(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.

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2. Trouble Shooting Chart VHF COMMUNICATION SYSTEM - ADJUSTMENT/TEST

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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 - Unpublished Work - See title page for details.  23-21-01 Page 103 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.

(2)  
The airplane should not be in or near a large metallic structure, such as a hanger or tower, which may attenuate RF energy. Airplane structure, such as landing gears, may block the VHF antenna when the airplane is in certain positions with respect to ground station antennas.

WARNING:  DO NOT KEY THE VHF TRANSMITTERS WHEN THE AIRPLANE IS BEING FUELED.
B.  Prepare to Test
(1)  
Provide ground power to the airplane and energize busses on load control centers P6 and P18.

(2)  
Ensure the radio bus master switches are closed.

(3)  
Ensure all VHF circuit breakers and interphone circuit breakers are closed.

(4)  
Allow 5 minutes for system warmup.

(5)  
Check flight interphone system and ensure audio selector panels are operable (Ref 23-52-0).

C.  Test VHF Communication System
(1)  
At captain's station, select VHF-1 with mike selector switch. Corresponding indicator light on audio selector panel shall come on.

(2)  
At first officer's station, select VHF-2 with mike selector switch. Corresponding indicator light on audio selector panel shall come on.

(3)  
Tune each VHF system to an unused frequency and operate the COMM TEST button on the VHF control panel. The receiver shall operate at maximum sensitivity as verified by an increase in background noise.

(4)  
Position the VHF control panels transfer switches to the left and tune each VHF system to an active frequency with the left set of controls and that the other frequency indicator window is flagged. Listen to communications traffic and compare receiver sensitivity. They should be approximately equal. All but the weakest signals should open squelch.

(5)  
Rotate VHF system volume control on audio selector panel and confirm control works smoothly through entire range.

(6)  
Tune each system to several different channels and ensure operation is good by channeling from both above and below the selected frequency. Compare reception between receivers; reception should be approximately equal.

(7)  
Position the transfer switch on each VHF control panel to the right and repeat steps C.(4) thru C.(6). Results shall be similar.

(8)  
Tune VHF-1 system to an authorized test frequency. Key the system and establish communication. Check TRANSMIT MONITOR light on front panel of transceiver comes on when transmitting and that the light flickers when modulating. Transmission and reception should be loud and clear. Sidetone should be clearly audible when transmitting.

CAUTION:  DO NOT KEY THE VHF TRANSMITTERS WHEN AIRPLANE IS BEING FUELED. AVOID KEYING THE TRANSMITTER IF RECEPTION IS NOT SATISFACTORY. (LACK OF CLEAR RECEPTION MAY BE AN INDICATION OF AN OPEN TRANSMISSION LINE.)

(9)  
Repeat step C.(8) for VHF-2 system.

(10)
Repeat steps C.(l) thru C.(8) for any combination of VHF system and audio selector panel, where a test is needed. Reception and transmitter sidetone shall be loud and clear.

NOTE:  Push up VHF toggle switches if only monitoring is to be done. The mike selector switch connects both receiving and microphone circuits to the systems.
　
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