Saturday, January 12, 2019

Assembling keyboard for DSKY substitute, resolving issues, and more checking on the memory module


Assembling plungers, springs and testing

After the batch of 19 printed plungers arrived, I put them all together and screwed together the switch stack. The plungers ride up and down in slots on the sidewalls of the honeycomb piece, giving them 1/4" travel. Coil springs sit inside the plunger and extend to the top of pushbuttons that are mounted on the printed circuit board. The honeycomb with its plungers and springs is screwed to the PCB and standoff legs to fit the stack at the proper height inside the DSKY enclosure.

The plungers had to be snapped into the honeycomb, which is a difficult procedure that is right on the edge of impossible, involving inserting the plungers at an angle and pressuring the sidewalls to allow the slider tang on the plunger to slip into a sidewall slot.

I did a test fit with the stack in the enclosure, the faceplate installed and keycaps placed on top of the plungers. It looks and works fairly well, but some issues still need to be addressed to finalize operation. These are excessive sidewall clearance and control of the spring position inside the stack.

Test fit, loose keycaps on switch stack
The clearance between the plungers and the honeycomb sidewalls is a bit too large, allowing the plunger to wobble as it rides up and down. This was essential to allow the plungers to be brute force installed in the honeycomb, but affects the operation of the plunger.

To fix this, I will be adding shims inside the gap between sidewall and plunger. About two thicknesses of construction paper in each gap led to excellent stability of the plunger as it moves up and down.

I need to work out a process that keeps adhesive only on one side of the shim while I insert it and press it sideways during the set time. I will experiment until I am confident in the method.

The coil springs are only about 1/4" diameter, considerably less than the rectangular solid cutout inside the plunger. This allows the spring to migrate to the sides of the cutout, thereby cocking the face of the plunger. In some cases, the spring can pop loose and rotate sideways so that it is jamming the operation.
Recess milled to hold coil spring in position
To resolve this, I am making two modifications to the stack. Marc milled circular depressions in the underside of the plunger to act as a cup, holding one end of the spring so that it can't shift from the center of the plunger to the sides. I will cut very short sections of a plastic rod and glue them to the top of the pushbottom, keeping the bottom of the coil spring from shifting off the face of the button.

Excessive sidewall gaps and spring shifting are visible here
Catastrophe and recovery

During the process of milling the cup inside the plungers, a freak accident occurred. The honeycomb fell off a table and partially shattered on the floor. The hard resin used to print the honeycomb is a bit brittle. It is not too bad, since I can bend it to force plungers into place, but it is brittle enough when dropped four feet to a hard lab floor.

This is a setback, since the nature of the broken areas of the honeycomb make repair infeasible. I need to order a new part, which adds weeks of delay and burns a bit more money. The part was immediately ordered and with luck I will have it before the end of the month.


We scheduled a visit to the Computer History Museum where we were given access to two artifacts that were listed in the records as a memory module and a cover. These were made available in the research facility, where we examined them with the hope of testing continuity of the pins as a cross check against the results with our defective module.

Alas, another small setback. The cover was indeed the proper part that fits onto the memory module, but the module itself was a partial assembly. It had the RCA built core stack with its foam potting, but the diode plate was not installed on the top so that this was not complete enough to test electrically.

More seriously, it was a block I memory module, not the block II type that we have. Block I was only 1K words in capacity and had a different configuration of pins across the base. The pins on the bottom would not have mated with a block II AGC even if the module were complete and working.

Since it was a different and incomplete module, we couldn't do any useful tests for comparison with our block II module. However, we are requesting access to the module that is on the wall, on display, in the main museum facility. We will have to remove it from the AGC and test it while the exhibition is closed to the public, but that is merely an inconvenience not a serious roadblock.

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