Reference: NAVSHIPS 91875(A), para 2i
The BFO circuit provides a heterodyning signal to the BFO-mixer, V1004 for the second i-f to produce the desired beat note when the RECEPTION control is set for A1 and F1 reception. The beat frequency oscillator (V1301) is a Hartley circuit with adjustable capacitor, C1309, operated by the FREQ. VERNIER front panel control. This capacitor is paralleled by C1307 and C1308, which are screwdriver trimmer adjustments (C1308 is active only in the FSK position of the RECEPTION control on the AN/SRR-12 and -13).
Output of the oscillator is electron-coupled to the plate, pin 5, of V1301. BFO signals are fed through J1301B, J1002A, to the third grid, pin 4, of the BFO mixer, V1004, where they heterodyne with A1 or F1 received signals (coming from V1003) to produce a beat note in the audio range. Capacitor C1309, labeled as the FREQ. VERNIER on the front panel, changes the resonant frequency of Z1301 to provide variation of 1500 cycles on either side of the center frequency which is zero cycle except in the FSK position of the RECEPTION control on the AN/SRR-12 or -13 receivers. In this case, the beat note at the center, (0) setting of the FREQ. VERNIER, is 2550 cycles.
Plate and screen grid potentials to V1301 are removed through the setting of S1301 when the RECEPTION control is set to A2 or A3, or by S252 of AN/SRR-11, S452 of AN/SRR-12, or S652 of AN/SRR-13 when the crystal calibrator is on.