"Harold E. Johnson" wrote in message
news:_TZhl.555961$yE1.439407@attbi_s21...
When I used to use them, we always repaired them by
several sharp bangs against against anything sturdy enough
not to be damaged by the "hammer".

W4ZCB
Depending on the design of the carbon cell there is a
tendency for the carbon to settle and "pack". Shaking or
rapping the microphone will distribute the carbon so that
the mic works again. Note that carbon mics are also position
sensitive. Some may not work well if used in some position
other than they were designed for. One of the design
objectives for the Western Electric T-1 cell, used in the
500 series telephones, was to make them as much as possible
independant of position and immune to packing.
Packing of a sort can also be produced by moisture
causing teh granules sticking together due to moisture or
because they have been fused by the application of to high a
voltage.
There are other causes of poor performance, such as
oxidation of the contact surfaces. Fixing that requires
opening up the mic and cleaning the surfaces, which may be
difficult where one of them is the diaphragm.
Carbon microphones were popular for communications
equipment because they are amplifiers and, thus, can have
very high output compared to a generator such as a dyanamic,
magnetic, or crystal microphone.


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Richard Knoppow
Los Angeles
WB6KBL