Complications during DSKY module testing and power up

INCORRECT SCHEMATICS LED ME ASTRAY

I had been working from a manual produced by AC Electronics, the prime contractor who managed Raytheon in producing the Apollo Guidance Computer and its Display/Keyboard (DSKY). It was ND-1021041, Project Apollo Command Module Guidance and Navigation System Manual, Volume II. On page 4-508, figure 4-233, it shows the connections for relays K24 and K30 which drive the four segments that light up the Plus and Minus sign in front of a five digit register.

Erroneous drawing from the AC manual

When I did some continuity testing with the relays both set and unset, I had several puzzling failures that seemed to require multiple failures inside the module, where the other 40 relays were working flawlessly. Fortunately, I had Mike Stewart to thank for steering me towards more accurate schematics of the relay module, drawing 1006161 which gave a more concise (and accurate) view of the module.

From proper NASA drawing

To interpret the diagram above, you need to match the legend on the right to the blocks on the left for the two relays and draw out the contacts. I did this by modifying the incorrect AC drawing to illustrate the difference.

Modified AC drawing to reflect reality

I realized that the two relays have their contact 7 tied together, thus when both are unset a path exists between 6 and 96, although not shown on the NASA document. When unset, contacts 4 and 7 of each relay are closed, thus connecting 6 and 96 together.  Otherwise, continuity was verified as shown on the NASA drawing.

In practice in a DSKY, only pins 56, 13 and 91 are used to control the sign digit. The other pins exist for testing purposes only, I believe.

POWER SUPPLY DRIVE SIGNAL DETERMINATION

The power supply is hooked to +28V and +14V supplies and is furnished an 800 Hz clock signal from which it generates the 275V 800 Hz AC output. This is not a sine wave, instead a digital square wave input. The two DC levels are easy to produce, but I needed to understand the specs for the clock particularly voltage or current levels.

Fortunately I know the Apollo Guidance Computer side, which generates this signal. It uses a open collector circuit that pulls down the 14V from the input transformer to ground as a 800 Hz square wave.  I had built a substitute for that AGC circuit as part of our interface to the computer, thus I can conveniently use a 5V TTL square wave source at 800 Hz into my interface and wire the output directly to the DSKY power supply.

One other aspect of the circuit needed analysis. The spacecraft has a potentiometer that acts as a dimmer for the DSKY illumination. It is wired across the input transformer of the power supply, shunting some of the power from the AGC clock pulse through the pot to lessen the flow inside the transformer. This serves to make the output change.

I don't know what value the potentiometer has, but looked it up in the Interface Control Documents that were defined for how the Command Module and AGC/DSKY were connected. This was a 10K linear taper potentiometer.

I did realize that at one end of the travel, the pot will totally bypass the input transformer causing the segments to be extinguished and at the other end it will set the maximum brightness based on the 10K resistance of the device.