TM 55-1510-220-10 3. Frequency controls (VOR control panel) Set for both receivers. 4. Volume   controls   (VOR   control   panel Turn   clockwise   to   activate   sets   and adjust volume. 5. NAV  A  audio switches (audio control panel,  fig.    3-1)  ON.    Confirm  proper signal, then OFF. 6. RMI and HSI Confirm proper indications. (a) Pilot/copilot  course  indicator switches (fig.  2-30).  Pilot select VOR-1, copilot select VOR-2. (b) To    determine    course    to    station    on pilot's HSI: TO-FROM pointer reads TO (up) position. (c) To  determine  bearing  from  station  on pilot's HSI: TO-FROM pointer reads FROM position (down). (d) To determine course to station on RMI: Select VOR, verify single needle points course to station. (2) Localizer (LOC) operation: 1. VOR    frequency    knob    (NAV    panel) Select frequency. 2. Pilot's, copilot's COURSE INDICATOR switches (instrument panel) Select VOR source. 3. Check glideslope flags unmasked. (3) VHF communications receiver operation: 1. VOR   frequency   control   knob   (VOR control panel, fig.  3-16) Select desired frequency. 2. Volume   control   knob   (VOR   control panel, fig.  3-16) Full on. 3. NAV   A,   audio   switch   ON.      Adjust audio. (4) Shutdown procedure: 1. Volume   control   (VOR   control   panel, fig.  3-16) OFF. 3-25.   MARKER BEACON RECEIVER. a. Description.  The marker beacon receiver is located inside  the  No.1  VOR  receiver.    The  marker  beacon  receiver obtains   power   through   the   VOR   receivers.      The   marker beacon  provides  visual  and  aural  indication  of  the  aircraft's position  over  a  75  MHz  marker  beacon  ground  transmitter. Upon   entering   the   range   of   marker   beacon   signals,   blue, amber,   or   white   annunciator   lights   will   illuminate,   and corresponding aural signals will indicate aircraft passage over the   (0)   outer,   (M)   middle,   (A)   inner   or   airway   marker beacons.  Range  is  vertical  to  50,000  feet.    On-off,  volume, and  sensitivity  controls  are  located  on  the  marker  beacon audio control panel (fig.  3-2). b. Controls/Indicators and Functions. (1) Marker    beacon    sensitivity    switch    (marker beacon audio control panel, fig.  3-2) Controls sensitivity of marker beacon receiver. (a) HI   position   enables   high   sensitivity operation of marker beacon receiver. (b) LO    position    enables    low    sensitivity operation of marker beacon receiver. (2) "0"    indicator.        Illuminates    when    aircraft passes over an outer marker beacon. (3) "M"   indicator.      Illuminates   when   aircraft passes over a middle marker beacon. (4) "A"    indicator.        Illuminates    when    aircraft passes over an inner or airway marker beacon. c. Marker Beacon Operation. 1. Marker beacon volume control (marker beacon  audio  control  panel,  fig.    3-2) As required. 2. Marker   beacon   hi-lo   switch   (marker beacon  audio  control  panel,  fig.    3-2) As required. 3. Marker beacon indicator lights (instrument  panel,  fig.    2-30)  Confirm beacon indication. 3-26.   AUTOMATIC DIRECTION FINDER (DF203). a. Description.  The Automatic Direction Finder (ADF) (fig.    3-17),  is  a  radio  navigation  system  which  provides  a visual indication of aircraft bearing Change 2 3-31