[BC] FM Exciters (Pic's)

[Mark Humphrey]

On 2/26/06, Phil Alexander <dynotherm at earthlink.net> wrote:
>
>
> I may be remembering it incorrectly, it has been 40 years since I've
> looked at a manual, but as I recall, both the tube type RCA exciter
> as used in the BTF-xxD transmitters and the Gates/Harris used in the
> FM-xxH/H3 transmitters directly modulated an oscillator that was
> frequency locked to crystal control by dividing the carrier frequency
> down to a little above 100 kHz, comparing it with a crystal at that
> frequency and generating a DC bias superimposed on the modulating audio
> supplied to a variactor in a ?? Colpitts ?? oscillator circuit. The RCA
> had a cute little 1" CRT for reading Lissajous patterns to make sure
> the divider was correctly tuned from stage to stage to get the correct
> total division otherwise the output could be entirely out of the band.
>
>
I have a manual for the RCA BTE-10B (with the cute CRT, successor to the
"Iron Fireman") in front of me.  The FM oscillator was a 6AQ5 Hartley type,
operating at 1/18 carrier frequency, with two 6AQ5 reactance tubes for main
channel in a "push-pull" arrangement, plus a 6CL6 reactance modulator for
the SCA.    The output of this master oscillator was divided by 240 and
compared with the crystal reference oscillator (divided by 5) with a phase
detector operating somewhere between 20 and 25 kc.  This phase detection
circuit was just a couple of transformers with a pair of 1N34A diodes and
filter network, to generate the DC error voltage which was fed back to the
reactance tubes.

This brings back some memories.   At my Class D college station, we bought
one of these used from WBVP-FM in Beaver Falls, PA to get on the air cheap.
AFC stability wasn't a big problem, but I could never get it to make the
full 10 watt rated power.  Let's just say its performance was not in
proportion to its weight.

WGRQ 96.9 in Buffalo had one on the air through the late '70s. I wonder how
many are still running.

Mark