[Milsurplus] WW2 Carbon Mic repair

[WA5CAB at cs.com]

I forget exactly what all was in K3HVG's ER article, and also recommend 
tracking it down.  But one problem that is fairly common in the T-17 is with 
the PTT switch contacts.  The switch is electrically a DPST.  One contact set 
connects together the wires for the Tip and Sleeve of the PL-68 plug.  This 
is used to key the transmitter.  The other set connects one side of the 
carbon element to the PL-68 Sleeve.  This allows bias current supplied by the 
transmitter to the Ring contact to flow through the carbon element.  If either 
or both of the switch contacts aren't working when you push the black 
button, the microphone can't work.  

A quick and dirty check can be made with an ohmmeter on the R x 1 scale 
(preferably an analog, not digital, one).  Connect one ohmmeter lead to the 
PL-68 Sleeve.  Connect the other lead to the Tip.  Press the PTT button.  
Ohmmeter should read 0 to maybe 1/2 ohm.  Move the second lead to the PL-68 Ring. 
 Press PTT.  Ohmmeter should read anywhere from 10 to 50 ohms or so.  If it 
does, blow or whistle into the microphone.  Ohmmeter pointer should move.  

If either test failed, open up the microphone by removing first the black 
button, then the side of the mic handle.  Check continuity from the three 
terminals of the PL-68 to the ends of the three wires where they terminate 
inside the mic body.  If any are open, determine why and repair/replace to fix.  
If all are good, check continuity across the two switch contacts with the 
switch operated.  If either or both are open, burnish the contacts.  Finally, 
check continuity through the carbon element.  If open, replace the element.

One thing to note is something that I had never seen in print before until 
I was reading the I-142 (telephone test set) the other day.  Maximum 
sensitivity of a carbon element occurs when the face of the element is held 
parallel to a vertical plane.  Sensitivity with element horizontal will be down as 
much as 50%.

In a message dated 3/31/2011 11:47:59 AM Central Daylight Time, 
bluebirdtele at embarqmail.com writes: 
> Hi:
> I'm looking for info in order to repair my WW2 carbon hand mic.  I think 
> it is called a T-15, T-16. Something along that line.  Has "SHURE" marked on 
> the side and has a big klunky PTT button on the side.  Color is 
> approximately OD green.  Very common in the WW2 radio pictures and probably was 
> issued to lots of radio sets.
> My question is:
> I tried wacking the mic element on the table top to clear the carbon 
> granuals but no luck in Mudville. It's Kaput.  What is a suitable replacement 
> for mic element? I would like to use the mic on a GF 11, ART 13 and GRC 9.
> Thanks
> Greg
> WA7LYO
> Kinston NC
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480