Audio receiving circuits.
Direct inputs 3 and 4. The audio signals present on these circuits are
continuously monitored. The signal passes through mixing resistors R41 and R40 and VOL
control R34 to headset amplifier AR1 and then to headset H-101A/U via T4.
Direct inputs 1 and 2. The audio signals from these receivers are continuously
monitored. Their level is not controlled by VOL control R34. The signal is applied to the
headset through resistors R38 and R39, transformer T3, headset amplifier AR2, and
Interphone. The interphone line connects to all communications controls in the
aircraft and is monitored at all times without any operator action required. Audio on the
interphone line is applied to headset amplifier AR2 through resistors R33 and R32, VOL
control R34, and transformer T3.
Receiver monitoring. Audio from the receivers is applied to a resistor mixing
network consisting of resistors R42 through R51. With all monitor switches set to the OFF
position, junctions of resistors R42-R43, R44-R45, R46-R47, R48-R49, R50-R51, from
receiver audio inputs 1, 2, 3, 4, and 5 respectively, are connected to SELECTOR switch S1B
front. With the SELECTOR switch set to ICS, all junctions are grounded, effectively preventing
audio signals from these receiver audio input lines from being applied through the headset
amplifier to the operator's headset. Resistors R42 through R51 are high in relation to the
receiver 150-ohm audio input lines source impedance and the 100-ohm input impedance of the
volume control network. Grounding these junction points does not affect audio levels or load
SELECTOR switch action.
SELECTOR switch S1 to position 1, the ground circuit at the junction of R42 and R43 is broken
and permits audio from the associated receiver 1 to be applied, through resistor R42, VOL
control R34, and headset amplifier AR2, to the operator's headset. The other transmitters and
associated receivers are selected in the same manner.
RECEIVER switch action. Seven toggle switches permit monitoring of eight
receiver audio input signals. Switches designated 1, 2, 3, 4, 5, and AUX control monitoring of
any one of six receivers. The NAV switch controls monitoring of two receivers.