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.
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...
ReplyDeleteThat'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.
DeleteRap them on a solid table top a couple of times, and they started working.
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.
ReplyDeleteIt's been years, but in the old days, they used to keep professional carbon mics in a heated "oven" overnight, not hot, just warm, to keep them from getting damp and "packing."
ReplyDeleteWhile a good, sharp rap was the old fix, and I wouldn't have much qualm about doing that with a telephone mic even now, the old pro mics could be Zamak, a cast zinc alloy that degrades over time and it could crumble. Even old brass and aluminum should be treated with respect. Don't be rough with them, you can't get a replacement.