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C.  On GJ B-2504 thru B2505, the VHF communication system controls consist of: a set of frequency selection knobs, a volume control and a test button. Rotation of the outer knob will set up the frequency which is then indicated through a window above the knob. The outermost knob has 18 positions numbered 118 thru 135. The next knob has 40 positions numbered 0.000 thru 0.97. (The 0.005-MHz position is not indicated in the frequency window.) Any of 720 channels with 25-kHz spacing may be selected on frequencies of 118.000 to 135.975 MHz. Pressing the TEST button disables receiver squelch circuits allowing an increase in background noise, thus indicating whether the receiver is operating properly.
D.  On GJ ALL EXCEPT B-2504 thru B-2505 and B-2509 thru B-2510, the VHF communication system controls consist of: a dual set of frequency selection knobs, a volume control, a test button, and a transfer switch. Each section of the dual set of frequency knobs control the same VHF system. A different frequency may be set up on each section. By positioning the transfer switch to the left, the frequency set up on the left will be used by the VHF transceiver and a transfer flag will negate the right frequency window. Positioning the transfer switch to the right will cause the VHF transceiver to tune to the frequency set up on the right and the left frequency window will be flagged. Rotation of the two outer knobs will set up the frequency which is then indicated through a window above the knobs. The outermost knob has 18 positions numbered 118 thru 135. The next knob has 40 positions numbered 0.000 thru
0.97. (The 0.005-MHz position is not indicated in the frequency window.) Any of 720 channels with 25-kHz spacing may be selected on frequencies of 118.000 to 135.975 MHz. Pressing the COMM TEST button disables receiver squelch circuits allowing an increase in background noise, thus indicating whether the receiver is operating properly.
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Jun 20/85  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  23-21-01 Page 3 

3.  Transceiver (Fig. 2)
A.  The transceiver is composed of a combination transmitter and receiver which share certain circuits. Frequency selection is available in the 118.00- to 135.975-MHz range with 25-kHz spacing. The transceiver is composed throughout of solid state circuits. Transient protection and voltage regulation circuits ensure correct operating voltages to all circuits.
B.  The receiver is normally on unless a push-to-talk circuit is completed. Received amplitude modulated signals are completed through a de-energized antenna relay, to the RF amplifier. The signals are amplified, mixed and converted as necessary, then passed through the detector, squelch and amplifier circuits. Audio output of the receiver is fed to the audio selecting system through the VHF control panel volume control. Selcal signals received are sent to the selcal system on separate circuits (Ref 23-28-0).
C.  The same selected frequency is used for both transmitting and receiving. Pressing the push-to-talk button energizes an antenna relay which switches the antenna to the transmitter circuits. The transmitter is amplitude modulated by audio signals from the flight interphone system after crystal oscillator frequencies are mixed and multiplied as required to obtain the final output radio frequency. Transmitted audio is passed through the receiver and sent back to the transceiver sidetone output circuits where it is then connected to output circuits to the VHF control panel volume control and then to the audio selector panels. A minimum power output of 25-watt RF energy may be applied to the VHF antenna.
D.  The front panel of the transceiver contains a TRANSMIT MONITOR light, a SQUELCH DISABLE button, and two jacks: PHONE and MIKE. The TRANSMIT MONITOR light comes on when the transmitter is radiating and it should flicker when the carrier wave is modulated. Pressing the SQUELCH DISABLE button disables the squelch circuits allowing the receiver to operate at maximum sensitivity.
4.  Antennas
A.  Each VHF communication system uses an all metal VHF blade-type antenna. Each antenna is used for both receiving and transmitting. The antennas are interchangeable between the upper and lower installations.
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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.
　
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