23-21-01
 ALL  ú ú 01 Page 401 ú May 28/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
 D.
E.
F.
A
757
MAINTENANCE MANUAL

 (b)  N1304 Encoder - SELCAL Ground Equipment
 Motorola Inc
 1000 Mittel Drive
 Wood Dale, IL 60191

 NOTE: This encoder is used with HF and VHF transmitters as
____
 part of a SELCAL ground Station.

Access
 (1)  
Location Zone 119/120 Main Equipment Center

 (2)  
Access Panels

 119BL Main Equipment CenterProcedure
 S 424-012
 (1)  
Install the SELCAL decoder (AMM 20-10-01/401).

 S 864-011

 (2)  
Remove DO-NOT-CLOSE tag and close this circuit breaker on the P11 panel:

 (a)
11G3, SELCALSELCAL decoder installation test:

 S 864-003

 (1)  
Supply electrical power (AMM 24-22-00/201).

 S 864-008

 (2)  
Have approved SELCAL-coded transmission sent from ground station.

 NOTE: If ground station is not available, do a SELCAL-coded
____ transmission with the SELCAL encoder. Make sure the transmissions are approved.
 S 204-007
 (3)  
Make sure applicable SELCAL light and chime come on.

 (4)  
Push applicable SELCAL switch.

 (5)  
Make sure the SELCAL light goes off.

 (6)  
Remove electrical power if it is not necessary (AMM 24-22-00/201).

 S 864-005
 S 204-004
 S 864-009
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 23-21-01
 ALL  ú ú 01 Page 402 ú May 28/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757 MAINTENANCE MANUAL
 PASSENGER ADDRESS SYSTEM - DESCRIPTION AND OPERATION
____________________________________________________
 1.  General_______
 A.  The passenger address (PA) system provides voice announcements, boarding music and chime signals for the passengers.
 B.  The PA system consists of a PA amplifier, tape reproducer and loudspeakers. The speakers are located in the PSU, galley, lavatory and attendant areas.
 C.  The PA amplifier gets power from the 28v dc battery bus, through a circuit breaker on the overhead circuit breaker panel, P11.
 D.  The boarding music tape reproducer gets power from the 28v dc right bus, through a circuit breaker on the overhead panel, P11. It also gets power from the 115v ac right utility bus, through a circuit breaker on the miscellaneous electrical equipment panel, P37.
 2.  Component Details (Fig. 1)_________________
 A.  Passenger Address Amplifier
 (1)  
The passenger address (PA) amplifier is a 2 MCU shelf-mounted unit weighing approximately 6.5 pounds. It is located on the E4-3 shelf in the main equipment center. Cooling air is from the equipment cooling system via inlet and exhaust ports at the bottom and top of the unit, respectively.

 (2)  
The PA amplifier provides audio power amplification for driving the passenger cabin speakers. Separate outputs provide audio to speakers and sidetone to the audio selector panels (ASP's).

 (3)  
Voice announcements and boarding music are amplified from priority inputs. Chime signals are generated from discrete inputs. External inputs automatically increase the PA volume level for flight mode operation and decompression situations.

 (4)  
The Pacific Electro Dynamics (PED) PA amplifier front panel has a three-position test switch, a five-position LED output display, an OVERLOAD light, MASTER GAIN and AUX GAIN potentiometers, and LOAD jack. The test switch provides a means for amplifier and speaker checks. The output display indicates output power during the test mode. The OVERLOAD light indicates when the PA amplifier is automatically current limiting. The MASTER GAIN and AUX GAIN are for bench adjustments. The LOAD jack is provided for using a test set to simulate a speaker load (not used).

 (5)  
The Collins PA amplifier front panel has a three character LED for test meter readouts. A four position function switch is spring loaded from LOAD back to TONE and from LEVEL back to OPERATE. The LED is off when the TONE and OPERATE positions are selected. In the LOAD position, the amplifier measures the magnitude of the load impedance at 587 Hz. In the TONE position, a 587 Hz test tone is produced in the cabin and at attendant stations. In the LEVEL position, the switch measures main power amplifier output at 587 Hz into an internal load with the speakers disconnected as a reference for setting the gain controls. Normal operation of the PA amplifier requires the switch to be in the OPERATE position.
　
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