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5.  Handheld Microphones
A.  The handheld microphones contain PTT switches which complete audio circuits and provide a ground for control circuits. The PTT switch must be pressed before messages are begun or no transmission can take place. The handheld microphones are plugged into jacks at each flight crewmember's station.
6.  Boom Microphones
A.  The boom microphones are part of a small headset with a microphone. The microphone is placed in front of the mouth by means of a boom connected to the headset. A control wheel or audio selector panel PTT switch must be pressed before transmission can take place. The boom microphone headset is plugged into jacks at each flight crewmember's station.
7.  Oxygen Mask Microphones
A.  The oxygen mask has a microphone built into the mask. Each mask is equipped with an oxygen connection and a microphone plug. A control wheel or audio selector panel PTT switch must be pressed before transmission can take place. The mask microphone is plugged into jacks at each flight crewmember's station.
8.  Push-To-Talk Switch
A.  Push-to-talk (PTT) switches are located at all flight interphone stations. The handheld microphone has its own PTT switch, and the control wheels have PTT switches and audio selector panels have R/T switches for the boom and oxygen mask microphone keying circuit. The switch must be pressed before messages are begun or no transmission can take place. Audio and control circuits to the audio selector panel are completed when the switch is pressed.
9.  Operation
A.  Flight compartment crewmembers access the flight interphone system to communicate with each other by positioning the flight interphone keying switch on their audio selector panel to the key position. The push-to-talk function can also be performed at either pilot's station by setting the control wheel PTT switch to the I/C position.
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's station, regardless of the position of their PTT switches or MIC SELECTOR selection.
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B.  Flight compartment crewmembers key a selected airplane communication radio system from an audio selector panel by depressing the system MIC SELECTOR switch and then holding the radio transmitter keying switch to the key position. The PTT function can also be completed at either pilot's station by holding the control wheel PTT switch to the R/T position. All MIC SELECTOR switches are mechanically interlocked to prevent dual operation. The MIC SELECTOR switch also enables the radio system audio. Audio volume is adjusted at the system audio control.
C.  Handheld microphones contain their own PTT switch. When connected to a mike input connection, pressing the switch will connect the microphone and control circuits to the system selected on the corresponding audio selector panel.
D.  The captain's and first officer's loudspeaker is turned on or off by a switch on the crewman's audio selector panel. The audio output from the loudspeaker is adjusted to a comfortable listening level with this volume control on-off switch.
E.  Ground crew personnel access the flight interphone system by plugging a handset into the FLIGHT INTERPHONE jack on external power receptacle panel P19. This allows the ground crew to communicate with the flight crew on the flight interphone system.
F.  Each crewman can select audio from navigation radio receivers for monitoring at his crew station. A push on/push off switch - volume control for each navigation radio receiver is provided. Dual operation of navigation radio receivers is permitted. This audio can be filtered to attenuate the range signals by placing the range switch to the V (voice) position. The range signal can be enhanced by placing the filter switch to the R (range) position. In the B (both) position no filtering is provided.
G.  The ALT/NORM switch is normally placed in the NORM position. In the ALT position the MIC selection is fixed for that crewman station with only one VHF radio working. With the corresponding crewman station switched to ALT, the captain or observer would be connected to VHF-1 or the F/O to VHF-2 with only headphone audio being received from the working radio at preset volume. Either boom or oxygen masks could be used and the working radio keyed from the control wheel or audio selector panel PTT. The ALT or NORM position of the switch affects only the communications of the crewman for that audio selector panel. 
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FLIGHT INTERPHONE SYSTEM - ADJUSTMENT/TEST
　
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