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 B.  VHF No. 1 antenna is at station 627.5, top centerline of the fuselage.
 C.  737-300/400; The VHF No. 2 antenna is located at station 511, bottom centerline of the fuselage.
 5.  Operation (Fig. 2)_________
 A.  The VHF system uses 28-volt power obtained from circuit breakers on the P18 circuit breaker panel and the P6 circuit breaker panel. VHF No. 1 system power is obtained from the hot battery bus on panel P18. VHF No. 2 system power is obtained from switched electronics bus No. 2 on panel P6. VHF No. 3 system power (if installed) is obtained from bus No. 1 on panel P18.
 B.  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.
 C.  To receive, a crewman must activate VHF audio on the audio selector panel (AMM 23-51-00) and adjust the VHF volume control to obtain a comfortable listening level. The volume control on the audio selector panel may also have to be adjusted. The crewman will then hear any messages that may be received on the selected frequency. The COMM TEST switch (if installed) on the VHF control panels 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.
 D.  To transmit over the VHF systems, the MIC selector switch on the audio selector panel must be pressed in for the desired VHF system. Pressing the microphone PTT (push-to-talk) button of the flight interphone system will operate control circuits in the transceiver which disconnect the receiver circuits and connect the transmitter circuits to the VHF antenna. Amplitude modulated transmission will begin when audio signals are present on the microphone circuits. Sidetone is rectified from the modulated rf to the VHF 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 PTT button is released, the system will return to the receive condition.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 E. Collins 618M-3/3A VHF Transceiver
 (1)  
The VHF transceiver may be broken up into three sections; the receiver, the transmitter, and the frequency synthesizer. The VHF synthesizer decodes the two-out-of-five frequency information from the VHF control panel and provides all internal rf signals required by the receiver and transmitter.

 (2)  
Receiver section

 (a)  
In the receive mode, a dc tuning control and rf injection frequency are applied to the receiver section. RF signals from the antenna pass through the T/R switch and are applied to the rf preselector. The preselector selects the desired signal from the antenna to provide an input to the mixer. In the mixer the preselector output is combined with the rf injection frequency to produce a 20-MHz difference frequency.

 (b)  
The 20-MHz signal is applied to the IF amplifier and detector. The 20-MHz IF amplifier is AGC controlled and provides the required selectivity and signal amplification. The detector output is applied to the AGC, squelch, and audio circuits. The output from the detector is also sent to the SELCAL amplifier which provides an audio output to the airplane SELCAL system.

 (c)  
The audio compressor amplifier is designed to hold the audio output level within a 3 dB range with a received signal modulation change from 40 to 90%. The audio amplifier and output impedance matching transformer provide the audio signal to the airplane audio circuits.

 

 (3)  
Transmitter section

 (a)  When the PTT (press-to-talk) control is applied to the VHF transceiver, the synthesizer provides transmitter excitation, power is applied to the transmitter amplifier, the transmitter is connected to the VHF antenna, and receiver squelch is defeated allowing the audio amplifier to be used for sidetone amplification. The transmitter is capable of providing 20 watts minimum output for 118.000 to 135.975 MHz. The rf output is low-pass filtered and applied through the T/R switch to the antenna. The transmitter also provides a side tone and transmitter power output indication. The sidetone signal is amplified by the receiver audio amplifier and sent to the airplane audio system. When the transmitter output is greater than 10 watts the TRANSMIT POWER light comes on. The light flickers slightly when the rf output is modulated with voice peaks.
　
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