[ARC5] Carbon Mic Replacements

[J Mcvey]

I was just going to suggest an electret for that purpose but you beat me to it.  In this case any junkbox electret will probably due, but you will have to add a dropping resistor to limit the current to around 1.5 ma which depends on what voltage you are running (12 or 28V)  . no other component required. The output form an electret is pretty high as mikes go...
The original circuit supplies quite a bit of current as R54 is only 300 ohms! For 28V you would need about 18K in series with the electret.
 

    On Thursday, March 24, 2016 9:44 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
 

 
You can buy an electret replacement for the telco and military standard
T-1 carbon mic element used in 500 series telephones.  I opened one up and
attached is a schematic.  That's simpler than some of the circutis I
have seen.  And in a military mic with a PL-68 you can probably eliminate
the diode bridge since the polarity is unlikely to get reversed as it
might in a telephone.

The replacement mic capsule is intended to replace the carbon mic in
modular cord telephone handsets, but with a bit of carving you can make
it fit into a non-modular handset.

There's been some discussion of T-17 mikes and how they are extremely
insensitive, even when fairly new.  It's my belief, and maybe someone
can confirm or deny, that it was intentionally insensitive because it
was meant to be used in an airplane with all the ambient noise, and
therefore the user was supposed to shout into it to get over the noise.

You can almost replace a T-17 carbon element with a T-1 telephone element;
the latter is just slightly too large in diameter.  Probably if you got
one of the electret replacements for T-1 and opened it up you could cut it
down a little to fit into the T-17 housing.

I got the T-1 electret replacements from www.sandman.com.  Sadly, Mike
Sandman is gone, but I'm told the company is still operating.

An alternative is to use the T-32 desk microphone, which looks like a
"candlestick" telephone with a push-to-talk switch in place of the
receiver hook.  Some of these have a carbon element typical of old
telephones, but others have, or can be fitted with, what's called a
"bulldog" transmitter housing which holds a standard Western Electric
F-1 element, the kind that was used in the 302 telephones that
preceded the 500 style.  You can probably find a viable F1 element, and
if not there is plenty of room to put in a T1 or its electret replacement.

Jim W6JVE

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