[Boatanchors] Carbon Mic Rejuvination

[Richard Knoppow]

----- Original Message ----- 
From: "mac" <w7qho at aol.com>
To: "milsurplus QSLNet" <milsurplus at mailman.qth.net>;
<grc-9 at yahoogroups.com>; <boatanchors at mailman.qth.net>
Sent: Friday, March 15, 2013 2:02 PM
Subject: [Boatanchors] Carbon Mic Rejuvination


> Heard a rumor about rejuvenating old carbon microphones
> with AC  published somewhere (ER maybe?).  Anybody know
> more?  My experience  with the old military mics (T17 &
> the like) is that they either play  or they don't, banging
> on the table , etc., never worked for me  anyway.  Damn
> things can surprise you though, just picked up a pair
> labeled "Microphone T-17  4857-CHI-42.  Rattiest looking
> units I've  ever seen and both work great!
>
> Dennis D.  W7QHO
> Glendale, CA
>
    Two things happen to carbon mics: first is the
absorption of moisture, the second is burning of the carbon
crystals.  The first can sometimes be cured by baking out
the microphone.  Use a relatively low temperature to avoid
damage to other parts of the mic.  I think up to maybe 150 F
is safe. It might take a couple of days to bake out.  If the
crystals are burned from excessive current about all you can
do is to replace them.  When telephone microphone capsules
were plentiful you could get enough crystals out of one to
fix most single button carbon mics.  Depending on the mic
opening up the carbon chamber may be simple or nearly
impossible. Some are effectively sealed and I don't know any
trick for opening them without damage.  Many old
double-button broadcast mics are simple to open. The Western
Electric types use a paper "book" to seal the edges of the
carbon chambers around the diaphragm and to damp it
somewhat.  The condition of the paper is important but I
don't know of a replacement.
     Some hand held military carbon mics use capsules very
similar to those in telephone handsets so that sometimes the
whole capsule can be replaced.
     I have one carbon button mic made I think by
Telephonics that works perfectly and another Shure made one
that works but is very noisy. The noise may be from burned
crystals.  I have not tried opening it.
     FWIW, a properly working double-button carbon mic can
have surprisingly good music reproduction.  A friend, now
SK, who was an audio engineer at NBC once used an old WECO
carbon mic on the Tonight Show orchestra during rehearsals.
The mic hisses a bit but the overall sound is of a good
condenser mic.  One should not be surprised since it uses a
very similar stretched diaphragm and has a good diaphragm
damping means.
    Probably the most carefully designed carbon mic is the
one used in the series 500 telephone set.
    Another afterthought, carbon mics were used in some 
early broadcasting and public address but NEVER for sound 
recording.  The early sound recording mics were all 
condenser types.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com