Antonio Vernucci wrote:

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

I have done this before, although not to a T-17. The idea here is to bake
all the moisture out of it, because the moisture causes the carbon granules
to cake up and stick together. It took me a lot longer than an hour or so.

180'F isn't really very hot.

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?

I do not. BUT, I would suggest first of all measuring the static resistance
of the microphone and comparing that with the resistance of a known-good
microphone.

I might also try using a telephone transmitter element (in the US we have
lots of Western Electric T-1 transmitters everywhere) and comparing that
with the test microphone in measured sensitivity.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

I do not. BUT, I would suggest first of all measuring the static resistance
of the microphone and comparing that with the resistance of a known-good
microphone.

The static resistance is about 100 ohm, but I do not have another carbon
microphone for comparison

I might also try using a telephone transmitter element (in the US we have
lots of Western Electric T-1 transmitters everywhere) and comparing that
with the test microphone in measured sensitivity.
--scott

I tried to power the microphone with 12 V through a 1200 ohm resistor. Talking
loud into the microphone and with the mouth very close to it, the scope (put
across the microphone leads) shows a peak voltage of about 600mV (or 1200mV
p-to-p). Perhaps it is good enough, but I am not sure on whether the bias
current is too low, and I should then try again using a lower resistance.

73

Tony I0JX

"Antonio Vernucci" wrote in message
. ..
I do not. BUT, I would suggest first of all measuring
the static resistance
of the microphone and comparing that with the resistance
of a known-good
microphone.

The static resistance is about 100 ohm, but I do not have
another carbon microphone for comparison

I might also try using a telephone transmitter element
(in the US we have
lots of Western Electric T-1 transmitters everywhere) and
comparing that
with the test microphone in measured sensitivity.
--scott

I tried to power the microphone with 12 V through a 1200
ohm resistor. Talking loud into the microphone and with
the mouth very close to it, the scope (put across the
microphone leads) shows a peak voltage of about 600mV (or
1200mV p-to-p). Perhaps it is good enough, but I am not
sure on whether the bias current is too low, and I should
then try again using a lower resistance.

73

Tony I0JX
Microphones vary but 100 Ohms is about right. Take the
audio from the series resistor rather than the microphone.
Set the voltage so that the drop across the mic terminals is
about one to two volts. You can run more but the least is
the best. Most devices, transmitters, etc., using carbon
mics use an imput transformer with a primary of somewhere
around 100 ohms and a secondary suitable for what you are
feeding, usually a high impedance. The exciting voltage for
the mic goes through the primary usually with a capacitor
across the battery to insure a low impedance audio frequency
path and minimum noise.
See what you get and post again.


--

--
Richard Knoppow
Los Angeles
WB6KBL