Air Force Receiver BC

The first of these receivers is a commercial unit. The Air Force designation is BC-779. The receiver is for ground installations.

Notice in the block diagram of the receiver above that it has two RF stages, three IF stages, and three AF stages. It has a separate channel AVC system. It uses a limiter stage but no squelch circuit. A separate power supply unit is necessary. The receiver is available in different frequency ranges, but all models use the same basic circuit.

Looking at the control panel below, notice first the five-position bandswitch. This permits selection of any one of five different frequency ranges. The switch also operates





Receiver BC-779, Front View


Receiver ßC 779 and Power Unit RA-94, Rear View a masking plate over the main tuning dial in order to expose only the calibration scale of the particular band selected.

The main tuning dial turns with the control beneath it. It is used to tune the receiver within the range of the band selected. The dial is illuminated from the rear.

The bandspread dial and the control beneath it are operated in conjunction with the tuning dial. The bandspread dial has a scale reading from 0 to 100. It is used to get fine tuning over a narrow band of frequencies.

The S-meter may be used when the receiver is operating on AVC. It can be used to log the strength of a received signal or to tune in a signal. It shows the greatest deflection when a signal is correctly tuned. The meter circuit is adjusted at the factory so that a change of one number on the calibration indicates a change in signal strength of approximately two to one.

The CRYSTAL SELECTIVITY control provides five degrees of crystal selectivity by varying the Q of the crystal filter circuit.

The PHASING control is used in conjunction with the crystal filter. With this control, the rejection notch in the crystal filter response can be moved in order to eliminate reception of image frequencies.

The BANDWIDTH control adjusts the bandwidth of the IF by changing the coupling of the IF transformers. Minimum bandwidth occurs at position 3, and clockwise rotation increases the bandwidth.

The LIMITER switch is an on-off switch which permits use of the limiter as desired. When the noise level is low, the limiter can be shut off.

The AVC-MANUAL switch permits disconnection of the AVC. Then manual gain control is available through the sensitivity control. In either position, the audio gain control can be used to vary the audio output.

The SIGNAL control permits a choice between modulated or CW reception. The BFO is turned on when the control is in CW position.

The BEAT OSCILLATOR control adjusts the frequency of the beat oscillator by varying a capacitor in the oscillator tuned circuit. Adjusting the oscillator varies the pitch of the audio tone signal.

The SEND-REC control can be operated when the receiver is used with an associated transmitter. In the SEND position, the receiver is silenced, though ready for instant use when the control is moved to the REC position.

The phone jack provides for connection of a headset to the receiver output. This jack is in parallel with the phone lugs on the terminal strip at the rear of the receiver.

Now look at the drawing of the rear of the receiver and power supply above. On the same terminal strip as the phone connections are the speaker connections and the phono connections. The phono terminals provide a means of using the audio section of the

260 234 210 190

260 234 210 190

130 117 105 95 a





0 0 0 00000 10 9876 54321

Power Unit RA-94C

receiver as an audio amplifier. Other terminal strips provide connections for the power cable, the antenna, and a relay to disconnect the receiver automatically during transmissions.

The overall schematic diagram shows the complete receiver circuit. The circuits for band switching are shown at the lower left. There are five sets of ganged input and output tuned circuits for the three RF amplifier stages and the high frequency oscillator. Each set of tuned circuits provides for one frequency band. The selection of the proper band is controlled by ganged switches, SW1A to SW1E.

Conversion is accomplished through a pent-agrid mixer and separate oscillator.

A crystal filter is used between the mixer and the first IF amplifier. C32 is the variable phasing capacitor which counteracts the capacitance of the crystal holder. It can be varied to move the rejection notch in the crystal filter response curve. Crystal selectivity is controlled by SW7, a ganged switch which provides five degrees of selectivity.

In the OFF position, the crystal is shorted. On position 1 the crystal filter is placed in the circuit. On each of the other four positions additional resistance is introduced into the crystal filter output parallel tuned circuit. Thus, in each position more and more resistance is placed in series with the filter (the parallel tuned circuit is purely resistive), and the Q of the filter decreases. The selectivity of the crystal filter circuit increases.

The bandwidth of the IF bandpass is controlled by ganged regulation of the amount of coupling in T2 and T3, the IF transformers.

The AVC has its own amplifier, operating with part of the output of the second IF amplifier. Plate current of the AVC rectifier flows through the S-meter when the AVC-MANUAL switch is in the AVC position.

The output of the BFO, a beat frequency , is injected into the plate circuit of the third IF amplifier. This means that the AVC voltage has already been coupled to an AVC amplifier and cannot be affected by the BFO. To insure that there is no stray coupling between os-





Schematic Diagram, Receiver ßC-779-A, ß

Schematic Diagram, Receiver BC-779-A, 8

cillator and AVC, the output lead of the BFO and the input lead of the AVC amplifier are shielded.

When the limiter on-off switch is ON, the limiter operates on the detector output. The limiter tube is a twin triode operated as a single triode. Grid and cathode voltages are taken from the negative voltage divider system of the detector output. Bias is self-adjusting, and, with a normal signal, the limiter is cut off. However, the time constants of grid and cathode circuits are such that a sudden negative voltage of high amplitude (noise) drives the cathode negative faster than the grid. The limiter conducts for the duration of the noise pulse. Since its cathode is connected to the detector output voltage divider, the limiter shorts the signal when it conducts.

The audio section uses two voltage amplifiers. The output of the second AF amplifier (the driver) is transformer-coupled to a push-pull power amplifier stage. The power amplifier, in turn, is transformer-coupled to headsets and speaker. The secondary of this transformer consists of a high impedance winding for headsets and a low impedance winding for ihe speaker.

The power supply unit used with this receiver bears the designation RA-94-C. It is usually mounted under the receiver. Look at its circuit diagram on page 76. It consists of a full wave rectifier (VT-145) with an output filter and voltage divider to supply plate and screen voltages; a half wave rectifier (VT-80) with a resistance-capacitance filter to supply negative bias voltage, and a secondary winding to supply AC heater voltage.

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