TM 1-1510-218-10 3C-40 (3)   Tuning   Knobs.      Two   concentric   tuning knobs,  located  on  the  right  side  of  the  NAV/TACAN control panel, are used to set the frequency or TACAN channel shown on the digital display, depending on the mode being used. (a)   Frequency or TACAN Channel Standby    Entry    Mode.       When   the   unit   is   in   the frequency or TACAN channel standby entry mode, the larger concentric knob is used to increase or decrease the first two digits to the left of the decimal point from 08   to   17 MHz   (frequencies)   or   01   to   29   (TACAN channels)  of  the  standby  digital  display.    When  the larger concentric knob is turned to increase the digits above  17  (frequencies)  or  29  (TACAN  channels),  the display    will    start    over    at    08    or    00    respectively.   Conversely, when the knob is turned to decrease the digits below 08 (frequencies) or 00 (TACAN channels) the  display  will  start  over  at  17  or  29  respectively.   When  in  the  frequency  mode,  the  smaller  concentric knob, placarded  PULL  Y,  is  used  to  set  frequencies with   50   kHz  spacing  (i.e.,  .75,  .80,  .85,  .90,  etc.).   When in the TACAN mode with the smaller concentric knob  turned  in  the  pulled-out  position,  the  TACAN   X designator will be changed to Y. (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    upper    digital    display frequency  or  TACAN  channel  rather  than  the  standby frequency. (4)   MODE    Switch.        A    momentary    push- button switch, placarded  MODE, located on the lower portion of the NAV/TAC control unit, is used to put the unit into the TACAN mode.  Momentarily pressing the MODE switch while in a frequency mode puts the unit into the TACAN mode. (5)   OFF    /VOL    /    PUSH    TST    Control.      A control  knob  on  the  front  of  the  NAV/TACAN  control unit, placarded  OFF /  VOL /  PUSH  TST, 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.   Pressing  PUSH TST overrides the automatic squelch circuitry.        Turning    the    knob    clockwise    past    the automatic    squelch    threshold,    while    the    automatic squelch  circuitry  is  overridden,  will  allow  background audio to be heard through the headphones or speaker and  demonstrate  whether  the  receiver  is  working  or not,  and  allow  setting  of  audio  volume.    To  return the unit to automatic squelch control, press the PUSH TST control knob again. c.   Modes   of   Operation.      The    NAV/TACAN control  unit  may  be  operated  in  TACAN  or  frequency mode in either standby entry or active entry mode. (1)   Standby     Entry     Mode     (Frequency     or TACAN  Channel).    When  the  unit  is  operated  in  the standby   entry   mode,   a   new   frequency   or   TACAN channel is set on the standby digital display, using the tuning   knobs.      When   the   operator   is   ready,   the frequency   or   channel   is   transferred   to   the   active display by pressing the transfer switch. (2)   Active Entry (Frequency or TACAN Channel).    When  the  unit  is  operated  in  the  active entry mode, a new frequency or TACAN channel 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. (3)   TACAN  Mode.    Enter   the  TACAN  mode by momentarily pressing the  MODE switch.  While in this   mode,   the   tuning   knobs   change   the   TACAN channel numbers. d.  NAV/TACAN Control Unit (KFS-579A) Operation. (1)   Turn On Procedure. 1.   Avionics    Master    Switch    (overhead control panel, Figure 2-15)  – ON. 2.   OFF / VOL / PUSH TST 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  I  Receiver  Audio  Switch   (audio control panel, Figure 3C-1) – ON (up). 4.   VOL Control – Adjust as required.