All receiver signals or any combination of signals may be monitored simultaneously by
operating the associated MONITOR switch to the ON position. Operation of MONITOR
switches S3 through S7, associated with receivers 1 through 5, respectively, disconnects the
junctions of R42-R43, R44-R45, R46-R47, R48-R49, and R50-R51 from SELECTOR switch
S1B front, breaking the ground circuit. The audio signals are applied to the operator's headset
through resistors R42, R44, R46, R48, R50, VOL control R34, and headset amplifier AR2.
Operation of AUX switch S8 removes ground from the junction of R52-R53 permitting
amplification of the AUX receiver audio signal. NAV switch S9 controls the input from NAV-1
and NAV-2 receivers. Resistors R55, and R56 connect the NAV-1 and NAV-2 lines,
respectively, to monitor switch S9 and mixing resistor R54. Operation of NAV switch S9
removes the ground from the junction of R54-R55-R56 allowing amplification of the NAV
receiver inputs. The mixing resistors act to provide the same output at the headset for all
levels of input signals applied to the communications control.
f.
Protective device and filter assembly VR1. This assembly filters the +27.5-volt
DC input, preventing it from introducing noise in the amplifiers of the communications control,
and protects the communications control from damage caused by application of reverse
polarity transients up to 80 volts. In addition, it maintains performance of the communications
control within specified limits for variations of input voltages from +21 to +29 volts and prevents
possible short circuits in the communications control from damaging other equipment in the
aircraft. Input is applied to the junction of R57 and the collector of transistor Q8 through
protective diode CR11. Diode CR11 prevents damage to the communications control if the
27.5-volt input polarity is reversed. Resistor R57, connected to zener diode CR10, supplies
reverse current to CR10 and maintains it in its constant voltage range of operation. Zener
diode CR10 is connected to the base of regulating transistor Q8 and holds it at constant
voltage when the input power varies from 21 to 29 volts. The emitter of Q8 supplies a constant
voltage (approximately 20 volts) to the amplifiers in the communications control when the input
power voltage varies from 21 to 29 volts. If the supply voltage drops from 27.5 to 21 volts, the
voltage at the collector of Q8 will fall, and the voltage at its emitter will also tend to fall. When
the emitter voltage decreases, the base to emitter voltage of Q8 decreases causing its
collector resistance to decrease. This decrease in collector resistance tends to maintain the
voltage at the emitter of Q8 constant. Capacitor C23 acts to filter ripple voltages which may
appear on the power line and limit the amplitude of ripple that is applied to the amplifiers in the
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