TM  55-1510-221-10 (2.)   Copilot's COURSE   INDICATOR switch.  Selects   VOR   receiver   to   control   copilot’s HSI. HSI. (a.) VOR 1. VOR 1 controls copilot’s HSI. (b.) VOR 2. VOR 2 controls copilot’s (3.)   NAV-A   switch   (audio   control   panel, fig. 3-1). Applies VOR audio to respective headsets. (4.)  MKR  BCN  HI-LO  (marker  beacon audio control panel, fig. 3-2). Controls sensitivity of marker beacon receiver. e. VOR  Operation. (1.)   Turn-on   procedure: 1. Insure  that  aircraft  DC  and  AC power is on. 2.   Avionics   master   power   switch (overhead control panel, fig. 2-12) - ON. 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,    then    NAV    B    audio switches (audio control panel, fig. 3-1) - ON. Confirm proper signal, then OFF. 6.  RMI  and  HSI  -  Confirm  proper indications. (2.)  Normal  Operation. 1. 2. 3. 4. Pilot/copilot course indicator switches  (instrument  panel)  - Select VOR source. To determine course to station on pilot’s   HSI:   TO-FROM   pointer reads TO (up) position. To  determine  bearing  from  sta- tion  on  pilot’s  HSI:  TO-FROM pointer  reads  FROM  position (down). To determine course to station on RMI:  Select  VOR,  verify  single needle points course to station. (3.)  Localizer  (LOC)  operation: 1. VOR  frequency  knob  (NAV panel) - Select frequency. 2.   Pilot’s,   copilot’s   COURSE   INDI- CATOR   switches   (instrument panel) - Select VOR source. 3.  Check  glideslope  flags  unmasked. (4.)  VOR  receiver  operation: 1.   VOR   frequency   control   knob (VOR  control  panel,  fig.  3-17)  - Select  desired  frequency. 2.   Volume   control   knob   (VOR   con- trol panel, fig. 3-17) - Full on. 3.    NAV    A,    audio    switch    -    ON. Adjust audio. (5.)  Shutdown  procedure:  Volume   control (VOR control panel, fig. 3-17) - OFF. 3-26. 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 receiver.  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. Volume, and sensitivity controls are located on the marker beacon audio control panel (fig. 3-1). b. Controls/Indicators and Functions. (1.)   Marker   beacon   sensitivity   switch (marker beacon audio control panel, fig. 3-2).  Con- trols sensitivity of marker beacon receiver. (a.)  HI  position.  Enables  high  sensi- tivity operation of marker beacon receiver. (b.)  LO  position.  Enables  low  sensi- tivity operation of marker beacon receiver. (2.)   "0"   indicator.   Illuminates   when   air- craft passes over an outer marker beacon. (3.)  "M"  indicator.  Illuminates  when  air- craft passes over a middle marker beacon. (4.)  "A"  indicator.  Illuminates   when   air- craft 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. 3-32