(8)  
Secure HF and VHF systems.

(9)  
Determine whether there is any further need for electrical power on the airplane; if not, remove external power.

5C8 
23-28-0 Page 502  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Jun 20/88 

PASSENGER ADDRESS SYSTEM - DESCRIPTION AND OPERATION
1. General (Fig. 1)
A.  The passenger address (PA) system provides means of communicating with the passengers and entertaining them with recorded music played over the speakers in the passenger compartment. It also provides chime signals for the Passenger Call Systems (Ref Chapter 33), Flight Crew Call System (Ref 23-43-0), and Passenger Signs (Ref Chapter 33) to attract the attention of flight attendants as well as passengers. Announcements may be made using any of the three hand microphones or using the microphones and audio selector panels in the flight interphone system.
B.  The passenger address system consists of a PA amplifier, loudspeakers, microphones, a tape reproducer, and interconnecting wiring.
C.  Power for the system is 28 volts dc and 115 volts ac provided through circuit breakers on the P6 panel.
2.  Passenger Address Amplifier
A.  The passenger address amplifier provides high-quality audio power to the speaker system of the airplane. It includes microphone inputs for use by the pilot and cabin attendants and two balanced inputs for use with tape reproducers for announcements and programmed music. The amplifier has high, high/low, and low chimes for use by the pilot and passengers.
B.  On GUN/XIA ALL EXCEPT B-2501-B-2503, B-2511, B-2512, B-2514-B-2516, and B-2524, the amplifier unit consists of a two-level chassis with an attached front panel and dust cover. A three-position rotary switch and three red LED's are located on the front panel. The center position of the switch (NORM) is used for normal operation with the red LED's connected across the output load. The spring-loaded CAL position allows the unit to be tested for a predetermined output and supplies a tone for checking the speakers.
C.  On GUN/XIA B-2501-B-2503, B-2511, B-2512, B-2514-B-2516, and B-2524, the PA amplifier is a 1/4 ATR short, shelf-mounted unit located on the electronic equipment rack E2, shelf 1. It contains solid-state components located on printed circuit card assemblies and a power supply. Two front panel switches and a meter enable unit and system self-test. Both switches are spring-loaded to the NORMAL (up) position. A multipin connector on the back panel provides signal interface with other units.
5C2 
Oct 20/89  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  23-31-0 Page 1 

K90172
Passenger Address System Component Location  5C2 
23-31-0  Figure 1 (Sheet 1)  Oct 20/89 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5C2  Passenger Address System Component Location 
Oct 20/89  Figure 1 (Sheet 2)  23-31-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

D.  The amplifier uses silicon semiconductors to provide 30 watts of continuous audio power to an 83-ohm loudspeaker system. It accepts audio signals from microphones, tape reproducers, or other call/alert devices. It amplifies them for reproduction through a loudspeaker system and through headphones for sidetone.
E.  The five inputs have individual level controls and solid-state priority switching. Photo switches are used in the input for dc voltage control of the audio inputs. Thus, key click is eliminated and isolation between control circuits and audio paths is provided. The pilot (flight compartment) announcements are the highest priority. The attendant announcements are the next highest priority. Prerecorded announcements (airplanes with prerecorded announcements) are the next highest priority. The lowest priority input is the boarding music. Boarding music is interrupted by prerecorded (airplanes with prerecorded announcements), attendant or pilot announcements. Pilot announcements override attendant announcements and interrupt prerecorded announcements (airplanes with prerecorded announcements) and boarding music. A compression amplifier, used with all microphone inputs, ensures an equal audio output level even though different levels are applied to the input.
F.  The volume level of the audio system is changed to overcome the variations between ground and flight noise by an engines on/off control switch which is installed to operate automatically. Operation of this switch results in a reduced audio output when the engines are off.
G.  The master gain control makes it possible to tailor the gain of the amplifier to a particular aircraft according to its size and acoustical characteristics. A 5-watt auxiliary amplifier in the amplifier minimizes acoustical feedback from the attendant's station and also allows desired chime tones to be sounded only in the attendant's speakers.
　
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