[ARC5] FMing the ARC-type-12 Transmitters

[David Stinson]

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