[Milsurplus] GI microphonia

[Mike Hanz]

Unserviceable but Repairable wrote:

>Multipart question
>
>1.   a. have RS38 (4 tcs) that's 380 ohms.  Telephonics.  Good original runner
>      b. T17's heart transplant F1 is 88 ohms
>      c. Have Continental F1 substitute that's 220 ohms
>    So do your elt.s vary all over like these 
>

Yup. Cementation, storage conditions, and original size of granules 
affect the resistance considerably.  The Summary Technical Report of 
Division 17, NDRC after the war records these complimentary observations 
about carbon mikes:

"Carbon-type microphones were adopted almost exclusively by the Armed 
Forces of the United States, despite the fact that all carbon 
microphones are essentially nonlinear devices.  Furthermore, their 
frequency-response characteristic depends on the level of the sound 
pressure for which the response is obtained.  They require a separate 
source of current, their performance is likely to vary with change of 
position, and they produce a certain amount of "burning noise" due to 
passage of current through the carbon granules.  In addition, there is a 
certain amount of deterioration of the carbon granules with age.  Their 
extensive use is due primarily to their very high sensitivity, far 
exceeding the sensitivity of magnetic and dynamic microphones."

>2.  has anyone ever seen the dynamic mic intended for the ART-13?
>

The intention for this capability was for British applications, so 
you'll need to get one of the WWII British aircraft mikes to be totally 
correct.  However, according to that Summary Technical Report, the 
Signal Corps MC-253-A button developed for the US mask microphone was 
"extensively tested and adopted for use by the British."   It looks like 
it's the same size as the ANB-M-C1 carbon button for the mask oxygen 
mask, so if you can find one of those buttons and stuff it in a T-17 or 
RS-38, you should be good to go.

>    Have Moto dynamic that looks like RS38 cookie that does OK &
>    looks 'period.'
>
>3.  How deep is the ocean?
>

11,033 meters at its deepest point.

73,
Mike