[Milsurplus] FMing the ARC-type-12 Transmitters

[B Smith]

Very nice modification Dave
We are all looking for alternate  uses for some of our equipment.
The Type 12 equipment was the best available for many years on
some of "lighter" our military aircraft and became a real work horse.
I was "wagon" mobile at the last MRCA meet with Type 12 equipment on 
2 meters.

Breck
  From: David Stinson 
  Sent: Sunday, January 13, 2013 9:26 PM
  To: milsurplus at mailman.qth.net ; ARC-5 List 
  Subject: [Milsurplus] FMing the ARC-type-12 Transmitters


  Some folks asked me about the simple, two-wire 
  swap that will put ARC-type-12 transmitters on FM.
  It removes PA modulation and instead, modulates
  the oscillator.  I get 4 KC deviation without any trouble
  using stock 8140 KC CR1 crystals.

  All the stock 8140 KC crystals I have oscillate "high-"
  some by quite a bit.  I installed a 5-40 pFd variable
  across the crystal to "rubber" it onto freq.
  IIRC, I actually put it at the osc. tube socket, 
  since I only wanted to use one channel (146.52) 
  and it was easier to put it there.

  Schematic of transmitter T-13 change:

  http://home.netcom.com/~arc5/TYPE12/T13FMMODb.jpg

  Wiring diagram of change:

  http://home.netcom.com/~arc5/TYPE12/T13FMMODa.jpg

  NOTE: Change only the 2 wires noted.
  Leave the other wires on terminals 4 and 1 intact.

  ARC-type-12 manual in .pdf is available at:
  http://jptronics.org/radios/Military/ARC12/mil_ib_arc-12_equip_1_55.pdf

  An important note on using ARC-type-12 transmitters:
  The PA output has a significant 73 MC componant.
  This is no problem when the transmitter feeds a 146 MC antenna,
  since it attenuates this.  It will show-up if you feed it into 
  a 50-ohm DC resistance and look at the spectrum.
  73 DE Dave AB5S

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