(e)  
Set the microphone selection on the ASP to the flight interphone microphone.

(f)  
Push the Control Wheel PTT switch to the INT position.

(g)  
Make a voice announcement.
1) Make sure you hear the announcement.

(h)  
Release the PTT switch. 1) Make sure the switch goes to the (center) off position.

(i)  
Put the ASP back to the usual condition.

(3)  
Remove electrical power if it is not necessary (Ref 24-22-00/201).

500 
23-52-21 Page 404  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/98 

AUDIO SELECTING SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
GJ ALL EXCEPT B2509 and B2510 and
Airplanes with Digital Flight Interphone

1. General
A.  The audio selecting system enables personnel in the control cabin to use the various airplane radio and communication facilities to perform normal flight communications and maintenance services. Audio selector panels may be used to complete VHF and HF (if installed) communications, communicate with personnel on the airplane with the interphone systems or make announcements to passengers with the PA system. Airplane VOR (VHF) navigation, marker beacon DME and ADF receivers may also be monitored.
B.  Three audio selector panels are installed on the airplane. The captain's and first officer's audio selector panels are installed on the aft electronic section of the control stand. The observer's audio selector panel is installed on overhead panel P5. The flight interphone system microphones and headphones are connected through jacks to the audio selector panels. All audio selector panel functions are connected in parallel with the paralleling network on electronics rack shelf E2-1 (Fig. 1).
C.  The dual power source for the audio selector panels is obtained from the 28-volt dc battery bus and unswitched electronics bus No. 2. The captain's and observer's audio selector panels power sources are from a single circuit breaker; and the first officer's audio selector panel power source is from a single circuit breaker. The two circuit breakers are on circuit breaker panel P6. The captain's and first officer's audio selector panels also complete power to the flight interphone system loudspeakers.
2.  Switching
A.  Each audio selector panel contains monitoring toggle switches, a rotary mike selector switch, a push-to-talk button, a boom mike/oxygen mask mike toggle switch, a volume control, an isolation amplifier, relays, diodes, filter, resistor circuitry, green indicating lights, and a rotary voice-both-range filter switch.
5C8
Jun 20/88 23-53-0 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

H48738
Audio Selecting System Component Location  5B2 
23-53-0  Figure 1  Sep 20/81 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5C8  Audio Selector Panel Switching 
Jan 20/83  Figure 2  23-53-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  Monitoring toggle switches connect audio signals from the various radio receivers and flight interphone system to the input of the isolation amplifier. Four monitoring toggle switches connect ADF and NAV audio to the rotary voice-range filter switch. ADF and NAV audio is either connected directly to the isolation amplifier input or filtered and then connected to the isolation amplifier input. The NAV-1 and NAV-2 toggle switches are provided with audio from VOR 1 and 2 systems respectively. DME audio is connected through the NAV-1 switch to the isolation amplifier. The marker beacon (MKR) toggle switch connects marker beacon audio directly to the isolation amplifier input. The OXY-BOOM toggle switch connects the oronasal mask mike or boom mike audio high to the audio selector panel relays. The PA toggle switch completes the muting control circuit for the flight interphone system loudspeakers whenever the switch is pushed up and the PA microphone (if installed) on the control stand is used.
C.  The rotary voice-range filter switch connects audio signals to the range filter only when positioned to RANGE or VOICE. The range filter is bypassed when the switch is positioned to BOTH. When in the RANGE position, only the NAV 1020 (VOR) cycle, DME 1350 cycle, and ADF (radio range) morse code tones will be heard. In the VOICE position, NAV and ADF voice will be heard.
D.  Relays complete microphone audio and control circuits to the rotary mike selector switch and complete the ground for muting the flight interphone system loudspeakers. The normally closed contacts of one relay are connected to a handheld mike jack. When the relay is energized, by pressing the OXY/BOOM push-to-talk switch or by using the MIC position of the INT/MIC switch at the captain's or first officer's station, the oronasal or boom mike audio and control ground are completed through the rotary mike selector switch. When the second relay is activated, by using the INT position of the INT/MIC switch at the captain's or first officer's station, the oronasal or boom mike circuit is connected directly to the flight interphone amplifier. The handheld mike push-to-talk button completes the ground for the loudspeaker muting circuits and system control circuits whenever the push-to-talk button is closed.
　
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