TM 1-1510-218-10 3A-8 NOTE To adjust volume when audio is not being received,  turn  squelch  switch  OFF,  adjust volume   for   comfortable   noise   level,   then turn squelch switch ON. 3.   Volume – Adjust. 4.   SQUELCH – As desired. (5)   Transmitter Operating Procedure. 1.   Transmitter selector – No. 3 position. 2.   UHF    control    panel    –    Set    required frequency    using    either    PRESET    or MANUAL frequency select controls. 3.   Microphone  jack  selector  switch   – As desired. 4.   Microphone switch – Press to transmit. (6)   Shutdown Procedure.    Function  selector switch – OFF. 3A-9.  VHF COMMAND SET (VHF-20B). a.   Description.    The  VHF  command  set  is  a line-of-sight radio transceiver which provides transmission   and   reception   of   amplitude   modulated signals in the very high frequency range of 116.000 to 151.975 MHz for a distance range of approximately 50 miles.    Two  VHF  radio  sets  are  installed  placarded COMM  1   and   COMM   2.    Audio  signals  are  applied through    the    pilot    and    copilot    transmitter    selector switches and through the pilot and copilot VHF-1 audio switches to their respective headsets.  The VHF radio sets are protected by a 10-ampere  VHF  #  1 and  VHF # 2  circuit  breakers  located  on  the  overhead  circuit breaker  panel,  Figure  2-16,  Sheets  1  and  2.    Figure 3A-4  illustrates  the  VHF  command  set  control  panel.   The associated antenna is shown in Figure 2-1. b.  Controls and Functions.  (1)   Frequency Indicator.  Indicates operating frequency of set. (2)   CONTROL   Indicator.      Indicates    which control head is in operation. (3)   COMM TEST Switch. Overrides automatic squelch circuit. (4)   Frequency   Selectors.      Selects   desired operating frequency of set. (5)   TRANS   Switch.      Selects   which   of   two control heads determines operating frequency of set. (6)   VOL   OFF   Control.      Adjusts   volume   of received audio and turns set on or off. c.   Normal Operation. (1)   Turn On Procedure. 1.   VOL control – Turn clockwise. (2)   Receiver Operating Procedure. 1.   Frequency   selector    –   Select   desire frequency. 2.   VOL control – As required. (3)   Transmitter Operating Procedure. 1.   Transmitter selector switch – VHF 1 or VHF 2. 2.   Microphone Switch – Press. (4)   Shutdown Procedure. 1.   VOL control – Counterclockwise, OFF. 1.  Frequency Indicator 2.  CONTROL Indicators 3.  COMM TEST Switch 4.  Frequency Selector 5.  TRANS Switch 6.  VOL OFF Control Figure 3A-4.  VHF Control Panel