[GreenKeys] Northern type 105 model 4A

[Jim Haynes]

There's a whole bunch of similar devices from various makers.  The
military O-5B/FR and there is a TMC one and some others.  All using
a reactance tube to shift the frequency of a 200KHz oscillator that
is mixed with a crystal or VFO to produce the desired output frequency.
I presume the reason for all that complexity is to get a linear
shift vs input voltage relatonship, perhaps for use with fax as well as
telegraphy.  Hams in the old days tended to use the much simpler diode
modulator shifting the frequency of the transmitter crystal or VFO.
Perhaps it was quite nonlinear frequency vs voltage, but for RTTY FSK
we didn't really need linearity.

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
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 		Meredith Willson, The Music Man