TM 55-1510-219-10 4.    Microphone  switch  -  Press  momentarily (interrupted   tone   from   voice   security unit should no longer be heard). NOTE No   traffic   will   be   passed   if   the interrupted tone is still heard after pressing and releasing the microphone switch. 5.  Microphone switch - Press (do not talk). Wait until beep is heard then speak into microphone. (5)  Shutdown. 1.  Function switch (UHF panel)-OFF. 2.      POWER   ON   switch   (Voice   security panel) - OFF. e.  UHF Command Set - Emergency Operation. NOTE Transmission on emergency frequencies   (guard   channels)   is restricted    to    emergencies    only. An  emergency  frequency  of  121. 500  MHz  is  also  available  on  the VHF command radio set. 1.      Transmit/interphone      selector      (audio panel) - No.  3 position. 2.      Mode   selector   switch   (UHF   panel)   - GUARD. 3.  Microphone switch - Press. 3-9.  BACKUP VOW (AN/ARC-164). A radio set identical in type and performance to the UHF command set (fig.  3-3) is installed on the pedestal extension  (fig.    2-10)  to  serve  as  backup  VOW  (Voice Order Wire).  This set provides the pilot and copilot with secure 2-way voice communications. Complete provisions only are provided for a KY-58 voice security device.    The  backup  VOW  set  is  protected  by  a  7  1/2 ampere BU VOW circuit breaker on the overhead circuit breaker   panel   (fig.      2-26).      The   backup   VOW   and Transponder share an antenna which is mounted on the aircraft belly (fig. 2-1). 3-10.  VHF-AM COMMUNICATIONS (VHF-20B). a.    Description.    VHF-AM  communications  provide transmission    and    reception    of    amplitude    modulated signals  in  the  very  high  frequency  range  of  116.000  to 151.975  MHz  for  a  range  of  approximately  50  miles, varying with altitude.  A dual head control panel (COMM 1,  fig.    3-4)  is  mounted  on  the  pedestal  (fig.    2-10) accessible   to   both   the   pilot   and   copilot.      The   panel provides    two    sets    of    control    indicators,    frequency indicators,   frequency   select   knobs,   a   single   volume control, and a single selector switch for quick frequency changing.    Transmission  audio  is  routed  by  pilot  and copilot #1 transmitter select switches.  Received audio is routed by pilot and copilot #  receiver audio switches to the respective headsets.  The VHF radio is protected by one  10-ampere  VHF  circuit  breaker  on  the  overhead control circuit breaker panel (fig.  2-26).  The associated antenna is shown in figure 2-1. b.  Controls and Indicators. (1)  VHF-AM control panel (fig.  3-4). CONTROL FUNCTION Left control Indicates set operating frequency Frequency (control TRANS switch left posi- indicator tion.) Frequency Selects desired set operating fre- selectors quency (control TRANS switch left position.) VOL-OFF Adjusts volume of received au- control dio, turns set ON or OFF. CONTROL Illuminates, if control TRANS indicator switch in left position. TRANS switch Selects right or left control head to control operating frequency of set. Frequency Indicates set operating frequency indicator (control TRANS switch right po- sition.) Frequency Selects desired set operating fre- selectors quency (control TRANS switch right position.) CONTROL Illuminates, if control TRANS indicator switch at right position. COMM  TEST Overrides automatic squelch cir- switch cuit. c.  VHF-AM Set - Normal Operation. (1)   Turn-on   procedure.      VOL   control   -   Turn clockwise. 3-8