Making the repair on the core stack - part 4

METHOD CHOSEN TO RESTORE THE COMMON CONNECTION FOR BIT 7 LOW SENSE

I judged the risk of taking apart the backplane and core stack too high, as it could cause another wire to come off or worse, trigger yet another continuity break on the bottom board of the core stack. Therefore I looked for the best means of repairing this with the stack still mounted on the backplane and in the frame.

My first approach was to solder a wire from the common pins on the bit plane, bridging the low and high core planes of bit 7. This was good because the two common wires, although separately routed to the jumper blocks, ended up tied together at pin D06 of the same sense/inhibit card in slot B3. Therefore I just changed the point where they were tied together to the far end, on the core stack. 

The ground wire that was twisted around the other two wires for bit 7 low 4K would therefore be connected through this jumper and provide the same shielding to the twisted wires up to the point of the jumper block pins. This seemed to preserve the impedance and noise suppression almost as well as the original connections. 

IMPLEMENTING THE FIX WITH THE JUMPER - FAILED

I pulled out many SLT cards in rows 2 and 3 to give me clean access to the top of the core stack where I was going to make the hack. All I had to do was tack a short wire from the common pin on the right side of the core plane to the common pin in the middle of the plane - adding to the wires already connected to those pins. 

However, this time I was utterly unable to get the wire to solder to the pin, in spite of all the flux and technique I could apply. It just would not stick at all; I believe that the odd metal, probably minimally magnetic, used for the connection points required spot welding at the IBM factory. 

SECOND METHOD DEVELOPED

If I couldn't solder to the pins on the bit 7 plane, I still had the black wire (ground) which was not connected at the jumper pin any more. Since it was loose, I hoped I could gently coax it back, sliding out between the blue and white wires it was braided with. If I got an end out of the backplane where I could solder to it, I would still have it wound around part of the distance of the wiring from bit plane to jumper block. 

I didn't have the ability to solder to the bit plane pins, but I did have access to the pins of the backplane in rows 1, 2 and 3 above the core stack. Most would be blocked by SLT cards and connectors but there were two slots unpopulated on the 1130 core memory compartment one of them at E2 not that far from where the wiring originally was routed to F4. 

I would connect the wire directly to the D08 pin in the empty card slot - which is ground - since the common ends route to their SLT cards (e.g. B3) which internally connects the common to D08 of the card. This loses some of the shielding from induced noise, but hopefully the sense circuitry can still reliably detect one bits during reads. 

During all this hacking, I used my ohmmeter to connect to the two bit 7 low 4K sense connections on the B3 card. As long as they continued to have continuity and low resistance, nothing I did damaged the connections any further. This was my patient monitoring equipment during my open heart surgery on the core stack. 

I gripped the blue and white wires while applying pressure on the black, but it was not moving. I suspect it was pinched between the backplane walls and the plastic block on the core stack, something that yanked it off the pin but wouldn't let it move. I had to shift gears a bit, so I just snipped off the black wire as close to its entry to the backplane as I could.

I did a test connection of the black wire to a D08 ground pin on the backplane and then tested the three connections for the bit 7 low on card B3 - these were pins D02, B02 and D06 for the two sense lines and the common connection respectively. The readings were perfect. The only open question is noise pickup during memory operation. 

Tomorrow I will finalize the connection to E2 D08 for the common/ground line of bit 7 low 4K sense/inhibit lines. This is mainly a mechanical process, ensuring the wire will route between the cards in slots E3 and F3 as it goes up to the open slot at E2, plus insulating the splice I had to make from the snipped short original black wire.