Apollo Spacecraft S Band Communication System, Part 17—Inside the Pre-Modulation Processor

With the microwave uplink and downlink restored and tested in previous episodes, now it’s time, with the aid of assiduous and generous collectors of Apollo gear, to examine how the signals were composed for transmission. The spacecraft Pre-Modulation Processor (PMP), designed and built by Collins Radio, processes voice and audio data from the crew and spacecraft and encodes them for transmission on the downlink to Earth. Just getting inside this box, sealed for decades, is an adventure.

Ken Shirriff disassembled the PMP into its component modules and posted a detailed analysis of them on his blog, “Talking with the Moon: Inside Apollo’s premodulation processor”. To the modern eye, the gear looks almost alien in construction: there isn’t a printed circuit board in sight, with components mostly spot-welded (but some soldered) to metal pegs with components and wiring on both sides of each panel.