(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.

(a)  
Communications should be loud and clear between stations.

(b)  
Varying the volume control on each audio selector panel should increase or decrease the associated headphone audio level.

(c)  
Varying the volume control on the captain's or first officer's audio selector panel should increase or decrease the associated loudspeaker audio level.

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(d)  
Varying the volume control on a loudspeaker should increase or decrease the loudspeaker audio level.

(e)  
Both loudspeakers should be muted whenever a PTT switch is pressed at the captain's, first officer's or observer's station.

(f)  
Ensure the loudspeakers can be turned off and on.

NOTE:  The second observer has a headphone jack paralleled to the first observer's audio selector panel. If necessary, adjust the flight interphone amplifier gain potentiometer on the front of the audio accessory unit to obtain a comfortable listening level.
(3)  
Select OXY on each audio selector panel and establish communications between the captain's, first officer's and first observer's stations using their oronasal mask mikes. Tap the side of the mike to produce audio signals and press the audio selector panel PTT button to operate the mike.

(a)  
Communications should be loud and clear between stations.

(b)  
Both loudspeakers should be muted whenever a PTT switch is pressed.

(c)  
The PTT switch should be spring loaded to an off position when released.

(4)  
Repeat step C.(3) between the captain's and first officer's stations using their oronasal mask mikes and their control wheel PTT switches held to the MIC position. Results shall be similar.

(5)  
Position the mike selector switch on all audio selector panels to OFF.

(a)  The INT light should extinguish.

(6)  
Select BOOM with the BOOM-OXY switch and push up the INT toggle switch on all audio selector panels.

(7)  
Establish communications between the captain's and first officer's stations using boom mikes and the INT side of the control wheel PTT switches.

(a)  
Communications should be monitored at all stations regardless of the position of the audio selector panel mike selector switches.

(b)  
Both loudspeakers should be muted whenever the PTT switch is pressed.

(8)  
Position the PTT switches to the off position.

(9)  
Using a handset plugged into the FLIGHT jack on external power receptacle panel P19, establish communications with each audio selector panel.

(a) Handset audio should be heard loud and clear with headphones at each station.

(10)
Turn off the interphone loudspeakers, and push down the INT toggle switch on each audio selector panel.

(11)
If no longer required, remove electrical power from the airplane.

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CONTROL WHEEL PUSH-TO-TALK (PTT) SWITCH - REMOVAL/INSTALLATION
1. General
A.  There are three tasks in this procedure. The first task is the removal of a PTT switch from a control wheel.  The second task is the installation of a PTT switch on a control wheel. The third task is a test of the operation of a PTT switch on the control wheel.
B.  Each control wheel has a Push-To-Talk (PTT) switch installed. You can use this procedure for the two PTT switches on a control wheel.
2.  Control Wheel Push-To-Talk (PTT) Switch Removal (Fig. 401)
A.  General
(1) This task removes the captain’s or the first officer’s PTT switch from the control wheel.
B.  Access
(1)  Location Zone
　
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