TM 1-1510-218-10 3A-27 4.   TUNE control  –  Rotate  for  maximum reading on tuning meter and zero BFO beat. 5.   GAIN control – As required. 6.   BFO / OFF switch – OFF. 7.   Single or double needle switches, RMI – As required. 8.   Single   or   double   needle   on   RMI   – Read course to station. (2)   Sense Antenna Direction Finding. 1.   Mode selector – ANT. 2.   Range switch – Select. 3.   TUNE control  –  Rotate  for  maximum reading on tuning meter. 4.   GAIN control – As required. (3)   Aural-Null Direction Finding. 1.   Mode selector – ANT. 2.   BFO / OFF switch – BFO. 3.   Range switch – Select. 4.   TUNE control – Tune desired station. 5.   GAIN  control   –  Adjust  for  minimum audio output. 6.   Single or double needle switches RMI – As required. 7.   BFO / OFF switch – OFF. 8.   Mode selector – LOOP. 9.   LOOP switch   –  L or   R.    Turn  left  or right  until  a  null  is  reached,  minimum sound in headsets.   NOTE The   true   null   and   direction   to   the   radio station  may  be  indicated  by  either  end  of the single needle.  This ambiguity must be solved to determine proper direction to the station. 10.   Single   or   double   needle   on   RMI   – Read course to station. (4)   Shutdown Procedure. 1.   Mode selector – OFF. 3A-24.  ADF RADIO (KR 87). a.   Description. The KR 87 automatic directional finder is a digitally tuned solid-state receiver which  provides  bearing  information  to  stations  in  the 200 kHz to 1799 kHz frequency band, along with audio reception.    Refer  to  Figure  3A-16.    The  unit  displays the  active  frequency  in  the  left,  display  window.    The right  window  will  display  either  the  standby  frequency (which can be transferred to the active window), or a flight timer, or programmable elapsed timer.  The flight timer  will  keep  track  of  the  total  flight  lime,  while  the independent programmable elapsed timer can be reset to  count  up  from  zero  or  be  preset  to  a  value  and count  down  to  zero.    The  system  is  protected  by  a 1-ampere  circuit  breaker,  placarded   ADF,  located  on the right subpanel.  The antenna located on the lower side of the aircraft contains loop and  sense antennas, preamplifiers,   and   modulators,   which   combine   the antenna signals into a single RF signal that is output to the receiver via a triaxial cable.  Refer to Figure 2-1. b.  Operating    Modes.        The    ADF    has    two operational  modes.    In  the  antenna  (ANT)  mode, the loop   antenna   is   disabled   and   the   unit   acts   as   a receiver, allowing audio reception through the speaker or  headphones.    The  indicator  needle  as  selected  on the  RMI's  will  park  at  a  90  relative  position  and  the ANT  message  on  the  left  side  of  the  display window will  display.    This  mode  will  provide  a  slightly  clearer audio  reception  and  is  used  for  station  identification.   In various parts of the world, some L/LM stations use an  interrupted  carrier  for  identification  purposes.    A Beat  Frequency  Oscillator  (BFO)  function  is  provided to  permit  these  stations  to  be  more  easily  identified.   Pushing the  BFO button will cause a 1000 Hz tone to be   heard   whenever   there   is   a   radio   carrier   signal present at the selected frequency.  It will also light the BFO message in the center of the display. With the  ADF button pressed, the unit is placed into the ADF mode and the loop antenna is enabled.   The   ADF   message   on   the   left   side   of   the   window display    will    display    and    the    indicator    needle    as selected on the RMI's will point to the relative bearing of  the  selected  station.    In  order  to  tell  if  there  is  a sufficient  signal  for  navigational  purposes,  place  the unit   back   in   the   ANT   mode,   parking   the   indicator needle as selected on the RMI's at 90.  When the unit is then switched to the ADF mode, the needle should slew   to   the   station   bearing   in   a   positive   manner, without excessive sluggishness, wavering, or reversals.