Monday, June 1, 2015


     I went through most of my collection of carbon mics tonight.  The old ones are nearly always non-working when found: they pick up humidity, the carbon packs solid, and that's it; or the carbon granules dribble out over time. 

     Not all of them.  I found several that still work, and a couple that I'd thought were dead (a Shure Brothers 3-A and a 3-B) aren't.  Here are the working ones:
      Two are telephone-types, the T-32 (a pretty common desk mic around WW II) and an odd old telco one from the wood-wall and candlestick era next to it.  The Universal X1 in its nice stand was a complete surprise; I assumed from condition it was a goner.  Nope!  And the little Philmore lapel mic next to it had been sealed up.  The Stancor 10-P -- which needs a carbon mic to do AM -- is behind them.

     Here's my test set-up.  The mic cable goes to a little mixer.  It's isolated with 0.68 uF series condensers, looking at current across a 150 Ohm resistor in series with the mic, a 1.5 V battery and an added resistor to limit current -- I used 820 Ohms and it worked okay for checking.  In practice, you trim microphone current for best fidelity with a carbon -- usually the lower, the better, but there's a point where it stops working.
     Mic cable to mixer at left, connections to carbon mic under test at top right, battery terminals and added current-limiting resistor at bottom right.  I think you can trace it from the photo.


  1. Wasn't the old trick with a poorly-working carbon mike to take the element and slam it onto the table/desk to loosen up the granules? Seems like we did that with the mike in an old telephone handpiece. IIRC, we smacked it with a fair amount of force, not just a little tap. Of course, that's been a long time ago, I could be mis-remembering...

    1. That's pretty much what I was taught early in my Ham carreer when these were still avalable from the surplus places for like $2.50.

      Rap them on a solid table top a couple of times, and they started working.

  2. How about the standard fix for carbon comp resistors that drift high in resistance? Bake them out. I don't recall times/temperature, but it wasn't bad. Rough guess, 150 to 180 degrees overnight. It's below boiling so that they don't pop.