(4)
Frequency synthesizer
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A 737-300/400/500MAINTENANCE MANUAL
(a) The frequency synthesizer uses a single phase-locked loop to generate the receiver injection frequency and transmit drive frequency. A 3.2-MHz crystal oscillator serves as the frequency standard and provides the necessary stability. The synthesizer also generates a dc tuning voltage to track the receiver preselector to the selected channel frequency. The two-out-of-five frequency information from the control panel is converted to BCD (binary coded decimal) format in the frequency synthesizer. The BCD information provides the synthesizer with the frequency select information.
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A 737-300/400/500MAINTENANCE MANUAL
VHF COMMUNICATION SYSTEM - TROUBLE SHOOTING
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1. General
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A. The very high frequency (VHF) communication system is referred to as the VHF system.
B. You can use an equivalent VHF component from a different VHF system to do trouble-shooting of the defective VHF system. Change the possibly defective component with the equivalent good component from a different VHF system. If the system operates correctly after the component is changed, you have found the defective component.
C. If a VHF system does not operate, make sure the proper voltages are available to the system. Make sure all VHF circuit breakers on load control centers P6 and P18 are closed (pushed in). Make sure the power is supplied.
D. To prevent trouble-shooting that is not necessary, make sure the specified problem continues to occur. If the flight report says that a VHF system does not operate on one or two specific frequencies, try to operate the VHF system at those frequencies. Airplane location or other conditions in the area at the time possibly caused the problem to occur. Make sure the problem occurs at your location. Also make sure the flight interphone system operates correctly. A problem with the VHF system can be a result of a defective component in an audio selector panel. Some checks to make before you change components in the VHF system are:
(1)
A loose coaxial connector will cause intermittent operation and noise in the system. Make sure all connectors are tight.
(2)
Frequency Check
(a)
Set the frequency several times from both above and below the problem frequency. Wafer switch contacts in the VHF control panel are possibly bent.
(b)
Compare the operation of the problem VHF system with the operation of another VHF system in the airplane.
(3)
Transmit Check
(a)
The transmitted sidetone is demodulated RF output. If the sidetone is good, the transmitter modulated output is usually also good.
(b)
Do this procedure to do a check of the frequency when the VHF system transmits: transmit the frequency on one VHF system while you receive the same frequency on a different VHF system.
(4)
Receive Check
(a)
Do this procedure to do a check of the frequency and operation when the VHF system receives: listen to the received voice from ground stations, other airplanes, or from a different VHF system.
(b)
The VHF control panel has a COMM TEST switch. When you push the COMM TEST switch, the operation of the receiver squelch circuits in the VHF transceiver stops. Listen to the noise level increase when you push the COMM TEST switch. You can use the COMM TEST switch to compare the operation (noise level) between different VHF transceivers.
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