An a-v-c circuit which is independent of the detector provides improved a-v-c action. In Radio Receiver BC-779-B the separate a-v-c circuit consists of an a-v-c branch i-f amplifier Tube JAN-6SK7 (VI1) and an a-v-c rectifier Tube JAN-6H6 (V12). The rectified and filtered output of V12 is applied as a-v-c voltage to both r-f stages and to the first two i-f stages.
a. A-V-C Amplifier Tube JAN-6SK7 (VI1). The output of the 2d i-f stage, V6, is applied simultaneously to the 3d i-f stage, V7 (fig. 29), and to the control grid (pin 4) of the a-v-c amplifier stage VI1 (fig. 31). V7 and VI1 have a common grid circuit, including fixed bias. The suppressor and cathode of VI1 are grounded; screen voltage is applied to pin 6 through 50,000-ohm series-dropping resistor R37; and the screen is bypassed by 0.05-mf capacitor C29. Plate (pin 8) voltage is applied through 2,000-ohm filter resistor R51 and the primary of transformer L34. The primary of L34 is tuned to 465 kc by 100-mmf capacitor C51, and capacitor C53 (0.01-mf) bypasses high frequencies to ground. The transformer secondary is untuned.
b. A-V-C Rectifier Tube JAN-6H6 (V12). The secondary voltage across L34 is applied to a-v-c rectifier Tube JAN-6H6, the two diode sections of which are connected in parallel to form a single half-wave rectifier. The diode (or rectifier) load is composed of 25,000-ohm resistor R53, 5,000-ohm resistor R54, 5,000-ohm resistor R55 and 1,000-ohm variable resistor R41. A 0.005-mf capacitor (C52), across the load, and 0.01-mf capacitors C54 and C55 filter the rectifier output.
c. S-Meter. When AVC-MANUAL switch SW4 is at AVC, the 0- to 200-microampere meter Ml is connected across the 1,000-ohm calibration resistor R41. The meter reading is a measure of the rectified current through the load. However, the meter is calibrated as a signal strength meter (S-meter). By adjusting R41 and S9 reading may be obtained on an input between approximately 10 and 10,000 microvolts. At the factory R41 is usually adjusted to produce a reading of S9 on a 50-microvolt signal at 3.5 mc. When so adjusted, a change of one S number on the meter indicates a change in signal strength of approximately two to one. The SENSITIVITY control R56 must be turned full on (10). When the S-meter is used for tuning, the BAND WIDTH control must be set at 3 for maximum accuracy. The S-meter is disconnected from the diode load circuit when the AVC-MANUAL switch is at MANUAL.
d. Bias Voltage Dividfr. Fixed bias voltage is obtained from the power supply (par. 7\d) and applied to a voltage divider composed of 3,000-ohm resistor R40, 1,700-ohm resistor R39, and 300-ohm resistor R38. The voltage drop across 300-ohm resistor R38, bypassed by 0.25-mf capacitor C30, is applied as fixed bias to the mixer and 3d i-f stages. A 50,000-ohm SENSITIVITY control, R56, is connected across the series combination of R40 and R39.
e. A-V-C Operation. With the AVC-MANUAL switch at AVC, the a-v-c control voltage is obtained from the junction of R52 and R19 and applied to the r-f stages and the first two i-f stages through the AVC-MANUAL switch and the i-f filter consisting of 1-meg resistor R52 and 0.05-mf paralleled capacitors C56 and C18B. This resistor-capacitor combination establishes the time constant of the a-v-c circuit. The grid resistors and capacitors in the controlled r-f and i-f stages also affect the time constant. The 2-meg resistor, R19, provides an interconnection between the avc and the fixed bias circuit. The SENSITIVITY control can therefore be used to adjust the gain of the receiver during a-v-c operation. This feature is very helpful in a noisy location. When avc is used for c-w code reception, a longer time constant is desirable and is secured by adding a 0.25-mf capacitor C17 in parallel with C56 and C18B. This additional capacitor is connected to one pole of the SIGNAL MOD-CW switch, SW3 (fig. 32), which controls the beat oscillator. Therefore, when the beat oscillator is turned on for c-w code reception, capacitor C17 is automatically added to the a-v-c system.
/. Manual Operation. With the AVC-MAN-UAL, the a-v-c line is disconnected from the a-vrc rectifier output and connected directly to the contact arm of the SENSITIVITY control, R56. Bias is then applied to the two r-f stages and the first two i-f stages. The bias is adjusted by means of the SENSITIVITY control and filtered by capacitor C18B. The bias cannot be reduced below the voltage drop (3 volts) across R38, and this voltage is applied as fixed bias to the mixer and 3d i-f stages.
g. Circuit Differences. The a-v-c, bias supply, and S-meter circuits are identical in Radio Receivers BC-779-(*), BC-794-(*), BC-1004-(*), and R-129/U (figs. 56, 57, 58, and 59), except that Radio Receiver BC-1004-D does not contain sensitivity meter Ml and capacitor C30 is included only in Radio Receiver BC-779-(*).
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