TM 1-1510-218-10 3C-42 (3)   Tuning   Knobs.      Two   concentric   tuning knobs, located   on  the  right  side  of  the  NAV  receiver control panel, are used to set the frequency or channel shown   on   the   digital   display,   depending   upon   the mode of operation being used. (a)   Frequency     Standby     Entry     Mode.   When the unit is in the frequency standby entry mode, the   larger   concentric   knob   is   used   to   increase   or decrease  the  first  two  digits  to  the  left  of  the  decimal point (08 to 17), of the standby digital display.  When the  larger  concentric  knob  is  turned  to  increase  the digits   above   17,   the   display   will   start   over   at   08.   Conversely, when the knob is turned to decrease the digits  below  08,  the  display  will  start  over  at  17.    The smaller concentric knob is used to set frequencies with 50 kHz spacing (i.e., .75, .80, .85, .90, etc.). (b)   Frequency Active Entry Mode.   When   in   the   active   entry   mode,   the   tuning   knobs operate  the  same  as  they  do  in  the  standby  entry mode,    but    will    change    the    active    digital    display frequency, rather than the standby frequency. (c)   Channel  Mode.    When the unit is in the   channel   mode,   turning   either   tuning   knob   will change the channel number in the upper display and the corresponding frequency in the lower display. (d)   Program  Mode.    When the unit is in the program mode, the tuning knobs are used to select the   channel   number   in   the   upper   display   (when flashing) or the channel frequency in the lower display (when flashing). (4)   CHAN Switch.  A momentary push-button switch, placarded  CHAN, located on the lower portion of the NAV control unit, is used to put the unit into the channel  and  program  modes.    Momentarily pressing the channel switch while in a frequency mode puts the unit   into   the   channel   mode.      Holding   the   channel switch for longer than 2 seconds puts the unit into the program mode. (5)   OFF  /  VOL  Control.    A  control  knob  on the front of the VHF control unit, placarded  OFF / VOL, controls the operation of the unit.  Turning the knob in the clockwise direction from the  OFF  position  (detent) applies  power  to  the  unit,  and  audio  volume  output  is increased as the knob is turned further to the right. f.    Modes   of   Operation.      The   NAV   receiver control  unit  may  be  operated  in  the  frequency  mode (either standby entry or active entry), channel mode, or program mode. (1)   Frequency   Modes.      The   NAV   receiver control  unit  may  be  operated  in  standby  entry  and active entry. (a)   Standby   Entry.      When  the  unit  is operated in the standby entry frequency mode, a new frequency  is  set  on  the  standby  digital  display  using the  tuning  knobs.    When  the  operator  is  ready,  the frequency  is  then  transferred  to  the  active  display  by pressing the frequency transfer switch. (b)   Active    Entry.       When   the   unit   is operated  in  the  active  entry  frequency  mode,  a  new frequency is set on the active digital display using the tuning  knobs.    The  active  entry  mode  is  entered  from the standby entry mode by pressing the transfer switch for longer than 2 seconds.  The standby digital display will be blank while the unit is in the active mode. (2)   Channel  Mode.    When the NAV receiver control  unit  is  in  the  channel  mode,  the  upper  digital display will show a channel number (1  through 9) and the   lower   digital   display   will   show   the   frequency assigned  to  the  channel  number  in  the  upper  digital display.    The  tuning  knobs  will  change  the  channel number  and  corresponding  frequency  shown  on  the displays.          The     channel     mode     is     entered     by momentarily pressing the CHAN button located on the VHF   transceiver   control   unit.      If   there   are   no   valid frequencies  programmed,  the  unit  will  display  a   CH 1 in the upper display and dashes in the lower display.   The unit can be changed from the channel mode to the frequency mode in three ways. (a)   Momentarily    pressing    the    transfer switch   will   return   the   unit   to   the   frequency   mode, placing the channel frequency in the active display and the previous active frequency in the standby display. (b)   Momentarily    pressing    the CHAN switch will return the unit to the frequency mode, and will return the active and standby displays to what they displayed prior to entering the channel mode. (c)   The  unit  will  return  to  the  frequency mode  if  no  knob  activity  takes  place  for  5 seconds after  the  unit  was  put  into  channel  mode.    When returned in this manner, the channel frequency will be put into the standby display with the active frequency remaining as it was before entering channel mode.  If no frequencies were programmed, the frequencies will remain the same as they were before entering channel mode. (3)   Program Mode.   The preset channels and frequencies    are    set    using    the    program    mode.   Pressing and holding the  CHAN switch for 2 seconds will place the unit into the program mode.  A  P and a