Tuesday, February 12, 2019

Preparing to add text decals and glue up the DSKY display, still working on keyboard


We were given access by the Computer History Museum to the Apollo Guidance Computer and DSKY that they have on public display. The computer is the first block II built, used at MIT to check out each change they made to hardware. The DSKY appeared to be an early block II DSKY, since the indicator panel had three lights (AUTO, HOLD and FREE) that were removed before the versions that were placed in all the spacecraft.

The plan for the DSKY was to record the serial numbers from the plate on the back, which was not accessible while the DSKY was mounted on the wall at the museum. The number matches a slightly later DSKY design, which should not have had the three removed indications I discussed above. Mike will be investigating all the paperwork to try to understand how this hybrid version came about.

I had the chance to work the keyboard, validating the amount of movement and the resistance of the keys when pushed. The actual DSKY feels indistinguishable from the keyboard action I achieved in my substitute, which made me happy.

Seeing the DSKY reinforced how deep is the real item. The enclosure I am fitting into is less than a third as deep. In some ways, that is not a problem because I can use higher density PCBs and components instead of the banks of crystal can sized relays in the real item. However, my interior is pretty crowded anyhow.

We removed the erasable memory module (2K words of traditional core memory) to test it electrically with a modern resistance meter (low applied voltage and current). We were able to confirm that all the addressing, inhibit and sense lines behaved as expected, with all the diodes operating properly in one direction.

That does confirm that our module from the AGC we are restoring does have an open inhibit line for bit 16. It is a final proof for what was already quite overwhelming evidence. There is a note on the memory module from the museum's AGC that indicates that bit 13 of a specific word address drops bits.

Thus, even if this otherwise perfect memory module were to be inserted into the AGC, we would have to carefully check all the software we want to run (e.g. Luminary) to understand the impact of bit 13 of that word becoming zero. At this time there is no expectation that we would be able to plug in the module, but we will do the advance research just to be prepared.

We then sat down to plan out means to test the memory drive electronics (current drivers, switches and sense amp modules) before we insert our memory module into the tray. We will design and build a PCB, populate it with some of the recreated connectors that are being made for us by a sponsor, and use it as a testing bed.



Marc and I worked out a method to glue cardboard shims into the honeycomb after the plungers are inserted in each cell. This lessens wobble and erratic movement of the key as it is moved up and down by user presses. He did a test on one key by hand cutting and maneuvering cardboard pieces, but to complete the other 18 keys would be too slow and labor intensive.

Thus, I designed the cardboard pieces to be laser-cut at Ponoko.com as well as refining my cardboard masks for the top of the displays. The parts arrived on Tuesday Feb 12th and I did a test fit. Now to do the careful gluing for all 19 keys. .

When the honeycomb is completed, with its shims, I will need to etch out some space on the bottom edge of the honeycomb to clear small features on the surface of the PCB, before I install the coil springs and join the board to the honeycomb assembly.


I refined my ideas for the light masks that sit under the acrylic panels on the DSKY. The new designs were shipped off to be laser-cut; these also arrived on Tuesday. After I satisfied myself with how everything fits together and looks while lit up, I can start gluing up the displays.

For the EL panel side, that involves gluing down the electroluminescent wires, gluing the sign digit light dam, gluing on the diffuser sheet, gluing down the main light dam, then gluing on the light mask at the top.

Quick and dirty test of stack, but still have mylar protective fil which is blurring the output slightly.
I had frosted the back of my acrylic panes, which I tested quickly on the board to see how the light dams, masks and frosting worked out. I was satisfied with the results - they are not perfect but good enough for me to complete the EL panel side. The indicator side is not sufficiently diffused to hide the two discrete LEDs that light up the space. While this is not great, I will proceed anyway.

Illumination behind the PROG indicator space - still have mylar film over acrylic panel

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