[ARC5] Carbon Mic Replacements

[Glen Zook]

Motorola started using "carbon compatible" microphones in the mid 1950s and there were conversion kits that consisted of a new microphone element and a small circuit board with a transistorized amplifier that were available for the older, oval shaped, MK-301 microphones.


The MK-601 microphone and other variants painted various colors, are what ZOW is calling a "toothbreaker" were the metal cased microphones with the same shape as the TMN-6013 / TMN6013A plastic case microphones used with the later Motrac, Motran, and Mocom-70 series equipment.  There was also one of this plastic microphone, but dark gray in color, that was used with the later low-band Mocom-10 series equipment.  The MK-601 series has a microphone cartridge with a separate amplifier board.  The TMN-6013 series uses a microphone cartridge with an encased amplifier all as a single unit.  For almost 2-decades, this unit was made of metal.  Then, Motorola went to a cheaper plastic encased assembly.


Here is a site that shows what the TMN-6013 series "looks like":


http://www.txsupplies.com/motorola-vintage-tmn-6013a-microphone-motrac-i2271667/




The same microphone, but with a different microphone plug, the TMN6054 series was used with the Micor and Mitrek series.


The only difference between the 4-pin microphone connector used on the MK-301, MK-601, and TMN-6013 series microphones is that the pins are rotated about 30-degrees from the "standard" military series that were used by General Electric and some of the minor two-way radio manufacturers.  There is a setscrew that holds the plastic insulator in place on both the military connector and the Motorola connector.  If this setscrew is removed, the pins can be rotated to make contact with the "normal" military series microphone jack.  One can drill a new hole for the setscrew or, if the microphone is not to be removed and reset very often, just left out!


The audio quality of the dynamic microphones, with the matching preamplifier to work with carbon microphone circuits, is, generally, a lot better than with just a carbon microphone cartridge.  


In March of 1965, my junior year in college (Georgia Tech), I was getting married in June and I got a job, as a two-way radio technician, at the Motorola Service Station that just happened to be a couple of blocks north of campus.  That was the time at which Motorola started supplying the TMN-6013 series plastic microphones instead of the MK-601 metal microphones.  There were a number of taxi companies "on contract" and we replaced microphones when needed in the mobiles.  However, the taxi drivers did not like the "new" plastic microphones even though they were considerably lighter.  One of the drivers explained that, with the metal microphone, if a passenger started giving the driver a "hard time", the metal microphone, when swung on the cord, made an excellent self defense weapon!  That explained why, sometimes, a cab driver would come in with a microphone that had dried blood, hair, and, sometimes, even pieces of teeth, in the microphone!


As to the quality of the audio between a "straight" carbon cartridge and the carbon compatible microphones:  In 1973, during the time I owned the Motorola reconditioned equipment center for the south-central United States (1970 to 1979), I got "talked into" purchasing a Cessna 150 airplane (N5572E).  The FAA tower, at the Addison, Texas, airport, kept complaining about the audio quality of the radios in the aircraft (it was a certified IFR trainer and had 2-radios, etc., installed).  Before I bought the aircraft, the radios had been serviced by 3-different radio shops at the airport.  Since all of my technicians, as well as myself, held the necessary radiotelephone licenses to work on the equipment, the radios were removed and brought to my facility for repair.  An electrolytic capacitor had been replaced and installed backwards!  That was corrected and then we put a Motorola TMN-6013A on both units.


After replacing the equipment in the aircraft, a test transmission was made to the FAA tower.  The controllers could not believe that the same radios were in the aircraft.  We had to actually taxi the Cessna over to the tower and the controllers came down to look at the equipment.  They said that the quality of the audio had gone from probably the worst of the several hundred airplanes at the airport to absolutely being the best audio of all those aircraft!

 Glen, K9STH 
Website: http://k9sth.net

      From: Michael Clarson <wv2zow at gmail.com>
 To: Mark K3MSB <mark.k3msb at gmail.com> 
Cc: ARC5 <arc5 at mailman.qth.net>
 Sent: Friday, March 25, 2016 8:12 AM
 Subject: Re: [ARC5] Carbon Mic Replacements
   
Another option. Motorola made a carbon mic replacement which they used on almost all their 2-way radios. At a hamfest, look for PLASTIC cased Motorola microphones (metal cased ones, known as "The Toothbreaker" may be carbon mics), either beige and grey, or a light grey. Inside you'll find a sealed dynamic mic and preamp assembly.that replaces a carbon element. It is polarity sensitive, and I do not know the voltage range, but they should be quite reasonably priced. WARNING: Many of these have a connector that looks just like the standard Amphenol 4 pin microphone connector. It is not. The key slot is in a different place. If you want to use one as is, pick up a control head for the mating connector.. Also, the wire inside the cable is almost impossible to solder to. --Mike, WV2ZOW
  
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