Making the repair on the core stack - part 1

REALIZED EXISTING TWISTED WIRES WERE LONG ENOUGH

A very fortunate coincidence is the two failed connections are on the jumper block connectors closest to the twisted wires hooked to Sense/Inhibit lines 7 for low 4K and 9 for high 4K. That meant that I could simply snip the wires from the S-clip connector. That connector is where the signals should have been routed across the bottom board to the pins. 

Blocks with bad connections are nearest top

The wires can be brought to the base of the pins directly, since they are just long enough, bypassing the S-clips and bottom board entirely. This eliminates the need to use a substitute twisted triplet of wire as I preserve the characteristics such as impedance of the original IBM wiring. 

Bit 9 high 4K clips are leftmost

Bit 7 lower 4K S-clips are third set of wires from right

The S-clips on those connectors fit over the bottom PCB board and make contact with a pad, which in these pictures would be on the top of the board. They seemed secure but there was no connectivity from them to the jumper block pins. 

Bit 7 wires can be reached and detached

Bit 9 wires can be detached after plastic coating is removed

VERIFIED ALL WIRES AND DIODES FOR X AND Y ADDRESSING

Before I invested time in a repair of the two sense connections, I had to verify that all the other wiring in the core was in good shape. In particular, I wanted to test the continuity of each X and Y addressing wire through the core stack - 192 of them - as well as the array of diodes that are part of the path. 

X and Y wires are able to be accessed by a smaller number of driving circuit through the use of diodes. 

X address wiring scheme

There are eight Read Gate/Write Drive connections as seen on the lower right, plus eight Read Drive connections to the lower diodes at the upper left and another eight Write Gate connections to the upper diodes at the upper left. Thus 24 wires allow selection of current through 64 X wires and the current can flow in two different directions for reading and writing. 

To test all 64 X address wires I hook the diode tester to each of the eight connections on the lower right, while hooking the other lead to eight Read Drive and to eight Write Gate connections on the upper left. A good test shows continuity with a diode drop in the appropriate direction and no flow in the opposite direction. 

This meant I did sixteen checks for each of the eight lower right connections, confirming 64 wires conducting in one direction and the same 64 wires conducting in the other direction. The Y checks were similar except that the lower right equivalent has 16 connections rather than 8, since the Y axis supports 128 wires. A bit more work than confirming the X axis but I was satisfied that all 128 Y wires and all 64 X wires worked and that the steering diodes that supported dual direction currents were all working properly. 

NOTCHING OUT ROOM FOR THE WIRE

The plastic block that has the pins embedded fits directly against the rear of the backplane when the core memory is attached. The backplane is a glass fiber that is brittle enough that it might crack if subjected to bending or other stress. It is mounted in a plastic stiffener before it is installed into a computer frame. 

Closeup of the glass backplane in its stiffener, with the holes for core visible

If I had soldered (or done wire-wrap) on the pins as they were, the wire and solder would stick up above the surface of the plastic block. Those would be stresses applied at small points, therefore potentially cracking the board as I tightened down the core stack mounting screws. 

To protect against that, I notched away plastic material to let the wires and any solder sit down below the plane of the plastic block, removing the high points that could cause damage. I would then solder the wires to the side of the gold plated pins, down in the recess I notched out. 

SOLDERING AND TESTING FIRST REPAIRED SET - BIT 9, UPPER 4K

I cut the wires for bit 9 from the S-clip connector and tested their fit to the H4 jumper block pins. 

Wire ends near H4 block

I completed the notching with the help of my stereo microscope, continuing to remove material until it was deep enough for the wire to fit below the surface of the block.

Beginning the notching

I then stripped the ends of the wires and placed them against the side of the pin barrel in the dug out area. They were easier than I expected to solder. I used the ohmmeter to verify the connectivity to the sense/inhibit wires and the proper resistance readings at the jumper block pins after my repair. 

Wiring complete for bit 9

NOTCHED AND ROUTED WIRES FOR SECOND SET, TO BE FINISHED TOMORROW

The wires for bit 7 must be connected to pins B12, B13 and D13, the pins on the F4 block furthest from the edge. The bit 9 wires connected to the nearest pins, thus the wire length was less critical.

After disconnecting the wires from their S-clip connector, I tested whether they could reach the pins with enough margin to make a safe repair. As you can see from the pictures below, these are long enough. 

I did the notching, so all that remains to do tomorrow is to strip the wires and solder them on. Testing will be done afterwards to ensure that the sense/inhibit wires are now properly connected with low resistance.