Ken Shirriff's Reverse Engineering Skills Produce a Schematic for the Apollo Premodulation Processor

Noted engineer Ken Shirriff has turned his reverse engineering skills on another piece of retro-technology from the Apollo space missions, this time concentrating on communications with the FM radio's premodulation processor.

"Many systems worked together for communication," Shirriff explains of the Apollo's radio system, "but I'm focusing on a single module: The voice detector inside the premodulation processor that performed the FM demodulation. "The premodulation processor (PMP) weighs 14.5 pounds and measures 4.7×6×10.5". It used 8.5W of power, supplied at 28V DC from the spacecraft's hydrogen/oxygen fuel cells or silver-oxide zinc batteries. The PMP was mounted in the Command Module's equipment bay, along with most of the electronic equipment."

With an actual pre-modulation processor in-hand, and aided by block diagrams from NASA's archives, Shirriff set about examining the technology. Inside the robust housing is nothing so simple as a modern printed circuit board with easy-to-follow traces. Instead, disassembly unveiled components and wiring soldered to metal pegs in something resembling a plate of spaghetti.

"Since the components and wiring are visible, it seemed like these modules should be easy to reverse engineer, but it's trickier than it seems," Shirriff explains. "The components are liberally covered in what looks like hot glue but is probably silicone. (I suspect that this was only used in equipment for ground testing, while modules for spaceflight were fully encapsulated to prevent short circuits.)"

While admitting that the result of the reverse engineering may thus have one or two errors, Shirriff was nevertheless able to ascertain the operation of the device — which implements a pulse-averaging discriminator to separate and extract the voice and data signals from the FM carrier. By the end of the process, Shirriff was able to match NASA's block diagram to the physical components and produce a schematic.

This isn't the first time Shirriff has investigated Apollo technology: Two years ago he was working on the launch vehicle computer from the Saturn V rocket, and last year looked at a computer buffer unit built using unusual diode-transistor logic (DTL) technology.

Shirriff's full-write up, and schematic, can be found on his website now; an earlier post offers an overview of the premodulation processor itself.