[ARC5] Carbon mic rejuvenation

[Richard Knoppow]

     Never heard of applying AC, perhaps an attempt to heat it 
enough to evaporate moisture.  Two things that can damage carbon 
mics are moisture and excessive current. The latter can burn the 
carbon particles at the edges where they make contact. Moisture 
can be removed from many types of carbon mics by baking them in a 
very slow oven, say not over 130F, for several hours. When the 
carbon particles are too moist they can pack together. Sometimes 
just rapping the mic on a hard surface will suffice to unpack the 
carbon but it often packs again after a short time.
    Ultimately, if nothing else works, replacing the darbon with 
particles taken from a telephone microphone may fix it but that 
can be a delicate operation.
     I have one RS-38A, it has quite high output and is 
relatively noise free. Other carbon mics I have are not in such 
good condition.
     Generally, a carbon mic should be run on the minimum current 
that gives reasonable output. They can be run on a transformer or 
as a partial cathode bias resistor.
     Carbon microphones were very widely used in telephone 
service and early broadcasting because they are amplifiers so 
have very high output. The electrical output can be far in excess 
of the acoustical power impinging on the diaphragm. They are also 
very rugged. Over the decades the Bell System Technical Journal, 
which is available as a free download from several sources, has 
had many papers on the design of telephone microphones, for 
telephone and communications use they must be relatively immune 
to moisture and positional effects, many approaches were devised 
to accomplish this. The T-1 microphone used in the Type 500 
telephone was one of the most carefully designed microphones in 
history (along with the ring armature receiver of Mott and Minor 
that accompanied it).

On 10/23/2018 8:34 AM, Robert Nickels wrote:
> I though the topic of rejuvenating carbon mic elements came up on 
> this list a while back but a check of the archives came up empty, 
> so maybe someone will remember or can direct me.   I have two 
> RS-38A  type carbon hand mics and both work but produce 
> substantially less audio than other mics.   I don't know if this 
> is due to some design difference or aging effects but I'd like to 
> see if I can improve the output.    If I recall the method 
> involved applying a AC voltage.
> 
> Thanks and 73,
> 
> Bob W9RAN

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL