V THEORY OF OPERATION
A. R.F. Amplifier
A signal entering the antenna is applied to the grid of the low noise
12BZ6 R. F. amplifier tube (Vl) through the tuned circuit formed by the
coil Tl and capacitors C21 through C31.
Here it is amplified and the output is tuned by the circuit formed by
coil T2 and capacitors Cl through C11. The signal is then applied to
the grid of the 6HS6 first mixer (V2).
B. First Mixer and Premixer System
A signal from the premixer system is applied to the cathode of the
6HS6 first mixer tube (V2), 5645 KC above the incoming signal frequency
applied to its control grid. This results in 5645 KC I.F. output.
The premixer system consists of a 4955KC to 5455KC solid state
permeability tuned VFO T13, a switchable overtone crystal oscillator
Ql, the premixer V8, and the premixer output circuitry composed of
T3 and T4 and their tuning and coupling capacitors.
The VFO signal is applied to the cathode of the premixer V8 and the
output of the crystal oscillator Ql, is applied to the grid.
The crystal frequencies are selected so that the difference frequency
output of the premixer is 5645 KC above the frequency of the desired
incoming signal.
For example, on 80 meters a 14.6 mc crystal is used in the crystal
oscillator. This frequency and the 4955 - 5455 KC VFO output com-
bine in the premixer stage to give output from 9145 KC to 9645 KC.
This signal is applied to the first mixer cathode through coils T3 and T4 and
their associated tuning and coupling capacitors to beat with the incoming
3.5-4.0 M
C
signal. This results in 5645 KC output.
Note that the RF coils Tl and T2 and premixer output coils T3 and T4 are
premeability tuned, and are ganged together. They are tuned by means
of the PRESELECTOR control.
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