Audio receiving circuits.

Troubleshoot and Repair Communications System Control
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(1)

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.

(2)

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

(3)

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.

(4)

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

impedance of the audio input lines or operation of the communications control.

(a)

SELECTOR switch action.

When transmitter 1 is selected by setting

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.

(b)

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.