[Boatanchors] Carbon Mic Rejuvination

[Richard Knoppow]

----- Original Message ----- 
From: "Jim" <jbrannig at verizon.net>
To: "Charlie , W5COV" <cvest at cox.net>; "milsurplus QSLNet" 
<milsurplus at mailman.qth.net>; <grc-9 at yahoogroups.com>; 
<boatanchors at mailman.qth.net>
Sent: Saturday, March 16, 2013 5:20 AM
Subject: Re: [Boatanchors] Carbon Mic Rejuvination


>I recall a telephone repairman WHACKING a handset on a desk 
>to loosen the carbon granules in the microphone.
>
> DON'T try this on a modern telephone!
>
> Jim

     Many carbon elements are subject to "packing" of the 
carbon granules, when banged or shaken hard the granules are 
separated again.  The vulnerability to packing depends on 
the mechanical design of the carbon chamber and to the 
condition of the carbon. Many mics are also position 
sensitive.  The Western Electric T-1, used in the 500 series 
phones, was designed to avoid both problems.  I don't know 
what WECO did to process the carbon to eliminate packing the 
carbon chamber is designed so that the overall contact of 
carbon to metal remains fairly constant with position.  The 
old carbon mics used in early broadcasting were very 
temperamental and often packed.  Packing can also come from 
excessive current.  In general, the current through the 
carbon should be the minimum that will produce sufficient 
output.
     Carbon microphones are actually amplifiers; the output 
is from a source external to the microphone rather than 
being directly absorbed from the airborne soundwaves.  It is 
this amplification that resulted in the universal use of 
carbon microphones in the telephone system for nearly a 
century.  When small, efficient, amplifiers became available 
carbon elements were pretty much replaced with electret 
mics.  The electret itself is an interesting development, a 
condenser microphone which does not need an external source 
of bias voltage. In combination with solid state amplifiers 
they pretty much eliminated the use of carbon elements for 
most applications.


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