[MRCA] Carbon Mic Checking

[D. Platt]

Bill Ramsey wrote:
> All,
>
>   I have a M52/U microphone with a M51/UR element I am trying to use on a
> T-195 transmitter. I am getting low modulation. Before I take out the
> modulator unit (not that bad a job) I want to check the microphone element
> for proper operation.
>
>   What would be the correct way of checking the element? 
>
>  
>
> Bill KA8WTK
>
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>
>   
Bill,  Been there and done that, as they say.  The M-51 element can make 
for a decent mic.  The T-195 has a modulation control on the lower, left 
side of the transmitter.  On one of mine, I plotted and drilled two 
small holes (sealed with OD-painted hole-plugs, later) so that I could 
get to the sidetone and mic gain pots without taking the chassis out of 
the cabinet.  The gain pot can be adjusted, normally all the way up.  
W3PWW related to me, and its true, that the front-panel Line Level pot 
affects mic gain.  Turning it all the way up seems to give just a bit 
more gain.  Finally, the plastic anti-spit membrane on the mic element 
can become brittle and cut down on acoustic transfer.  I have cut these 
out on a few elements in trying to get a bit more out of them.  That 
element was used on a whole generation of aircraft headsets and they did 
a nice job.  I used them on my Cherokee for a long time...!  As for 
electrically testing the elements, using a conventional (vice digital) 
ohmmeter on the x100 scale, you should get good swing with moderate 
voice input.  As for the actual readings?  They can vary a lot.  Its the 
amount of the swing of the resistivity that gives you an idea of the 
actual output, rather than the static resistivity.  I have a Telex mic 
tester that came from an FBO, years ago.  It has a batter-powered 
transistor amplifier and a meter and indicates mic "output.  I 
calibrated it with a known good mic.  I now just plug in a mic and see 
what the meter says.  It does not, however, give any clue to quality....hi!!

Jeep/K3HVG