Can You Hear Me Now? Lunar Edition

Despite what it looks like in the movies, it is hard to communicate with astronauts from Earth. There are delays, and space vehicles don’t usually have a lot of excess power. Plus everything is moving and Doppler shifting and Faraday rotating. Even today, it is tricky. But how did Apollo manage to send back TV, telemetry, and voice back in 1969? [Ken Shirriff] and friends tell us part of the story in a recent post where he looks at the Apollo premodulation processor.

Things like weight and volume are always at a premium in a spacecraft, as is power. When you look at pictures of this solid box that weighs over 14 pounds, you’ll be amazed at how much is crammed into a relatively tiny spot. Remember, if this box was flying in 1969 it had to be built much earlier so there’s no way to expect dense ICs and modern packaging. There’s not even a printed circuit board. The components are attached to metal pegs in a point-to-point fashion. The whole thing lived near the bottom of the Command Module’s lower equipment bay.

The processor, or PMP,  played a key role in multiplexing different streams in different configurations and passing them to (and from) the onboard S-band transmitter. Inside the box, [Ken] found four subassemblies nicely labeled and connected to a thin backplane. Along with discrete components, the modules also employed off-the-shelf assemblies that predated ICs and offered functions like filters or oscillators in one convenient package.

One thing that further complicated the design was the need for redundancy. For example, there are two switching regulators inside — yep, a switching regulator in a piece of gear from the 60’s — and the crew could select between the two power supplies.

[Ken] takes us through each module. The voice and data detector module extracted voice on a 30 kHz FM subcarrier. There’s also a bi-phase modulator, voice clipping, and a relay module to pass signals from the lunar module back to Earth.

If you want a closer look at the Apollo comm system, [CuriousMarc] has a series about it and was part of the group and stripped down this PMP. Radio signals are fun, of course, but the best footage came back as film. However, modern technology has sharpened up some of that old footage.

9 thoughts on “Can You Hear Me Now? Lunar Edition

    1. Point-to-point construction is something I can’t quite believe even when I see it in real life. In this case, the whole assembly seems to be made out of conductive material, like one great short circuit, which doesn’t make it any easier to accept as “how they actually did things back then, even at NASA”. I suppose it’s the liberally applied snot that makes it work, somehow.

      The PMP also looks remarkably fresh for something that’s at least 50 years old.

        1. Those are super neat, but the relative simplicity and spaciousness of the designs make it easier for me to accept them as real working devices built by skilled people. It’s the sheer messiness (to my eyes) and crowdedness of classic point-to-point work that astounds me.

      1. Circuit boards at the time were for mass production. You didn’t need many if these.

        It looks like a hole is drilled and a terminal inserted. They look like things I knew, metal terminals in teflon insulation. Fifty years ago I was getting scrap from someone working at RCA, and it was built with these. The terminals came in multiple forms, it almost seemed like competing systems but was probably to accomodate different needs. Thru terminals so the conducting came out the other side, others just a terminal on one side.

        The scrap was a mix, prototypes but also more finished items. I pulled the parts off and reuse the terminals, until I started making ircuit boards, and also using circuit board as a ground service, the parts mounted above it.

  1. Flopsy said ” In this case, the whole assembly seems to be made out of conductive material,”
    The board is conductive but the components are soldered onto pins which are inserted into an insulating ring. you can see them if you zoom on the picture. The snot is here to protect the components from vibrations.

    1. Thanks for pointing out the rings to me. I suspected that something like that was keeping the paths separate. Still seems like a lot of tight places for things to go horribly wrong in, but I guess that applies to electronics in general.

      1. Pretty amazing they managed to cram so much functionality into something so “simple”.
        Just think of how complex even an early IC is, let alone the latest devices!

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