TM 1-1510-218-10   3C-43 channel  number  will  be  shown  in  the  active  display and   a   frequency   or   dashes   will   be   shown   in   the standby    display.        Immediately    after    entering    the program mode, the channel number will begin to flash, indicating that rotating the tuning knobs will change the channel  number.    Momentarily  pressing  the  transfer switch will cause the channel number to stop flashing and  cause  the  frequency  to  begin  flashing,  indicating that    rotating    the    tuning    knobs    will    change    the frequency.  Momentarily pressing the CHAN switch will return  the  unit  to  the  frequency  mode.    The  standby frequency will return to what it was prior to entering the program  mode.    The  unit  will  automatically  return  to the   frequency   mode   if   no   front   panel   activity   takes place for 20 seconds. g.  NAV/TACAN Control Unit (KFS-564A) Operation. (1)   Turn On Procedure. 1.   Avionics     Master     Switch     (overhead control panel, Figure 2-15)  – ON. 2.   OFF/VOL Knob  –  Turn  clockwise  out of    detent,    then    press    to    turn    off automatic  squelch  circuit.    Continue turning knob clockwise until background noise is heard in headphones or speaker, assuring that receiver  is  operating,  then  set  audio volume  to  desired  level.    Press  knob again    to    return    unit    to    automatic squelch control. (2)   Receiver Operating Procedure. 1.   MODE     Push-Button     Switch     –     As required. 2.   Tuning Knobs – Set desired frequency or channel. 3.   NAV  1  Receiver  Audio  Switch  (audio control panel, Figure 3C-1) – ON. 4.   VOL Control – Adjust as required. (3)   Shutdown Procedure. 1.   OFF / VOL Control – Turn counterclockwise to OFF position. 3C-22.  TACAN/DME (KTU-709). a.   Description.    The TACAN system is a polar coordinate    UHF    navigation    system    that    provides relative  bearing  and  slant  range  distance  information with respect to a selected TACAN or VORTAC ground station.  The effective range of the TACAN is limited to the  line-of-sight.    Actual  operating  range  depends  on the  altitude  of  the  aircraft,  weather,  type  of  terrain, location  and  altitude  of  the  ground  transmitter,  and transmitter power. TACAN   audio   is   applied   to   the   audio   control panel.  The TACAN system also has  a self-test mode for    both    bearing    and    distance    functions.        The NAV/TAC  control  unit,  Figure  3C-17,  tunes  the  KTU- 709 to all  252 TACAN channels and all DME channels.   TACAN course deviation information may be displayed on  the  pilot's  or  copilot's  EHSI's.    When NAV Source Selector  is  set  to  radio  position,  the  TACAN  radio  is tuned  to  a  TACAN  station  and  the  radio  is  supplying valid   navigational   data.      TACAN   or   DME   distance information  is  displayed  on  the  pilot's  and  copilot’s EHSI’s  in  the  upper  right-hand corner of the displays.   The system is protected by a 2-ampere circuit breaker, placarded   TACAN,   located   on   the   overhead   circuit breaker panel, Figure 2-16. b.  DME    Hold    Switch.        A    push-on/push-off switch, placarded  DME  HOLD  PUSH/SW,  located  on the audio control panel   T2  and right of the radio call sign placard  T1  ,  controls  selection  of  the  DME  hold function.    When  the   DME  HOLD  switch  is  pushed  to the  on  position,  the  DME/TACAN  distance  frequency in  the  DME/TACAN  receiver/transmitter  will  be  held constant  regardless  of  the  frequency  selected  on  the NAV   1   control   unit.      The    DME   HOLD   switch  will illuminate when the DME hold function is selected and will  be  extinguished  when  the  DME  hold  function  is deselected.      The   DME   HOLD   switch  is  inoperative when  the  NAV/TA C is in the TAC mode or if the FMS is controlling the DME. 3C-23.  ADF RECEIVER (KDF-806). a.   Description.    Two ADF navigation receivers are   installed   to   provide   a   visual   indication   of   the relative  bearing  to  a  selected  ground  radio  station.   The  units  are  airborne  low  frequency  radio  direction finders that receive signals from transmitters in the 190 to   1799   kHz   range,   and   the   international   distress frequency of 2182 kHz.  The ADF receiver can also be used for homing and position fixing.  The ADF receiver is  equipped  with  a  Beat  Frequency  Oscillator  (BFO) which  is  used  to  more  accurately  tune  weak  signals.   Reception  distance  of  reliable  signals  depends  upon the  power  output  of  the  transmitting  station  and  the atmospheric conditions.  Bearing information from the number  1  ADF  receiver  may  be  displayed  visually  by the  single-needle  pointer  on  the  pilot's  and  copilot's EHSI.    Bearing  information  from  the  number  2  ADF receiver   may   be   displayed   visually   by   the   double- needle pointer on the pilot's and copilot's EHSI.  ADF