TM 1-1510-218-10   3C-39 Table 3C-11.  Heading Reference Indicator Values HEADING SWITCH SELECTION ACTIVE FREE / SLAVE SWITCH FREE / SLAVE SELECTION HEADING REFERENCE INDICATOR DISPLAYED All on Compass 1 Pilot Free DG1 (amber) Slave MAG1 (amber) All on Compass 2 Copilot Free DG2 (amber) Slave MAG2 (amber) Normal On-side Free DG (white) Slave Blank 3C-21.  NAVIGATION RECEIVERS (KNR-634A). a.   Description.   Two VOR localizer / glideslope/ marker  beacon  navigation  receivers  (KNR634A   T1  , KNR-634 T2  ) and a TACAN/DME unit (KTU-709) are controlled  by  two  control  units  (KFS-579A  and  KFS- 564A) mounted in the instrument panel. The  NAV  receivers  receive  and  interpret  VOR range   and   LOC   signals   in   the   frequency   range   of 108.00   to   117.95   MHz,   glideslope   signals   in   the frequency  range  of  329.15  to  335.00  MHz,  and  75 Hz marker beacon signals.  In addition, the KFS-579A #1 NAV/TAC   control   tunes   the   KTU -709   TACAN/DME system    to    all    252    TACAN    channels.        The    NAV receivers  are  powered  through  two  2-ampere  circuit breakers, placarded  NAV  #1 and  #2,  located  on  the overhead circuit breaker panel, Figure 2-16. b.  NAV/TAC Control Unit (KFS-579A) Controls,    Indicators,    and    Functions.        Refer    to Figure 3C-17. (1)   Frequency  Display.    Liquid  crystal  digital readouts   provide   a   continuous   display   of   both   the active   frequency,   or   TACAN   channel,   and   standby frequency  or  TACAN  channel  (bottom  line,  placarded SBY)   when   the   system   is   in   the   frequency   mode.   Display brightness is controlled by a switch, placarded VHF / NAV / ADF, located right of the altitude alert on the instrument panel. (2)   Frequency Transfer Switch. The frequency transfer switch is a momentary push-button switch,  placarded  with  a  two-headed  vertical  arrow, located to the right of the digital display. 1.  Frequency Display 2.  Frequency Transfer Switch 3.  Tuning Knobs 4.  MODE Switch 5.  OFF / VOL / PUSH TST Control Figure 3C-17.  NAV/TAC Control Unit (KFS-579A) (a)   Standby  Entry  Mode.    Pressing the frequency transfer switch with the unit in the standby entry  mode  causes  the  frequency  or  TACAN  channel displayed    in    the    active    (upper)    digital    display    to interchange   with   the   frequency   or   TACAN   channel shown in the lower (standby) digital display. (b)   Active Entry Mode.  While the unit is in the standby entry mode, pressing the transfer switch for longer than 2 seconds will cause the unit to enter the active entry mode.  While in the active entry mode, momentarily pressing the transfer switch will return the unit to the standby entry mode.