Antonio Vernucci wrote:
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
Tony, I just looked at a couple of single-button mics. The best one is
an old military T-32 that used an early. large telephone-type element.
Using a Simpson 260-8 VOM, I get a 400-500 ohm upward swing, with a
whistle into the mic. Several "known good" T-17s get 200-400 ohm
swings, less that the T-32. I find that I can get a very good idea of
what's up by this simple test. I find also that its not so much the
absolute static resistance but the amount of "swing" and hence, activity
of the element. Of course, an element with several Kohms worth of
static resistance would probably indicate a suspect element.
Regarding T-17s, I've never been able to truly resurrect an element. I
have found that the bypass capacitor, inside, can be leaky or shorted
and can cause the problems. That, and some pretty corroded plugs and
PTT switches. If you have a T-17D, they can't practically be fixed and
require a new element be fitted. If you disassemble one of those, the
carbon granules will spill out owing to the construction of that
particular model. The straight T-17, the A, B, and C models can be more
easily retrofitted. The H-33/PT handset element or one from one of
those 1950's style telephone receptionist boom-mic/headsets work very well.
de K3HVG
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