C.  If an oxygen mask microphone or boom microphone is used at the captain's or first officer's station, the MIC-INT push-to-talk switch on the outboard horn of their control wheels may be used to complete the microphone circuits. Pressing the MIC side of the switch connects the microphone circuits to the system selected on the associated audio selector panel. Pressing the INT side of the switch connects the microphone circuits directly to the flight interphone system amplifier regardless of the system selected on the audio selector panel. However, if the flight interphone system has not been selected on the audio selector panel, the flight interphone monitor toggle switch must be pushed up to supply flight interphone audio to the associated headset. The push-to-talk switch on the audio selector panels may be used at either station to connect oxygen mask microphone or boom microphone circuits to the system selected on the audio selector panel. All push-to-talk switches at both stations are spring loaded and will return to the off position when released.
D.  If an oxygen mask microphone is used at the observer's station, the PTT button on the observer's audio selector panel is used to connect the microphone circuits to the system selected on the audio selector panel.
E.  The pilots' loudspeakers may be turned on or off with a switch on the loudspeaker assembly case. Loudspeaker output may be adjusted to a comfortable listening level with the volume control adjacent to the on-off switch. Both loudspeakers will be muted if the captain, first officer or observer uses his microphones. The loudspeakers will also be muted if an announcement is made using the PA microphone on the control stand (when installed) while monitoring the PA system at the captain's, first officer's, or observer's audio selector panel.
F.  Ground personnel may communicate with control cabin crewmembers through the flight interphone system by using a handset plugged into the flight interphone jack on external ground power receptacle panel P19.
G.  A flight interphone amplifier gain potentiometer on the front of the audio accessory unit may be used to adjust audio signals to a suitable level.
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FLIGHT INTERPHONE SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
Airplanes without Digital Flight Interphone
1.  General
A.  Trouble shooting the flight interphone system is relatively easy due to duplication of components within the system. Interchanging suspected faulty components with known good components will quickly isolate the faulty components. If the flight interphone system is completely inoperable, the flight interphone amplifier is faulty. If the amplifier is faulty, replace the audio accessory unit. 
B.  Interphone loudspeaker failure can usually be detected by comparison between headphone audio and the loudspeaker audio. If audio is not available to either one, the audio selector panel is faulty.
C.  Isolation of a faulty control wheel PTT switch can be determined by a comparison with the control panel PTT switch. If oronasal mask mike audio can be completed using the control panel PTT switch but not when using the control wheel PTT switch, the control wheel PTT switch is faulty. If audio can not be completed with either one, the audio selector panel is faulty.
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Jun 20/88 23-52-0 Page 101
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

FLIGHT INTERPHONE SYSTEM - ADJUSTMENT/TEST
EFFECTIVITY
GJ ALL EXCEPT B2509 and B2510 and Airplanes With Digital Flight Interphone
1.  Test Flight Interphone System
A.  General
(1)  
The following tests should be conducted whenever a system component has been replaced or repaired.

(2)  
Communications and navigation audio, which is monitored with the flight interphone system components, is checked as part of the requirements of the respective communication and navigation systems test.

B.  Prepare to Test
(1)  
Provide power to the airplane and energize busses on load control center P6.

(2)  
Ensure all interphone and the dual power source circuit breakers are closed.

(3)  
Turn on both flight interphone loudspeakers.

C.  Test Flight Interphone System
(1)  
Position the mike selector switch to INT on the captain's, first officer's, observer's and, if installed, electronic equipment compartment audio selector panels.

(a) The green INT light should illuminate on each audio selector panel.

(2)  
Using the handheld mike at the captain's station, establish communications with each station which has an audio selector panel.
　
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