Tuesday, June 25, 2019

Another diode bites the dust in the AGC Current Switch Module

Setback with erasable memory, failure in Current Switch module

Near the end of our time with the AGC, we were running various portions of the Apollo missions such as LM powered descent initiation, LM rendezvous calculation to rejoin CM, or CM burn calculation to hit the earth entry interface.

Suddenly, we were seeing hardware restarts and the software was not running correctly. Using Mike's test monitor we quickly realized that erasable memory was not working properly. We could rapidly trace this down to a failure in the Current Switch module (again).

Testing diodes after powering down the computer led us to a third diode that failed open, just as the first two had been. We didn't have enough time left to mill out new windows in the polyurethane foam encapsulation, drill out the bad diode and insert a replacement.

This module has 28 such diodes - three have already failed and NASA documents show that Raytheon switched from the original diodes to new types due to failures similar to what we experienced. The original diodes had a mesa construction, something the semiconductor industry quickly moved beyond.

We kept the Current Switch Module here so we could remove all the encapsulation on the module, then replace the 26 original diodes with modern 1N914B diodes like we did with the first two we found. This should be completed so that we can rejoin the module with the AGC for live demonstrations we will do in July.

Back to Houston until our next session

The AGC flew back to its home in Houston, until our next session and subsequent demonstrations.

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