016 01:59 PM, Mattis Lind wrote:
>>> 
 I have now concluded that the fault is in the core memory module itself.
 The sense winding is broken on bit plane 7.
 
 
 Have you actually ohmed out the sense/inhibit wire?
>> This is by the way a four wire stack. Separate sense and inhibit wire.
> OH!  I didn't know any PDP-8 had 4-wire planes.  Very interesting. Hmmm, you 
> could rig a pulse transformer to the inhibit wire to bring signals over to 
> the sense amp.
>> Come to think of it, since the inhibit wire is OK, would it be possible to
>> arrange it as a three wire stack somehow. Change the sense amp and inhibit
>> drivers so that they use the same wire?
> Ah, you already thought of that.  Well, it SHOULD be possible. You'd probably 
> put a resistor in series with the pulse transformer so that all the select 
> current went through the inhibit wire.  Then, the only problem would be that 
> the sense amp gets hit with a big pulse during the writeback.  As long as the 
> sense amp recovers from the overload before the next read cycle, it ought to 
> work.  Some 4-wire planes were set up so the polarity of half the sense wires 
> were opposite to the inhibit, so the coupled inhibit current balanced out to 
> zero in the sense winding.  Any mid-sized 1:1 pulse transformer should do the 
> coupling, the currents might be pretty high, but the duration isn't very 
> long.  (Minor nit, the inhibit wires only get pulsed one direction, so you 
> might need a resistive path across the secondary to discharge the flux build 
> up so it doesn't saturate.)
>> Maybe Brent can come in with some advice if this is possible or just
>> stupid.
> It is NOT stupid, I think it could really work!

I'd tend to be more pessimistic about this working.

There are different requirements in winding a wire for purposes of inhibit and 
sense.
In the 3-wire arrangement the winding of the combined wire has to meet both
sets of requirements.

Specifically, for this case, in a 4-wire mem, where the inhibit wire was woven 
just for
the purposes of inhibit, I don't anticipate it's going to have the noise 
cancellation topology
needed to function as the sense wire.

Sense wires were woven as a floating loop feeding a differential amplifier. The 
loop is kept
quite closed or otherwise woven in a very balanced manner so that the magnetic 
fields from the
large select currents (and other influences) will cancel out or be rejected as 
common-mode influence at the
differential amp inputs.

The select current magnetic fields trigger the magnetic field reversal of the 
core, you want to sense the latter distinct from the former.
Unless you have circuitry with the wherewithal to distinguish those induced 
currents in the sense wire in time
(delay of the core field reversal from the select field), you otherwise have to 
minimise the influence of the select current fields on the sense wire.

If you look at the diagrams in my article you can see the sorts of differences 
in weaving topologies between 3 & 4 wire arrangements, 
as well as examples of the tortured topologies resorted to to balance the sense 
loop.
In the 3-wire example there you can see how the S/I wire was split in half with 
a special resistor network at one end to allow inhibit current flow
while at the same time configuring it as a balanced loop for the sense function.

My article certainly isn't the last word on the variety of implementations, I 
believe there were 4-wire designs with sense wires parallel to
select wires as in the 3-wire designs for example, so you never know until you 
examine the specifics at hand,
but I think it unlikely you'd have much success getting the inhibit wire to 
function for sense, not without going to as much trouble 
messing with the stack as if you tried to fix the sense wire.

If the sense wire is open I'd guess there's a good chance it's at one of the 
end points where it's soldered to a terminal or at an existing splice
from manufacture time and might be repairable if access could be had.

Might the stack have a parity bit array that could be redirected to replace the 
faulty bit array?,
perhaps foregoing the parity checking on that board-set if the parity logic is 
present.

Re: PDP-8 core memory problems.

[Jon Elson]

On 09/05/2016 05:46 PM, Mattis Lind wrote:

måndag 5 september 2016 skrev Jon Elson :


On 09/05/2016 01:59 PM, Mattis Lind wrote:


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


Have you actually ohmed out the sense/inhibit wire?

This is by the way a four wire stack. Separate sense and inhibit wire.
OH!  I didn't know any PDP-8 had 4-wire planes.  Very 
interesting. Hmmm, you could rig a pulse transformer to the 
inhibit wire to bring signals over to the sense amp.

Come to think of it, since the inhibit wire is OK, would it be possible to
arrange it as a three wire stack somehow. Change the sense amp and inhibit
drivers so that they use the same wire?
Ah, you already thought of that.  Well, it SHOULD be 
possible. You'd probably put a resistor in series with the 
pulse transformer so that all the select current went 
through the inhibit wire.  Then, the only problem would be 
that the sense amp gets hit with a big pulse during the 
writeback.  As long as the sense amp recovers from the 
overload before the next read cycle, it ought to work.  Some 
4-wire planes were set up so the polarity of half the sense 
wires were opposite to the inhibit, so the coupled inhibit 
current balanced out to zero in the sense winding.  Any 
mid-sized 1:1 pulse transformer should do the coupling, the 
currents might be pretty high, but the duration isn't very 
long.  (Minor nit, the inhibit wires only get pulsed one 
direction, so you might need a resistive path across the 
secondary to discharge the flux build up so it doesn't 
saturate.)

Maybe Brent can come in with some advice if this is possible or just
stupid.

It is NOT stupid, I think it could really work!

Jon

Re: PDP-8 core memory problems.

[Mattis Lind]

måndag 5 september 2016 skrev Jon Elson :

> On 09/05/2016 01:59 PM, Mattis Lind wrote:
>
>>
>> I have now concluded that the fault is in the core memory module itself.
>> The sense winding is broken on bit plane 7.
>>
>>
>> Have you actually ohmed out the sense/inhibit wire?
>

This is by the way a four wire stack. Separate sense and inhibit wire.

Come to think of it, since the inhibit wire is OK, would it be possible to
arrange it as a three wire stack somehow. Change the sense amp and inhibit
drivers so that they use the same wire?

Maybe Brent can come in with some advice if this is possible or just
stupid. I promise to read his excellent article on core memories once again
tomorrow when I am less tired to see If I can figure something out...

Re: PDP-8 core memory problems.

[Mattis Lind]

måndag 5 september 2016 skrev Jon Elson :

> On 09/05/2016 01:59 PM, Mattis Lind wrote:
>
>>
>> I have now concluded that the fault is in the core memory module itself.
>> The sense winding is broken on bit plane 7.
>>
>>
>> Have you actually ohmed out the sense/inhibit wire?


Yes. I have measured at at the tabs where the red arrow points in the image
below:

http://i.imgur.com/x5VVh2F.jpg

Unfortunately I am pretty sure that this will rule out pulse transformers
or whatever.

/Mattis