(2)
KEY INTERLOCK light that is activated as a result of HF antenna coupler faults.
(3)
CONTROL INPUT FAIL light that is activated as a result of control faults.
(4)
SQL/LAMP TEST switch which tests the above indicator lights and disables squelch of receiver audio.
(5)
PHONE jack, a 600-ohm headset plug-in to monitor test audio.
(6)
MIC jack, a carbon microphone plug-in to enable voice modulation tests at the HF transceiver.
(7)
On HF transceivers with adapters: GND/UNGND switch, which selects an audio input ground between a balanced and unbalanced output to ensure correct grounding of the microphone circuits. If the switch is not in the GND position, the flight interphone system would experience an increase in hum.
D. HF TRANSCEIVERS WITH ADAPTERS; The SQL/SENS switch is set to correspond to the squelch or RF sensitivity control (for receiver gain) on the HF control panel.
4. Coupler Status Annunciator Panel________________________________
A. Airplanes CAC B-2517 through B-2522, B-2531, and B-2532 have a coupler status annunciator panel. The coupler status annunciator panel is located in the electronic equipment compartment between the E1 and E3 racks. The coupler status annunciator panel has five indicator lights to present the status of the HF antenna coupler.
5. HF Communication Antenna________________________
A. The HF system transmits and receives thru a flush-mounted shunt-fed slot type of HF antenna. The HF antenna is located in the vertical fin leading edge. The antenna dielectric portion is constructed of fiberglass. The HF antenna is the metal leading edge of the vertical fin.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
6. HF Communication Antenna Coupler
________________________________
A. Two HF antenna couplers are located just below the HF antenna vertical fin leading edge. Each HF antenna coupler has miniaturized transistor circuits contained in a rack-mounted pressurized case. Electrical connection is provided through a multipin connector located on each unit front. The RF connection is through a coaxial plug designated RF INPUT also located on the front panel. The antenna connection is on the unit rear panel.
B. The pressurized case has two sections divided by an RF shield which provides thermal protection and 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 HF antenna 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 HF antenna coupler function to open the HF transceiver key interlock and remove RF power from the servo-amplifier circuits when any one of the following occurs:
(1)
The HF antenna coupler fails to home within 15 seconds.
(2)
The HF antenna coupler fails to tune within 15 seconds.
(3)
An arc is detected in the HF antenna coupler.
7. Operation
_________
A. HF SYSTEMS WITH HF TRANSCEIVER ADAPTERS; In the P18 load control center, a 3-phase, 115-volt ac circuit breaker and a 28-volt dc circuit breaker supply power to HF-1. In the P6 load control center, a 3-phase, 115 volt ac circuit breaker and a 28-volt dc circuit breaker supply power to HF-2.
B. HF SYSTEMS WITHOUT HF TRANSCEIVER ADAPTERS; In the P18 load control center, a 3-phase, 115-volt ac circuit breaker supplies power to the HF-1 transceiver. In the P6 load control center a 3-phase, 115-volt ac circuit breaker supplies power to the HF-2 transceiver.
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