1.  General
A.  The airplane communication systems consist of: high frequency (HF), very high frequency (VHF) communication systems, selcal system, passenger address (PA) system, service interphone system, flight and ground crew call systems, flight interphone system, audio selecting system, and voice recorder system. They provide facilities for air/ground and air/radio communication, a system for recording such communications, plus air-crew intercom, air crew/ground crew intercom, and passenger address and entertainment systems (Fig. 1). SELCAL is a passive radio alerting system.
B.  The major components of the HF, VHF, Selcal, PA and interphone systems, including audio accessory unit and paralleling network, are installed on electronic equipment rack E2 in the electronic equipment compartment. The voice recorder is in the aft cargo compartment. Systems control panels are installed on the control stand and overhead panel P5 where the pilots' have convenient access to them. External entrance into the electronic equipment compartment may be gained through an access door located immediately aft of the nose wheel well.
C.  The audio accessory unit contains modules for various communication systems. Amplifier modules for the flight interphone system and service interphone system are installed in the unit. A module for the passenger address system contains diodes and loading resistors. Loading resistors are installed on a module for systems which may be monitored through the audio selecting system. Potentiometers are installed on the front of the unit for adjusting PA sidetone and flight and service interphone system amplifiers. Detailed description of the individual modules is covered in the description of the associated system.
D.  The majority of the communication systems circuit breakers are on load control center P6, which is on the bulkhead behind the first officer's seat. Only the VHF-1, HF-1 and selcal-1 systems circuit breakers are on load control center P18, which is on the bulkhead behind the captain's seat. System power is taken from the dc battery bus, the dc standby bus, unswitched ac electronic bus No. 1, switched dc electronic bus No. 2 and ac-dc unswitched electronic buses No. 2. The switched electronic buses are energized through electronic bus No. 1 and electronic bus No. 2 master switches. Therefore, systems on the switched buses may be quickly isolated from the main power buses in case of emergency (Fig. 2). The master switches are on circuit breaker panel P6.
E.  Static dischargers are installed on all airplanes to reduce precipitation static interference in their radio receivers. They are located on the wing, vertical fin, rudder, and elevator trailing edges and tips. Four dischargers are installed on each wing and on the vertical fin and rudder. Three dischargers are installed on each elevator.
550 
May 01/75  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  23-00 Page 1 

K55121 K55124
Communication System Block Diagram  557 
23-00  Figure 1  May 01/76 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

557  Communication Systems Power Schematic 
May 01/76  Figure 2  23-00 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

HF COMMUNICATION SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
Airplanes with Collins 628T-1 Transceivers
1.  General
A.  Two high frequency (HF) communication systems are installed in the airplane. The HF communication systems provide amplitude modulated and single sideband voice communication between the airplane and ground or other airplanes. Communication in  the 2.800 to 26.999 MHz frequency range on any of 24,200 channels may be accomplished. Propagation characteristics of the HF band are most suitable for long distance communications.
B.  Each HF communication system is composed of one transceiver, antenna coupler, coupler status annunciator panel and a single antenna. The HF control panels are installed on the electronic control stand. A single flush-mounted antenna is located in the vertical fin leading edge. The antenna couplers are installed inside the vertical fin leading edge approximately under the antenna. The transceivers are installed on electronic equipment rack E2. The component locations are shown in Fig. 1.
C.  The HF communication system control panels enable the desired communication channel to be selected and the system to be turned on or off. Sidetone is connected to both HF system audio switches in the flight interphone system. The antenna coupler unit matches the characteristic antenna line impedance to the impedance of the transceiver at the particular frequency on which the system is operating. The tuner control unit and tuner will automatically keep the voltage standing wave ratio (VSWR) to a level of 1.3 to 1.0 or better thus providing the best possible power transfer. Communication on the HF system is completed through use of microphones and headphones of the flight interphone system (Ref 23-52-0).
　
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