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D.  Power is obtained through circuit breakers on load control center P6. Audio selector panels obtain 28-volt dc power from the dual power source of unswitched electronics bus-2 and battery bus.
2.  Switching (Fig. 2)
A.  The microphone selector switches connect microphone high and low to the selected communication system. The switches are mechanically interlocked so that only one switch can be depressed at a time. Headphone audio is selected by combination on-off/volume controls. Push the audio switch in to turn it on and push it again to turn it off. Rotate the knob to adjust volume. No audio from any system can be heard at a station until the audio control for the associated system is turned on.
B.  If a boom microphone or oxygen mask microphone is used for voice transmission, that microphone must first be selected by the BOOM-MASK switch. Voice transmission can then take place after a PTT switch is actuated. The handheld microphone does not require prior selection and contains its own PTT switch.
Sep 20/81 23-53-01 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

H44669
Flight Interphone System Component Location  5C8 
23-53-01  Figure 1  Jun 20/85 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

505  Audio Selector Panel Switching 
Oct 20/83  Figure 2  23-53-01 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

C.  Relays connect the boom or oxygen mask microphone to the system selector switches. The relays are controlled by the PTT switch on the audio selector panel and by the remote PTT switch at each crewmember's station.
D.  The INT microphone selector switch is for the flight and/or service interphone systems. The microphone input and audio for the flight interphone system is also controlled by the INT audio switch. The microphone input and audio for the service interphone system is also controlled by the SERV INT audio switch.
E.  When the I/C-R/T PTT switch on the audio selector panel is set to the I/C position, the previously selected boom or oxygen mask microphone is connected directly to the flight interphone amplifier.
F.  Access to the flight interphone system can also be obtained by setting either pilot's control wheel PTT switch to the INT position. This causes a relay in the audio selector panel to energize and connect the previously selected boom or oxygen mask microphone directly to the flight interphone amplifier.
G.  When a microphone selector switch is actuated, the crewmember's microphones will be connected to that system when the appropriate PTT switch is actuated. When the PTT switch on the handheld microphones is actuated, the handheld microphone will be connected directly to the system selected on the audio selector panel.
H.  When the I/C-R/T PTT switch on the audio selector panel is held to the R/T position, a relay in the panel is energized to connect the selected boom or oxygen mask microphone to the system selected on the audio selector panel.
I.  Access to the selectable systems can also be obtained by holding either pilot's control wheel PTT switch to the MIC position. This will cause the same relay to energize and connect the boom or oxygen mask microphone to the selected system.
J.  All flight compartment PTT switches are connected to a speaker mute line so both cockpit speakers will be muted whenever a PTT switch is actuated. This prevents audio feedback from the speakers to the microphones.
K.  The filter switch connect audio signals from the navigation receiver to the voice-range filter. The switch allows the selection of voice only signals (V), range only signals (R), or both voice and range signals (B).
L.  An amplifier is contained in the audio selector panel to provide audio signals to the output circuits.
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AUDIO SELECTING SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
Airplanes with Digital Flight Interphone
1. General
A.  The audio selector panels enable the control cabin crewmembers to transmit or receive on the radio communication systems, communicate using the flight and service interphone systems or make announcements over the PA system. Airplane VHF navigation, DME, marker beacon and ADF receivers may also be monitored.
B.  An audio selector panel is provided at the flight crew station for the captain, first officer and first observer.
C.  Audio selector panels are located on the aft control stand at the captain's and first officer's stations. The observer's audio selector panel is located on the aft overhead panel P5.
　
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