[Milsurplus] [Boatanchors] Carbon Mic Rejuvination

Sat Mar 16 18:44:30 EDT 2013

Gents,

This is the procedure I wrote some time ago, based on info from my 
friend who had learned this during his days at Western Electric. The 
document evidently has migrated around  a bit since then.  I should 
point out that the wording is entirely my own, and was not taken 
directly from any official Western Electric procedure.  It is, however, 
an accurate description of the technique as described to me.

Based on my own experience, I would recommend one slight change, 
however, and that is essentially to combine steps 2, 3, and 5. That is, 
apply bias and begin noise blasting at the same time, while rotating 
element as described.   I never found any advantage to biasing and 
rotating first, then repeating the process with noise.

Regarding the advice against element overheating, no specifics were 
given to me.  Surely anyone with a little experience will have no 
difficulty with this point.  Simply exercise a little common sense.

You may have to experiment with the magnitude of AC bias current--I 
don't think the values I supplied are anything magic, and different 
elements may respond differently to the treatment. A rheostat in place 
of the fixed resistor would be a nice touch. Or, if you have a variac 
handy, use it to drive the transformer primary.  The object is simply to 
apply some reasonable AC current, agitate the diaphragm acoustically, 
and don't abuse the element in the process.  I use a General Radio 
random noise generator and a small audio amp driving a small speaker 
coupled closely to the mic element, but anything reasonable will do the 
job just fine.

Have fun...

73,

John K9WT

On 3/16/2013 9:20 AM, C.Whitaker wrote:
> de WB2CPN Again,
> In Germany 1946 and after I saw their broadcast quality
> carbon mikes.
> They were large, made from a 4 by 6 inch, or larger, piece
> of 1 inch thick ceramic.  These were hollowed out about
> 3/8 inch deep, (1/4 inch edges), and filled with granular
> carbon.  Two flat electrodes were glued into place side
> by side before the carbon was poured in.
> Then a removable cover made of thin plastic, or mica, was
> fitted on to the top.  The mic was stood on its edge, and
> I suppose the vibrations caused by the sound made the
> resistance of the carbon vary.
> Trivia, but from time to time the broadcast engineer would
> remove the front cover, and regrind the carbon granules.
> They developed a better mic, the condenser mic, which
> had a vacuum tube amplifier close by, and was the 4 by
> 10 inch tube that you saw on the old German pre-WW II
> propaganda films.
> The "Western-Electric Approved Method" .pdf I got
> some place recently summarizes:
> 1.  Connect the element to 6.3 VAC, 60 cps, through a
> 500 Ohms 1 Watt resistor.  (They call this the Bias Current.)
> 2.  With the bias applied, hold the element in the vertical
> plane.
> 3.  For approximately one minute repeatedly rotate the
> element +/- 120 degrees around its horizontal axis.
> 4.  Do not strike or tap the element. Do not allow the
> element to overheat.  (What do they mean, overheat?)
> 5.  Then, for approximately 1 - 2 minutes "Blast" the
> element with acoustic white noise, and if white noise
> is not available just blowing into the element will work.
> 6.  This procedure may be repeated 2 - 3 times.  If the
> element still has low output, excessive distortion, or
> popcorn noise after several attempts, it should be
> discarded.   (I never knew that Ma Bell ever discarded
> anything, I put 22 years with AT&T Long lines.)
> 73  Clete
> ++++++++++++++++++++++++++++++++++++
>
> On 3/16/2013 2:25 AM, Richard Brunner wrote:
>> Modulating spark transmission has been done.  You have to get the 
>> pulse rate/buzzer up high enough.  It sounds pretty good, but there 
>> is a loud buzz and rushing noise in the background.  It was judged 
>> not satisfactory.
>>
>> A carbon mic in the antenna lead works good, but beware of RF burns 
>> on your nose.
>>
>> Richard, AA1P
>>
>>
>> On 03/15/2013 10:44 PM, COURYHOUSE at aol.com wrote:
>>> the only thng that  sounds  worse is when you try to  modulate spark 
>>> this
>>> way
>>>     In a message dated 3/15/2013 7:41:32 P.M. Mountain Standard Time,
>>> w7qho at aol.com writes:
>>>
>>> Technique used in the very early days for some of the first voice  and
>>> music transmissions....
>>>
>>> Dennis D.   W7QHO
>>> Glendale, CA
>>>
>>> *************
>>> On Mar 15, 2013, at 3:50 PM, Rob  Atkinson wrote:
>>>
>>>> you can put a carbon  mic in series between  the rig and antenna and
>>>> use it to modulate the RF.    Supposed to work but sounds really bad.
>>>> Only a few watts I  guess.
>>>
>>
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>
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