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D.  Headphone, handheld microphone, and oxygen mask microphone jacks are installed at the first observer's station. A headphone jack for a second observer is provided and is paralleled to the first observer's headphone jack inside the first observer's audio selector panel. An oxygen mask and boom microphone PTT button is on the first observer's audio selector panel (Fig. 1).
E.  The flight interphone amplifier and loudspeaker assemblies use a 28-volt dc dual power source from the battery bus and unswitched electronics bus No. 2. Circuit breakers are on load control center P6. The loudspeaker assemblies use 28 volts dc from the same circuit breakers that supply power for the audio selector panels (Fig. 2).
5C8
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H43826
Flight Interphone System Component Location  5C8 
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5C8  Flight Interphone System Circuit 
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2.  Microphones
A.  Handheld microphones incorporate a transistorized preamplifier within them for amplification of audio signals to a level suitable for use by the various communication systems. A push-to-talk switch on the microphone will complete power circuits to the preamplifier and complete systems control grounds whenever the switch is pressed. The handheld microphones are equipped with plug connectors so that they may be easily replaced or used at any station which has a handheld microphone receptacle. Hooks are provided at each station for microphone stowage.
B.  The oxygen masks have a carbon microphone built into the mask. Each mask is equipped with an oxygen connection and a microphone plug. The PTT switch on an audio selector panel may be used to complete mask microphone audio and control circuits to the system selected on the audio selector panel. The MIC-INT switch at the crewmember's station may be used to complete circuits to the selected system or directly to the interphone system without prior interphone selection on the selector panel. Stowage bins for the masks are provided at each station.
C.  The earphone/boom microphone headset incorporates an earphone and a microphone connected to an adjustable headband. The PTT switch on an audio selector panel may be used to complete the headset assembly microphone audio and control circuits to the system selected on the audio selector panel. The MIC-INT switch at the crewmember's station may be used to complete circuits to the selected system or directly to the interphone system without prior interphone selection on the selector panel. The headset assemblies are stowed on a hook at each station.
3.  Headphones
A.  Magnetic headphones equipped with plugs enable crewmembers to monitor audio signals directed through the audio selector panels. Headphones with an impedance of approximately 600 ohms may be used at any station equipped with a headphone jack. Headphones are stowed on hooks at each station.
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4.  Loudspeaker Assemblies
A.  Each loudspeaker assembly consists of a transistorized amplifier, speaker, muting circuits, an on-off switch and a volume control. The on-off switch and volume control are mounted on the case of the assembly where they are convenient to the pilots. Audio input to the amplifier is paralleled from the associated pilot's headphone circuits. Muting circuits attenuate the loudspeaker output whenever a push-to-talk button is pressed at the captain's, first officer's or observer's station to prevent audio feedback into the systems. The loudspeaker will also be muted if the PA microphone on the control stand is used and the PA monitor switch on the captain's, first officer's or observer's audio selector panel is turned on. The loudspeaker amplifier uses 28 volts dc obtained from the associated audio selector panel.
5.  Amplifier
A.  A transistorized amplifier module is installed in the audio accessory unit. The amplifier has two separate input and output circuits, thus providing isolation between the jack on the external power panel and the remainder of the flight interphone system. Design of the amplifier permits a temporary short to occur on either one of the input or output circuits without catastrophic failure of the amplifier. Either carbon or transistorized microphones may be used with the amplifier. The amplifier uses 28 volts dc and completes the necessary dc voltage to the systems microphones. Since no on-off switch is provided for the amplifier, it will be operable whenever the battery bus or unswitched electronics bus No. 2 are hot.
　
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