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 - Unpublished Work - See title page for details.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 If the system is operative after an assumed malfunctioning component has been replaced by a known operative component, the faulty component has been isolated.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.
(b)
Transmitter frequency may be checked on one VHF system by receiving on the other VHF system.
(c)
Transmitter output and modulation may be checked by means of the TRANSMIT POWER light on the front panel of the transceiver.
(4)
Receiving
(a)
Receiver operation and frequency may be checked by monitoring signals from ground stations, other aircraft, or from the other VHF system.
(b)
The control panel has a COMM TEST button which, when pressed, disables the receiver squelch circuits. The button may be used to compare the operation of the two VHF receivers by comparing the resultant increase in the respective receiver's background noise when its COMM TEST button is pressed.
5C8
Jun 20/85 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 23-21-01 Page 101
577
23-21-01 Page 102 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Nov 15/79
2. Trouble Shooting Chart VHF COMMUNICATION SYSTEM - ADJUSTMENT/TEST
532
May 15/77 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 23-21-01 Page 103
1. Test VHF Communication System
A. General
(1)
The following test should be used to check the operation of a VHF system after a system component has been replaced or repaired. Conduct test using only authorized test frequencies and check to ensure frequency is not in use before transmitting.
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