(4)
All flight compartment PTT switches are connected to a speaker mute line so both cockpit speakers will be muted when a PTT switch is actuated except when the MASK/BOOM switch is in the MASK position or oxygen mask is in use. This prevents audio feedback from the speakers to the microphones.
(5)
All audio warning messages from ground proximity and altitude alert are heard by all flight crewmembers without being selected on the audio selector panel.
Oxygen Mask Microphone Selection
(1)
The oxygen mask microphone can be selected by setting the BOOM/MASK to MASK position on the ASP. Cockpit speakers cannot be muted when oxygen mask is selected.
Verification of Radio Navigation Identification
(1)
Identification of navigation ground station is accomplished by selecting the appropriate navigation selector switch, pushing and turning the switch to adjust the volume.
(2)
Selecting the Voice-Both-Range (V-B-R) filter switch connects audio filters for the NAV and ADF radio received signals. In Voice (V), the 1020 Hz coded identification signals are filtered out leaving the broader voice spectrum essentially unchanged. In Range (R), the 1020 Hz coded identification signals are passed while the remaining audio is attenuated. In Both (B), no filters are used.
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737-300/400/500MAINTENANCE MANUAL
I. Alternate Mode
(1)
The audio selector panel controls are functional when the ALT/NORM switch is placed to the NORM position.
(2)
When the ALT/NORM switch is placed in the ALT position, the panel controls do not function and are not usable for selections or adjustments. Instead there is a limited fixed selection independent of selector switches or volume controls.
(3)
Only one VHF communications radio is usable at any flight crew station with the audio selector panel positioned to ALT. In the case of the captain or observer station the VHF radio is VHF No. 1. In the case of the first officer station the VHF radio is No. 2. Only headphone audio, sidetone or receiver audio from the designated VHF radio, would be available at this crewmember's station.
(4)
This limited fixed capability is accomplished by switching action of the ALT/NORM switch or power failure to the corresponding station card in the remote electronics/audio accessory unit.
J. Cockpit Speaker Volume Control
K. Communication between Flight Deck and Ground Service Crew
(1) 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.
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737-300/400/500MAINTENANCE MANUAL
FLIGHT INTERPHONE SYSTEM - TROUBLE SHOOTING
___________________________________________
1. General_______
A. Trouble shooting the flight interphone system is easy due to the 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. The amplifier is replaced by replacing the audio accessory unit or remote electronics unit.
B. Interphone speaker failure can usually be detected by comparison between headphone audio and the speaker 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 first making a comparison with the PTT switch on the audio selector panel or the PTT switch on the handheld microphone. If microphone can be keyed using the control panel PTT switch and the handheld microphone PTT switch but not when using the control wheel PTT switch, the control wheel PTT switch is defective.
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