HFO/Local Oscillator

Reference:    NAVSHIPS 91875(A), para 2e(4)

See figure 2-1 for similar circuit connections.  The oscillator used in the AN/SRR-13A receiving set is a Hartley circuit.  The cathode and control grid, pin 1 of the oscillator tube V601 are connected in the oscillatory circuit through contacts on S601A, which is controlled by the band selector.  The tuned circuit consists of coils L601, L602, L603, L604, or L605 fixed and trimmer capacitors, and oscillator sections of the ganged tuning capacitor, C651A.  the E tap of one of coils L601 to L605 is connected to the control grid (pin 1), as selected by a portion of S601A, through the grid leak network C622 and R601.  The D tap of one of the coils, as selected by a portion of S601A, is connected in the cathode circuit in band 1, to improve the stability of the circuit.   A portion of S601B shorts unused coils to prevent absorption of signal on the band in use.

The plate (pin 8) is connected to the bottom side (ground) of the tuned circuit, through capacitor C624.  This arrangement along with the cathode tap mentioned previously, provides proper feedback to sustain oscillations.

The plate voltage of V601 is regulated to insure stable output from the oscillator.  In order to reduce hum on bands 4 and 5, one side of the heater is connected to the cathode through a portion of S601B.  Choke L606 and ballast resistor R1605, located in the power supply, are provided in the other heater line to minimize heater current variations.  The oscillator tracks 1600 kilocycles higher than the signal frequency on all bands.

Josh Rovero,
Aug 3, 2011, 6:15 PM