COMMUNICATIONS - DESCRIPTION AND OPERATION
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
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).
D. The HF communication system uses 3-phase, 115-volt, 400-Hz ac and 28-volt dc power
obtained from load control center P18 and P6 respectively. AC power is obtained through a
single 3-phase circuit breaker.
2. Control Panel
A. The HF control panels can select any one of 24,200 channels, spaced 1 kHz apart in the 2-
to 30-MHz range. Each panel contains four frequency selector knobs, a frequency display
window, RF SENS knob and a mode selector switch. The left frequency selector knob selects
MHz, the two center knobs select 100- and 1-kHz parts and the right knob selects 1-kHz parts
of the frequency. The RF SENS knob controls rf gain in the receiving section of the transceiver.
The mode selector switch turns the system off (OFF), selects upper sideband (USB), and
amplitude modulation (AM) modes.
3. HF Transceiver (628T-1)
A. The transceiver is used for airborne, narrow-band upper sideband (USB), voice and data
communications. It is capable of transmitting (200 watts pep [peak envelope power] output in
USB and 100 watts carrier) and receiving at 2.800 to 26.999 MHz on 24,200 channels with
1-kHz spacing. The transceiver consists of modules, subassemblies and a chassis assembly
housed in a 3/4 ATR case. The function of the major modules and assemblies is as described
below.
B. The frequency synthesizer generates 19.8 kHz, 69.3-MHz, and 72.6- to 96.799-MHz signals
that are derived from a 9.9-MHz frequency standard. The 19.8-kHz signal is produced by a
divide-by-500 circuit and is supplied to the power supply as a switching signal. The 500-kHz
signal is produced by a phase-locked loop and is supplied as a carrier to the modulator and
product detector in the receiver/exciter. A harmonic generator produces the 69.3-MHz signal
that is used as a mixer injection signal in the receiver/exciter. The 72.6- to 96.799-MHz signal is
produced by dual phase-locked loops and is also used as a mixer injection signal in the
receiver/exciter.
C. The frequency module converts reentrant coded frequency data into bcd frequency data. Logic
gates are used as decade decoders for the 1-, 10- and 100-kHz digits. A programmable readonly
memory (PROM) is used as a decoder for the 1- and 10-MHz digits. The module also
contains a frequency error detector and a frequency band decoder.
D. The receiver section of the receiver/exciter module is a completely solid-state, dual-conversion
receiver that receives, amplifies, and detects incoming AM and/or USB signals from 2.800
to 26.999 MHz. The front end consists of a fixed tuned RF bandpass filter and an AGCcontrolled
RF amplifier. Frequency translation is accomplished using 72.6- to 96.799-MHz
balanced mixers. Separate AGC-controlled IF sections are used for the AM and USB signals.
The receiver has both receiver and SELCAL audio amplifiers. The exciter section is a
completely solid state, dual conversion exciter that produces either USB or AME
(full sideband) 2.800- to 26.999-MHz signals. The initial stages are a balanced modulator
and 500-kHz if amplifiers. Frequency translation is accomplished using the same mixers that
are used in the receiver section. A low-pass LC filter reduces spurious signals from being
amplified in the ALC (automatic load control) controlled output RF amplifier.
E. The power amplifier amplifies the RF input from the exciter to a 200-watt pep level. Four stages
of amplification are used, with both the driver and final amplifiers operated in push-pull
configurations. Protective circuits produce the ALC signal, which is used to control the RF drive
level to the input of the power amplifier. The final amplifier transistors are secured to conicalshaped
mounts that fit into finned heat sinks. An internal blower provides cooling air for the
power amplifier.
F. The low-pass filter module consists of six filters, directional wattmeter, motor control, a tune-inprogress
attenuator, and the transmit-receive relay. A 3-deck motor-driven bandswitch selects
the filter required for the selected operating frequency. The directional wattmeter consists of a
forward power detector and a reflected power detector. These detectors produce dc analog
signals that are representative of the measured RF power levels. The motor control contains
transistor switches that control dc voltage applied to the motor.
G. The chassis contains ac and dc power supplies, rear connectors, front panel controls and
indicators and interconnect board. The front panel components include phone and microphone
jacks, a SQL/LAMP TEST PUSHBUTTON, AND R/T FAULT and KEY INTERLOCK lamps. The
ac power supply, mounted in the lower tray, converts 3-phase, 115-volt, 400-Hz input power
into 46- and 28-volt dc power. The dc power supply, mounted in the upper tray, contains
regulators that produce +5-, +10-, +11-, and +20-volt dc power. The interconnect board
contains input audio amplifier, fault monitoring, and key interlock circuits.
H. Front Panel Controls and Indicators
(1) The front panel of the transceiver has two lamps and a pushbutton switch. The front panel
of the adapter has a 2-position switch that is operated using a screwdriver.
