Standby
(a)
In the standby position, the HF antenna coupler is set up to receive and the HF antenna coupler is ready to start a tune cycle upon receipt of a key line ground. The receive path is from the HF antenna through a relay switch, the isolation amplifier, and from the descriminator to the receiver. If the other HF antenna coupler is keyed, the relay switch de-energizes and shorts the receive signal to ground. When the HF antenna coupler receives the key line ground return, the sequence counter advances the system to the tune position.
Tune
(a)
Tuning is performed in three steps.
(b)
In step A, the series capacitor is servo-tuned to a zero-phasing error position. At some frequencies, the series capacitor or the shunt coil will be inserted and positioned due to band information commands so the series capacitor can produce a zero phasing error. The sequence counter then advances to step B.
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737-300/400/500MAINTENANCE MANUAL
(c)
Step B determines whether or not the shunt coil is needed for tuning. If, after step A, the system impedance is less than 50 ohms (negative loading) the shunt coil will not be able to resolve the loading to 50 ohms. In this case, the HF antenna coupler will advance to step C. This is typical of frequencies with the 2- to 10-MHz range. If the loading error was positive after step A, the shunt coil will be switched on the line and servo-tuned to obtain negative loading. The phasing error will be kept at zero during step B by the series capacitor. When the phasing error is zero and the loading error is negative, the sequence counter advances to step C.
(d)
During step C, either the shunt capacitor or the shunt coil (as selected in steps A and B) is servo-tuned to produce zero loading error. The series capacitor is fine-tuned to retain a zero phasing error. At this point, the sequence counter advances to the operate position.
(8)
Operate
(a) Upon reaching the operate sequence, the tune power and key lines have unlatched and the HF antenna coupler is ready to transmit and receive. When the operator selects a new frequency, the sequence counter advances to the home step and the complete tuning sequence is repeated.
(9)
Fault
(a) AIRPLANES WITH COUPLER STATUS ANNUNCIATOR PANELS; The HF antenna coupler has circuits that detect faults and provide such indications on the HF fault detection panel. COUPLER FAULT circuit - indicates a failure of the HF antenna coupler to home or tune within 15 seconds. RF FAULT circuit -indicates a lack of RF power due to a defective HF transceiver, an open coaxial, or insufficient power to allow the HF antenna coupler to tune. PRESSURE FAULT circuit - indicates low pressure in the HF antenna coupler case. TUNE IN PROCESS circuit - indicates when the HF antenna coupler is in a tune mode. OPERATE circuit - indicates when the HF antenna coupler has tuned.
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737-300/400/500MAINTENANCE MANUAL
HF COMMUNICATION SYSTEM - TROUBLE SHOOTING
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1. General_______
A. The high frequency (HF) communication system is referred to as the HF system.
B. Problems with the HF system are most easily corrected if you replace a possibly defective component with a good component. If the HF system does not operate after the component is replaced, do a check of the wires for the HF system.
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