5. Operation
A. A crewman may transmit or receive over a VHF system after the desired channel is selected on the VHF control panel. Receiving and transmitting are completed on the same frequency. VHF must be selected on the audio selector panel. Position the transfer switch to the left or right to connect the transceiver to the desired frequency selected on the dual controls.
B. To receive, a crewman must turn on the VHF toggle switch on the audio selector panel (23-53-0) and adjust the volume control on the audio selector panel to obtain a comfortable listening level. The crewman will then hear any messages that may be received on the selected frequency. The COMM TEST pushbutton on the VHF control panel may be operated to check if the receiver is operating properly; pressing the switch disables the squelch circuits and allows an increase in background noise.
C. To transmit over the VHF systems, the mike selector switch on the audio selector panel must be positioned to the desired VHF system. Pressing the microphone push-to-talk button of the flight interphone system will then operate control circuits within the transceiver which will disconnect the receiver circuits and connect the transmitter circuits to the antenna. Amplitude modulated transmission will begin when audio signals are impressed on the microphone circuits. Sidetone is rectified from the modulated RF to the antenna and returned to the headphones of the flight interphone system to indicate proper operation of the transmitter. The VHF control panel volume control may be used to adjust the sidetone audio to a comfortable listening level. When the push-to-talk button is released, the system will return to the receive condition.
5C8
Jun 20/85 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 23-21-01 Page 5
VHF Communication System Schematic 5C8
23-21-01 Figure 2 Jun 20/85
Page 6
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
VHF COMMUNICATION SYSTEM - TROUBLE SHOOTING
1. General
A. Trouble shooting the VHF communication systems is comparatively simple due to duplication of components between systems. Trouble spots can be most easily checked by interchanging common components between systems. If the system is operative after an assumed malfunctioning component has been replaced by a known operative component, the faulty component has been isolated.
WARNING: DO NOT KEY THE VHF TRANSMITTERS WHEN THE AIRPLANE IS BEING FUELED.
B. If a system is completely inoperative, the first check should be to ensure proper voltages are available to the system. Ensure all VHF circuit breakers on load control centers P6 and P18 are closed (pushed in) and power is available to them.
C. To decrease the number of unscheduled removals, confirm that the condition stated in the flight report exists prior to replacing system components. If a VHF system is reported to be inoperative on one or two specific frequencies, a precautionary check of the equipment should be made to confirm the report and that the problem is repetitive. The trouble may be due to the area and position of the airplane in respect to the ground station, or to other conditions existing at the time. Also ensure the flight interphone system is operating properly so that poor communication attributed to the VHF systems is not actually due to faulty circuits in an audio selector panel. Some checks to make prior to exchanging components in the VHF systems are:
(1)
A loose coaxial connector will cause intermittent operation and noise in the system; therefore, ensure all connectors are tight.
(2)
Channeling
(a)
Rechannel the system several times from both above and below the reported inoperative frequency. Wafer switch contacts in the control panel may be bent.
(b)
Compare system operation against that of the other system in the aircraft.
(3)
Transmitting
(a)
The transmitter sidetone is demodulated rf output; therefore, the presence of good sidetone is consistent with good transmitter modulated output.
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