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

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've never heard that, but back when telepehones uses carbon mikes, it was
popular to fix them by removing the cartridge inside, and banging the
cartridge on a table top.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

"Geoffrey S. Mendelson" wrote in message
...
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've never heard that, but back when telepehones uses carbon mikes, it was
popular to fix them by removing the cartridge inside, and banging the
cartridge on a table top.

Geoff.



Additionally, that procedure is a legitimate and it is intended to break up
and separate the clumped up carbon particles. A frequent procedure for
outside pay phones back in the old days. You would smack the mic on the
metal plate under the phone.

Paul P.

I've never heard that, but back when telepehones uses carbon mikes, it was
popular to fix them by removing the cartridge inside, and banging the
cartridge on a table top.

Geoff.

Oh, gosh! I didn't know the cartridge was supposed to be removed before
banging. I thought that the fix was to just whack a carbon microphone
on the nearest indestructible object. The mikes that came with the BC-191
and/or BC-375 often had metal housings and could be used as hammers
in an emergency.

Old Chief Lynn, W7LTQ

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 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.


--
--
Richard Knoppow
Los Angeles, CA, USA

"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

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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Maybe we need to discuss what you really want to accomplish.

My experience with T-17 microphones 50 years ago is that they are
just not very sensitive. I assumed this is deliberate - to make
yourself heard over the noise in an airplane you have to yell into
the mike.

Now if you want to preserve an authentic T-17 then I guess something
like baking the transmitter is about your only hope, and you can only
hope to make the mike as good as it was when originally manufactured.

I remember lots of guys in the old days, who were not interested in
historic preservation, simply took the carbon element out of the
T-17 and attached the wires to a Western Electric F-1 element and taped
it in place. F-1 was the element used in the 302 telephones, forerunner
of the later T-1 used in the 500 type sets.

If you want to preserve the appearance of the T-17 but not the
authenticity, then maybe you could get the original element out and
replace it with either a T-1 or an electret mike behind the faceplate
and nobody will know the difference (except you will sound a lot
better on the radio).

I was working with some military handsets a while back that use the
equivalent of a T-1, and replaced the old transmitters with new T-1s.
I also got an electret T-1 replacement from Mike Sandman - it was
designed for use with a modular-corded handset, but with a little
hacking I got it to fit into the older kind of handset.

Has anyone worked out the circuit to use an electret like you get from
Radio Shack - they have 2 and 3 terminal models - to replace a carbon
element?

Jim W6JVE