"Antonio Vernucci" wrote in message
. ..
I have a NOS T-17 carbon microphone that I use with my
BC.-191.

The modulation level is rather low, but I cannot easily
determine whether the problem is with the transmitter or
with the microphone.

Two questions:

1) I have been told that carbon microphones can be
revitalized by leaving them in an oven for an hour or so
at a temperature of about 180 F. Did anyone try to do
that? It would be easier to put the entire microphone in
the oven rather than just the capsule, but I am not sure
on whether this could cause damages to the non-metallic
parts

2) I would like to test the microphone by measuring the
output voltage with an oscilloscope while talking. Does
anyone have an idea on how much (peak) voltage I should
see on the scope when powering the microphone with 12 V,
through a resistor of known value?

Thanks & 73

Tony I0JX

Carbon microphones are not generators, they need an
external source of power. Typical voltage is around a volt
or two, the less the better. The simplest way to test the
mic is first to measure its resistance and see how much that
varies when the mic is shaken or tapped. It will change some
even on properly working mics. In the absense of a proper
transformer check the audio quality using a battery and
series resistance. Pick up the audio across the resistor.
Carbon mics are prone to "packing" which is when the
carbon granules stick together. Sometimes this is caused by
moisture, sometimes by being subjected to too high a voltage
which causes the granules to fuse. To get rid of moisture
the element must be baked. If it is possible to remove the
carbon granules and bake them separately that is best
because a much higher temperature can be used. It might also
work to put the element in a sealed box with a moisture
absorber in it for an extended period.
Nearly all carbon mics need to be stirred up pretty
frequently. About the only ones that seem immune from this
are the elements used in late WE telephones. They are
designed so that they are not position sensitive and are
well sealed so they don't much pack. Other types are
position sensitive and won't work in some positions, for
instance facing up or down. Its normal for most of these
mics to need shaking pretty frequently.
Carbon mics were very popular for many uses because they
are amplifiers. This results in very high output. The vices
are high noise and high distortion, at least in
single-button types.
The highest voltage that should be applied to a carbon
mic is probably around six volts, but the lower the voltage
that will result in adequate output the better since higher
voltages tend to cause arcing and fusing.


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Richard Knoppow
Los Angeles, CA, USA