SP-210 HF Oscillator

(from TM 11-866)


a. The HF oscillator operates at a frequency exactly 465 kc
(the frequency for which the IF amplifier is adjusted) higher than
that of the incoming signal. The oscillator section of the variable
tuning capacitor (C1D) has the same capacitance and plate shape
as RF sections (C1A, C1B, C1C) and the constant 465 kc frequency
difference is maintained by means of a padding capacitor in series
with the variable, together with appropriate values of oscillator
inductance and parallel trimmer capacitance. Due to the very slight
difference in frequency ratio between the HF oscillator and RF
amplifier circuits in the 20-40 mc band of receiver BC-794-B, no
series padding capacitor is necessary. With the exception of the
200-400 kc and 100-200 kc bands in receiver BC-779-B (L24, L25),
fixed padding capacitors are used, and exact tracking is accomplished
by adjusting oscillator inductance and trimmer capacitance. In
these two bands, the oscillator inductance is fixed, and tracking is
accomplished by adjusting the variable padding capacitors (C86, C87)
and variable trimmers (C77,C78).

b. The oscillator circiut is a modified Hartley employing a
triode-connected VT-91 (V4). The plate, screen and suppressor grid
are tied together and by-passed to ground (chassis). Injection
voltage for the 1st detector (V3) is taken from the oscillator cathode,
which is connected to a tap well down on the oscillator tuning coil.
This minimizes oscillator frequency changes caused by a reaction from
the 1st detector signal grid circuit.