PHYSICAL STRUCTURE OF CORE STACK
The core stack consists of a sandwich, having a bottom printed circuit board, eighteen core planes in the middle and a diode board on the top. As the core stack sits in the IBM 1130 memory compartment, the diode board is facing you when you look inside the compartment and is at right angles to the floor and parallel to the backplane of the compartment. Side A is facing the ceiling and side C is facing the floor. Therefore D is to the left and B is to the right when you are looking in the compartment at the core stack.
The core planes are parallel to the backplane of the compartment. Their long edge (B and D) is vertical to the floor and has 128 wire connections for addressing the with bits 9 to 15 of the memory address. The shorter edge (A and C) has 64 wires for addressing with bits 3-8 and is parallel to the floor. In addition to the addressing connections on the sides of the core plane, there are connections for the sense/inhibit wires on the A and C sides of the plane.
Wiring from the addressing and sense/inhibit lines are attached to the bottom board with a clip, contacting the copper trace of the printed circuit on that bottom board. This core memory uses the S type clip.
Pins stick out of the bottom board. These fit through holes drilled in the compartment backplane and the connection of those pins to the backplane traces is via the jumper blocks I mentioned previously.
The path from sense/inhibit wires for a particular bit (core plane) begins on the A or C side of the core stack. Each core plane has four sense/inhibit wires, each handling 2K of the 8K bits on that plane. The core plane is divided vertically, with the B side of the plane consisting of addresses 0 to 4095 and the side near D consisting of addresses 4096 to 8191.
Two sense wires further divide each half of the core plane, so we can think of it has having four quarters divided vertically. Each pair of sense wires are tied together at one end, called the common end and the other side of each wire is brought out separately.
Each 4K half of the core plane has three terminals, the two sense wires plus the common wire between them, which are soldered to a trio of wires tightly wound as they are routed away from the core plane. The wires are blue, white and black with black used for the common connection.
In the closeup above from side A, most of the contacts you see are the ends of the X addressing wires which are daisy chained from one core plane to the next, so that each X wire runs through all 18 core planes. The blue and white wires are soldered to the two ends of the sense/inhibit lines for the B side of the core plane and the black wire going to the common terminal of those lines which is beyond the right edge of this picture. The red oval shows the connections for the core plane of bit 7.
The diagram above corresponds to the picture of the wires to bit 7 but the diagram is rotated 180 degrees from the perspective we see looking at the core plane from the front. The bottom of this diagram is the bottom board of the stack, which is furthest away from our view in the pictures. The ovals show where the white and blue wires are soldered in the picture above this.
The other end of the white, blue and black trio of wires runs to a connector on the left side (from our perspective). The other red oval indicates where the three wires are soldered. These numbers positions are on the bottom board and connect via the S clip to a printed trace.
This diagram shows where those connections fit onto the bottom board. The blue oval shows the connectors where the wires are soldered, and the picture below shows our wires on the connector as viewed from my perspective working on the core stack.
That connector has the S clips behind it, as is a bit more visible in the picture below:
We don't know the exact route of the trace from the wire but it ends up on the pins of the bottom board where they will fit through at card slot F4 of the backplane. This is where the jumper connects those bottom board pins to the backplane pins at F4. From there, traces on the backplane route the signals to card slot B3 where the sense/inhibit circuit for bit 7 is implemented.
We found that the pin D13 at F4 card slot, from the bottom board of the core stack, is not connected to the sense/inhibit wire on the A side as shown in the first photo. The question was where the break occurred and then how best to rectify it.
LIKELY CONTINUITY BREAK LOCATION IS ON THE BOTTOM BOARD OF STACK
I used the ohmmeter to check continuity between the blue and white wires where the connect to core plane 7 and the other ends where they are soldered to the bottom board connector. I had a solid low resistance connection, and further more when I touched the S clip I had good continuity to that.
Therefore, I presume that the break is somewhere from the S-clip to the pin D13 for F4 on the bottom board of the stack. It might be the trace on the bottom board, it might be a cracked connection at the pin, or it might be a failed connection with the S-clip to the pad of the bottom board.
WAYS TO CORRECT THIS BREAK IN CONTINUITY
The core stack is designed with a nice impedance matched connection from the sense/inhibit wire ends all the way to the SLT card at B3. I don't want to throw that off by routing a single wire to bridge around the defect, unless it is done with full regard for the impedance of the circuit.
The straightforward way to repair this is to remove the core memory stack from the memory compartment. This is done by pulling all the jumper blocks off the backplane, then removing four screws that hold the core stack to the backplane. I would then lift it out extremely carefully and move it to the workbench. There I could find the connectivity fault and make the most organic repair.
However, this poses some practical problems. There is quite a bit of force on the jumper connections. I was able to remove two of the jumper blocks using a chip puller but the side by side placement of the remaining jumper blocks won't allow the use of that tool.
IBM mentions a special tool that the engineer should use to pull these. Of course I don't have such a tool, but might have to manufacture one. It would need to fit over the top and bottom of the blocks, not need too much of a gap since they are still spaced closely in the vertical direction, but impossibly close horizontally.
The other possibility is to route a similar tightly wound triple wire cable from the top (side A) of the core stack and somehow get the signal around to the backplane. There is an unused connector slot T2 at the top of the backplane, thus I have a choice of 24 pins to use. I would have to choose pins that have NO connection to any other part of the backplane, then use wirewrap to bring the signals to card slot B3 which is not far away.
The new wire from the side A connections would have to run up between SLT cards probably in column E to reach the back of the connector 2 where I would terminate them.
I have to mull over the relative strengths and weaknesses of the two repair methods, then execute.
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