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

Jim Haynes wrote:
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

Jim, I'm surprised you didn't need an amplifier with that electret
element? That's what the carbon-compatible mics use in 2-way and
avionic installations to get the required output level and the correct
low-Z. Also, the larger telephone elements won't fit inside the T-17
housing and would have to be mounted, up front, as you mentioned. I did
a military mic article for the June 2007 issue of Electric Radio, if
anyone's interested. DE K3HVG



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"K3HVG" wrote in message
...
Jim Haynes wrote:
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

Jim, I'm surprised you didn't need an amplifier with that
electret element? That's what the carbon-compatible mics
use in 2-way and avionic installations to get the required
output level and the correct low-Z. Also, the larger
telephone elements won't fit inside the T-17 housing and
would have to be mounted, up front, as you mentioned. I
did a military mic article for the June 2007 issue of
Electric Radio, if anyone's interested. DE K3HVG


Since T-1 elements are fairly plentiful one can get the
carbon granules out of one to repack the T-17. I did this a
long time ago to repack a Western Electric 375 double-button
broadcast mic. It worked fine but was, of course, tedious.
Where the original carbon granules have been fused due
to excessive current (too much voltage) simply drying them
out will not fix them, they must be replaced. I don't
remember how the T-17 element is constructed but most carbon
mics are made so that replacing the carbon can be done.


--

--
Richard Knoppow
Los Angeles
WB6KBL

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

"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.


--

--
Richard Knoppow
Los Angeles
WB6KBL

On Feb 2, 6:48*pm, "Richard Knoppow" wrote:
"K3HVG" wrote in message

...



Jim Haynes wrote:
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

Jim, *I'm surprised you didn't need an amplifier with that
electret element? *That's what the carbon-compatible mics
use in 2-way and avionic installations to get the required
output level and the correct low-Z. *Also, the larger
telephone elements won't fit inside the T-17 housing and
would have to be mounted, up front, as you mentioned. *I
did a military mic article for the June 2007 issue of
Electric Radio, if anyone's interested. *DE K3HVG

* * Since T-1 elements are fairly plentiful one can get the
carbon granules out of one to repack the T-17. I did this a
long time ago to repack a Western Electric 375 double-button
broadcast mic. It worked fine but was, of course, tedious.
* * Where the original carbon granules have been fused due
to excessive current (too much voltage) simply drying them
out will not fix them, they must be replaced. I don't
remember how the T-17 element is constructed but most carbon
mics are made so that replacing the carbon can be done.

--

--
Richard Knoppow
Los Angeles
WB6KBL
- Hide quoted text -

- Show quoted text -

Yeah; doesn't 12 volts sound a bit high? While approx 100 ohms plus
1200 phms = 1300 ohms and therefore 12/13000 = about 9 mlliamps
current, just wondering if a combination of packing, moisture and
maybe a bit of carbon granule sparking at too high an energising
voltage could be contributing to the problem?
Energising voltage of around 3 volts was typical in many dry cell
telephones sets; or even on sets where 48 volts is/was fed out from
the telephone Central office .

On 2009-02-08, terryS wrote:
On Feb 2, 6:48Â*pm, "Richard Knoppow" wrote:
"K3HVG" wrote in message

...
Jim, Â*I'm surprised you didn't need an amplifier with that
electret element? Â*That's what the carbon-compatible mics
use in 2-way and avionic installations to get the required

The electret elements from Radio Shack include an amplifier, which
is why you have to power them; but I don't know if it brings the output
level up to that of a carbon mike. The electret T-1 replacement I got
from Mike Sandman obviously has enough amplification built into it
somewhere, since it directly replaces the T-1. But I didn't tear it up
to see what is inside.

Jim Haynes wrote:
On 2009-02-08, terryS wrote:
On Feb 2, 6:48Â*pm, "Richard Knoppow" wrote:
"K3HVG" wrote in message

...
Jim, Â*I'm surprised you didn't need an amplifier with that
electret element? Â*That's what the carbon-compatible mics
use in 2-way and avionic installations to get the required

The electret elements from Radio Shack include an amplifier, which
is why you have to power them; but I don't know if it brings the output
level up to that of a carbon mike. The electret T-1 replacement I got
from Mike Sandman obviously has enough amplification built into it
somewhere, since it directly replaces the T-1. But I didn't tear it up
to see what is inside.

All those electret elements have a FET-IC in them, which is a J-FET with
controlled leakage so that they don't need a grid leak resistor inside them.
Apply voltage across them through a resistor, pick the modulated signal
off through a capacitor. With a little tinkering with series and shunt
resistances you can make one fit right in place of a carbon element.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."