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D.  The flight interphone system hand held mikes, boom mikes, and oxygen mask mikes provide the mike audio inputs from the flight crew to the remote electronics unit. The MIC SELECTOR switches on each crew station audio selector panel determine which one of the communication or interphone systems will receive the mike input for transmission.
NOTE:  HOT MIC cockpit voice recording is used. With HOT MIC recording, audio is recorded on the cockpit voice recorder from boom or oxygen mask microphones, whichever is selected at each crew member station, regardless of any switch positions.
E.  System audio is activated by the MIKE SELECTOR switches or by audio monitor controls below the MIC SELECTOR switches.
F.  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
A.  The audio selector panels contain MIC SELECTOR switches audio monitor controls and other switches.
(1)  The MIC SELECTOR switches across the top of the audio selector panel, determine the communication system to be operated. A switch is provided for each airplane radio communication system and both FLT (flight) and SERV (service) interphone systems. Only one switch can be closed (depressed) at a time. A second selection will cancel any previous selection. The switch depressed will be noticeably lower and backlighted to indicate the selection made. The system selected by the MIC SELECTOR will be in transmit whenever a microphone at that crew station is keyed with the R/T - I/C switch on the audio selector panel in the R/T position or the control wheel MIC - I/C switch in the MIC position. When no microphone is keyed at that crewman station the crewman will be in the receive mode for the selected communication system.
5C8
Jun 20/88 23-53-02 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

Audio Selecting System Component Location  5C8 
23-53-02  Figure 1  Jun 20/88 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

(2)  
The R/T - I/C switch is a system keying switch. When in the R/T position the switch will key the communications system selected by the MIC SELECTOR into transmit. When in the I/C position the switch will key the flight interphone system into transmit regardless of the MIC SELECTOR selection. This switch is spring loaded to center off.

(3)  
The BOOM/MASK switch will determine which flight crewman mike will provide the audio for transmission. This switch in the MASK position will disable muting of the pilots overhead speakers.

(4)  
The audio monitor switches are two rows of rotary on/off volume control switches in the center of the audio selector panel. The switches are push-on/push-off and rotate clockwise for increasing volume. A switch when depressed will be backlighted to indicate the selection has been made. Any number of audio switches may be closed at one time permitting monitoring of more than one system.

(5)  
A Voice-Both-Range filter switch connects audio filters for the NAV and ADF radio received signals. In Voice, the 1020 Hz coded identification signals are filtered out leaving the broader voice spectrum essentially unchanged. In Range the 1020 Hz coded identification signals are passed while the remaining audio is attentuated. In Both, all audio is passed.

(6)  
The SPKR switch is the off-on/volume control for the pilots overhead speaker. This control is not used on the observer audio selector panel.

(7)  
The ALT/NORM mode selector switch is placed in NORM for normal operation. When in the ALT position the normal selection process is inhibited and a predetermined MIC selection is provided. The captain's PTT will key the number one VHF radio and the F/O's PTT will key the number two VHF radio. No navigation radio audio or overhead speaker audio is available.

B.  Keying (Mode Selector in NORM)
(1)  After the desired communications system has been determined the system can be made to transmit with any of the following PTT features:
(a)  
placing the R/T - I/C switch on the same audio selector panel to the R/T position.

(b)  
plugging a hand mike into the hand mike jack at the same crew station as the audio selector panel and keying the mike.

(c)  
placing the MIC - I/C switch to the MIC position on the control wheel of the same crewman as the audio selector panel (captain and first officer only).

C.  Keying (Mode Selector in ALT)
(1)  
In the ALT mode, the mike and audio selection for this crewman's station is inoperative. Only one VHF radio is operative at this crewman's station regardless of switch selection.
　
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