(2) The two lamps on the transceiver are the KEY INTERLOCK and R/T FAULT lamps. The
KEY INTERLOCK lamp will light when keying transmitter if the external antenna coupler
has failed to tune properly. When this lamp is on, transmission is inhibited. The R/T
FAULT lamp will light when the internal monitoring circuit detects a transceiver fault. The
faults that will light this lamp are: the frequency synthesizer is out of lock; lock transmitter
power output (60 watts pep nominal threshold) in AME transmit mode; and low voltage
from the +5-, +10- or +20-volt dc power supplies.
(3) The pushbutton switch on the transceiver is the SQL/LAMP TEST switch. When pressed,
the KEY INTERLOCK and R/T FAULT lamps should light and the squelch circuit is
disabled.
(4) The switch on the adapter is the SQUELCH/RF SENS switch. This switch must be
positioned so that the transceiver will interface properly with the particular type of remote
radio set control that is being used.
4. Transceiver Adapter
A. The adapter is used to adapt the transceiver, to the mounting and wiring provisions existing in
present airplane configurations.
5. Coupler Status Annunciator Panel
A. The coupler status annunciator panel located in the electronic equipment area, contains 10
indicator lights. The panel provides remote visual indications of faulty HF system operation.
Five indicator lights are for HF system No. 1, and five identical indicators are for HF system
No. 2. A decal adjacent to the indicator designates the system and circuit function for each set
of the following five indicators:
(1) COUPLER FAULT - A failure of the coupler to home or tune the circuits within 15 seconds
causes the indicator light to come on.
(2) RF FAULT - The following cause the indicator light to come on: transceiver malfunction,
an open coaxial cable, or insufficient power. These faults all indicate a lack of RF power.
(3) PRESSURE FAULT - Low pressure in the coupler causes the indicator light to come on.
(4) TUNE IN PROCESS - Using the coupler in a tune mode causes the indicator light to come
on.
(5) OPERATE - Placing the coupler in an operate condition causes the indicator light to come
on.
B. Each indicator has a self-test feature which operates to check the lamp when the indicator
shroud is pushed in. Both sets of indicators receive 28V dc from the respective antenna coupler
when power is applied to the system. When the coupler senses a fault, the sequence counter
freezes and operates the fault circuit to provide a ground return that causes one of the three
fault indicator lights to come on. The TUNE IN PROCESS indicator receives a ground return
from the coupler to cause the indicator light to come on whenever a tuning sequence takes
place. The OPERATE indicator receives a ground return after the coupler has completed a
sequence of home, standby, and tune, and is in the operate condition.
6. Antenna Couplers
A. Two identical antenna couplers are located at approximately station 1016 just below the HF
antenna vertical fin leading edge. The coupler has miniaturized transistor circuits contained in a
rack-mounted pressurized case. Electrical connection is provided through two multipin
connectors located on the front of the unit. The RF connection is through a coaxial plug
designated RF INPUT located on the front panel. The antenna connection is at the rear of the
unit.
B. The pressurized case has two sections divided by an RF shield which provides thermal
protection as well as RF isolation. The forward compartment contains tuning control modules, a
dual servo-amplifier, and the power supplies. Integrated circuit construction is used in the
servo-amplifier and tuning control modules. The rear section of the coupler (the RF
compartment) contains the tuning element, a discriminator module, and an isolation amplifier.
The tuning elements consist of two variable vacuum capacitors and a variable inductor,
associated motors, gear trains, switches, and RF relays.
C. Protection circuits in the coupler function to open the transceiver key interlock and remove RF
power from the servo-amplifier circuits when any one of the following occurs:
The coupler fails to home within 15 seconds.
The coupler fails to tune within 15 seconds.
An arc is detected in the coupler.
7. Antenna
A. The HF communication system transmits and receives through a flush-mounted shunt-type
antenna. The antenna is located in the vertical fin leading edge. The antenna dielectric portion
is constructed of fiberglass. The antenna is the metal leading edge of the vertical fin.
B. The antenna conductor, which is located at a low impedance point in the circuit, acts as a type
of inductive coupling so that the airframe operates as an antenna.
8. Operation (Fig. 2)
A. Internal transmitter excitation and receiver injection frequencies are generated by a dual-loop
phase-locked digital synthesizer. The transmitter output is nominally 200 watts peak envelope
power (PEP) with a maximum average power of 100 watts. During the tune cycle of the external
antenna coupler, the transmitter output is nominally 50 watts. The transceiver has a dualconversion
receiver that produces 200-mW received voice audio and 0.5-volt SELCAL audio
outputs, and a 200-mW sidetone audio output. Operating voltages are produced by internal
power supplies that operate from a 3-phase, 115-volt, 400-Hz primary source.
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