23-31-00
ALL ú ú 08 Page 4 ú May 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
G. Microphone Jack
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3. Operation
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A. Functional Description
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(1) General (Fig. 2)
(a)
The PA system supplies boarding music, entertainment audio, voice messages, and chime signals to all passengers and attendants in the passenger cabin through various speakers. Also, PA sidetone goes to the flight crew through the audio selector panels.
(b)
Each PA input has an order of priority. The pilot (flight compartment) announcements are the highest priority. The attendant announcements are the second highest priority. The third highest priority is reserved for pre-recorded announcements. The lowest priority input is the boarding music. Boarding music is interrupted either by attendant or pilot voice announcements. Pilot announcements will override the attendant announcements.
(c)
The PA amplifier makes high-tone and low-tone chime sounds in response to discrete signals from the cabin interphone system (AMM 23-42-00). In that system the audio accessory unit sends different combinations of discrete signals to the chime inputs of the PA amplifier to signal different types of interphone calls.
(d)
Discrete signals from the passenger signs system cause a low tone chime signal to sound on all speakers. A single low chime sounds whenever the NO SMOKING or the FASTEN SEAT BELT signs are switched on or off.
(e)
Voice announcements from the pilots are made via the flight interphone system (Ref 23-51-00). The PA amplifier supplies sidetone to the pilots' headset during voice announcements. Voice announcements from the attendants are made via the cabin interphone system (Ref 23-42-00). Amplification by the PA amplifier is enabled by PTT inputs.
(f)
The PA amplifier has circuitry to provide system testing. All PA speakers can be checked. Output levels of the amplifier can be determined to allow for adjustments.
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23-31-00
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757
MAINTENANCE MANUAL
C26937
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757
MAINTENANCE MANUAL
C26869
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(2)
(3)
(4)
757
MAINTENANCE MANUAL
(g)
The system controls amplifier gains and compensates for increased noise levels when the oxygen masks are deployed and whenever either engine is operating.
AIRPLANES WITH PED AMPLIFIERS; Passenger Address Audio
(a)
Each audio input into the PA amplifier has a PTT control signal. Priorities within the PA amplifier are in numerical order with No. 1 as the highest priority input.
(b)
Compression and amplification circuits within the PA amplifier have internal and external adjustments. The MASTER GAIN and AUX GAIN on the front panel set the sound levels. The PA amplifier is capable of providing up to 120 watts from the main output and up to 20 watts from the auxillary output.
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