Repairing an Astatic D-104 microphone

I got this mic at a local hamfest with the intention of putting it on the Central Electronics 10B. The seller said the mic was good but that the amplifier in the base was “disabled.” This is perfect for my use, because I’d rather just use a straight Hi-Z element and use the preamp inside the 10B. Of course, we don’t know what we’re getting into until until we open it up and find out what “disabled” really entails….

Under the base as I bought it.

I don’t know what the history was, but I think this one tried to be a “D 10-4, good buddy” at one point in its life. It looks like some homemade solder connections, and in the condition I got it, the mic element was grounded no matter the state of the PTT switch! The PTT line (Red on the mic cable) was opened when you squeezed the handle. Then there was a mysterious green wire that someone had cut. I’m guessing someone tried to somehow make the T-UG8 “better”…nothing like 120% modulation to be loud on channel 19, eh? Maybe someone tried to bypass the amp but didn’t quite get it right. I looked for something close in the pile of schematics, but couldn’t quite figure it out. Oh well.

Seeing that the base was a bit of a basketcase, I thought it wise to check the mic to make sure I wasn’t wasting my time. (I see this seller regularly around here, so I wasn’t worried about it–mistakes happen and memories fail when we trade in old stuff.) With the high-tech test jig below there was plenty of buzz on the audio, but my voice came through the receiver loud and clear, so at least the mic element itself was working well enough. This meant it was going to be a matter of tracing wires and putting Humpty-Dumpty back together in a recognizable form both for my sanity and that of posterity.

Quick and dirty check to see if the mic would work

Since I was going to rip out the amplifier anyway, I just unscrewed everything and took off all the wires and tried to figure out what was connected to what.

The version I’m aiming for. (From Ted’s D-104 page

The only way to figure out what was what was to the remove the terminal strip from the chassis and start buzzing it out with a meter to see which colors are on which switches and mic connections. Inside there is a DPDT switch and the colors mostly matched the schematic above, but the “blue” wire was really more of a “gray” and some of the other colors had faded, but it wasn’t too hard to figure out. A couple wires were not connected to the “proper” locations on the terminal strip. Go figure. I found that pins 1 and 2 are connected up in the stem like the T-UG8 stand, so it’s likely that is how this mic was originally manufactured. Perhaps the amplifier died and someone flailed around trying to bypass it and didn’t quite figure out the logic puzzle. No matter; what’s done is done. In the end it behaves just like the UG8 stand.

Back to unamplified simplicity in a recreated UG-8 stand.

I then attached a new (old) shielded Hi-Z cable that I found at the same hamfest two tables down. The paint job on the base is a little rough, but I’m not going to worry about that right now.

Finished mic with the new cable.
With the Central Electronics 10B.

With the new cable there wasn’t any indication of buzz in the audio. My 10B has the QT-1 VOX option, so PTT isn’t totally necessary, but given that I’m only going to be using this in occasional long-winded quasi-monologues on a antique radio net I’m just fine with solder-to-talk turning the knob when I want to transmit. I may choose to put a filter/capacitor on the output to roll off the highs. I understand that’s a good idea with these phasing rigs since the sideband suppression is not that great > 3000Hz or so, but we’ll see what the spectrum shows before I optimize something like that. We do want to be a good neighbors…

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