[ARC5] FMing the type-12 Transmitters

[David Stinson]

On the easy-reverse, two-wire change to FM
a type-12 transmitter:

A member asked:


> Which two wires? I have a stack of T-11 and T-13 transmitters and 
> was considering using them for 2-meter packet or APRS.

Turn the transmitter bottom-up with the front facing you
and the relays in the rear. The mod tranny is the one on the right.
It's T201, 401, 2301 or 2401 depending on your model.
Move the wire from pin 3 to pin 4.
This moves the PA over to the B+ buss and off the mod tranny.

There is a wire on pin 4 (not pin 1!)  that runs to a 180-ohm
resistor mounted on an RF tuning coil right in the middle of
the chassis, between the middle vertical coluum of tubes.
Move this wire to pin 3 on the mod tranny.
This modulates the oscillator.

That's all there is to it.  Using a telephone carbon mike
element and not other changes, my rig modulates right
at 4 KC, which is fine for talking to plastic radios.
146.52 crystals are either 8140 KC or 12210 KC.

Two important notes:
Every crystal I've put in these rigshas oscillated a bit high
adn no two of the older, original crystals oscillates
in the same place.  That was OK in a 1950 VHF AM rig,
but not so much when talking to today's plastic radios.
I put a 25 pf trimmer cap from the "select" wire on the relays
to ground (because it  was easier to put it there than
at the crystal socket) and it "rubbered" down to .52 OK.

The type-12 transmitters double in the final.  The resonant
antenna is supposed to be part of the spurious surpression.
If feed to a 50-ohm resistive dummy load, it has a whopper
of a 70 MC product- only about 15 db down.
Use a resonant antenna rather than some broad-band
Chinese thing.

GL ES 73 DE Dave AB5S