Service Interphone
A total of twenty-two hand set jacks are located in various parts of the airframe in order that ground
crew can communicate with one another using the service interphone system. The system is rather simpler than those considered above. Mic. audio from all handsets with press to talk depressed are combined in and amplified by the service interphone amplifier in the interphone audio accessory box. The amplified signal is fed to all handset tels. Volume control adjustment is provided by a preset potentiometer. With the flight engineers interphone switch selected to ON the input summing networks for both service and flight interphone systems are combined. All mic inputs from either system are amplified and fed to both systems
Passengers address'
The systemc omprises three PA amplifiers tape deck, annunciator panel, attendant's panel, PA accessory
box, control assemblies speakers witch panel and fifty-three loudspeakers The various PA messages
have an order of priority assigred to them pilot's announcemets attendants announcements, prerecorder danno uncements and finally boarding music. All PA audio is broadcast over the speaker system and also, except for boarding music, over rides
entertainment audio fed to the passengesr tethoscope headsets A prerecordede mergency announcement
may be initiated by the pilot or an attendant, or automatically in the event of cabin decompression'
A chimei s generated when the pilot tums on 'fasten seat-belto' r 'no smoking' siglts. The passenger address amplifiers are fed via the flight or cabini nterphone systems for pilot oratiendant announcements respectively. Distribution of audio from the amplifiers to the speakers in variouszones depends on the class configuration, sinces omenoun. it ntnts may be intended for only a certain class of passengers. The necessaiy distribution is achieved by means of switches on the speakers witching panel. Audio is also fed to the flight interphone system for side tone purposes. Number 2 and number 3 ampliliers ere slaved to number I for all'class announcements. Should separate class announcement bse required the parallel control relayis energizeds, oseparating the number I audio from that of number 2 and 3. The control assemblies in the PA accessory box contain potentiometers used to set the gain of the PA amplifiers. When the aircraft is on the ground with gear locked down and ground powgl applied the lev-efof speaker audio is reduced by 6 dB' The tape deck contains up to five tape cartridges apartf rom the necessary tape'{rive mechanism' playback head and a pre-amplifier' Boarding music is Ltectea at an attendint's panel while prerecorded imnouncements are selected by means of twelve push buttons on the annunciator panel'
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