We powered up the block that converts a temperature reading to a gear rotation but nothing happened. So I'm reverse-engineering the circuitry to figure out what's gone wrong. Maybe an old capacitor since some of the old capacitors have fungus or corrosion or something.
The servo loops use magnetic amplifiers. Mag amps look like transformers, but they are inductors that can be saturated to pass current. I wrote an article on mag amps in the IEEE Spectrum a while ago: https://spectrum.ieee.org/the-vacuum-tubes-forgotten-rival