Sunday, January 31, 2016

Astatic D-104 Lollipop Mic Modernization

Objective was to procure an all-vintage D-104 and make it appear to my modern solid-state rig as a modern stock active mic. My first desktop mic was a non-amplified D104 variant that I bought with a 23-channel CB from a friend, and I always loved the design. 

A lot of learned souls have written about these mics and I found a bevy of sage advice, special shout out to K3DAV. 

They boast a clear, strong audio, they just plain look cool, and are an icon of American radio culture.

The restoration was spread out over a year thanks to my own indecision at times and waiting for inspiration to hit...
  • The one I found was made to order for about 40 bucks on eBay. Chrome had some pitting but not going for a looker, this is about feel. This had been sitting for years, all original wiring and parts. We can rebuild it. We have the technology. Thankfully it doesn't cost six million dollars.
  • First order of bitness is to inspect and clean up,
    • Disassemble the barrel section and Deoxit the switch and mic plug contacts
    • Remove bottom plate an Deoxit the trimmer pot, Elec/Relay switch, etc
    • Exterior - tried some steel wool on the chrome and it took the pits down to spots looks some better, good enough for my shack.
  • The original crystal mic elements are known to not age well due to the formulation used back then, but replacements are available for about ten bucks. The one I got is physically smaller than the original, but thanks to some spare foam and speaker grille cloth it fits just fine up there in the ol' crow's nest.
  • To make it plug-in ready to a ham rig,
    • Replaced the cord - the original is high-quality but for a few bucks it was worth while
    • Get an 8-pin plug
    • Tap into the DC power supplied by the rig - for Icom this is 8V which is perfect for replacing the 9V battery, after all, what is that 8V there for?
    • Add a 33k resistor in series with the audio line going to transceiver, to bring it down from tube-drive to levels consistent with solid state equipment.
  • At this point we tested it and found some noise...
    • Part of it was crackling in the 5k trimmer pot. Took it apart to find a worn spot in the paint, right where it needed to be set. Found it easiest to just swap around the end connections so the sweep would rest on solid carbon.
    • After the trimmer was back in play, found it still had low audio, bad fidelity, and a popping sound. Basically it was like someone talking from the other room while someone near by was drumming on an empty beer can. I think I cleaned the switch contacts once more and took some emery to them to make sure, but that didn't clear it up.
  • Finally decided the amp board had to be bad and to rebuild it -
    • Subbing a 2N3904 for the original 2SC945, which appears to be a legacy part these days
    • Wired on plain breadboard, since that's what I had. Some have made very nice etched boards for this but the redneck way will do just fine, if...
    • ...there's a way to mount the board and provide a chassis ground, as the original board does with lugs. At some point realized some 12-guage solid wire would do the trick, just need to drill holes in the board and bend it properly, and avoid cracking the board in the process. The solid wire is in a U-shape with the ends looped for mounting stubbs and the crossbar serving as a ground strip for the circuit.
    • Somehow this all came together, and once the new board was in place we are in bitness.

Should note, I don't do a lot of voice mode, mostly digital, but I've gotten some great reports with this mic, even on FM, which doesn't lie. Seems I no longer have to repeat information as often and sure is nice not having to worry about the battery.

Preamp Circuit:
  • Note the wire colors are for my particular base, which should be fairly universal but at least useful for reference.
  • The 5k variable is the trimmer that's accessible from the bottom. Generally this is just set once then left alone.

Base schematic sticker for this TUG8 stand:

73 de N8WWY

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