[Milsurplus] Carbon Mic Rejuvination

[John Vendely]

Dennis,

Yes, the standard Western Electric procedure for rejuvenating telephone 
carbon mics involved applying a low voltage 60 cps AC bias, and 
acoustically "blasting" the microphone diaphragm with white noise while 
rotating the element about its horizontal axis. The combination of AC 
bias and acoustic "blasting" breaks up the carbon granules without 
damage, and frequently restores function.

A friend of mine who was an engineer at the Western Electric plant in 
Indianapolis years ago, described the method to me.  I should note that 
he strongly cautioned against employing the usual barbaric expedient of 
bashing the microphone on the table to loosen the carbon granules.  Yeah 
I know, I know, we've all done it and sometimes it works after a 
fashion.  However, he was quite adamant in insisting that this generally 
does more harm than good, and experience has proven him correct.  These 
old mics aren't getting any more plentiful,  and doing it right is easy 
enough.

I wrote up a detailed procedure and circuit diagram based on his 
description, and can provide a .pdf file of it if someone will suggest a 
suitable place to send or upload it.  I think it will be of general 
interest.  I built up a test box, and have used it for years to 
successfully rejuvenate a variety of carbon mic elements.  Though some 
elements are beyond rejuvenation, the technique has brought back quite a 
few--including a couple of T-17 types...

73,

John K9WT

On 3/15/2013 5:02 PM, mac wrote:
> Heard a rumor about rejuvenating old carbon microphones with AC 
> published somewhere (ER maybe?).  Anybody know more?  My experience 
> with the old military mics (T17 & the like) is that they either play 
> or they don't, banging on the table , etc., never worked for me 
> anyway.  Damn things can surprise you though, just picked up a pair 
> labeled "Microphone T-17 4857-CHI-42.  Rattiest looking units I've 
> ever seen and both work great!
>
> Dennis D.  W7QHO
> Glendale, CA
>
>
>
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