TM  1-1520-240-10 3-3-2            CONTROLS/ INDICATOR FUNCTION MB SENS Labeled HI and LO. LO sensitivity is used for positive identification of marker beacon passage. HI is used for station passage identification at the outer marker or en- route. Marker Beacon Indicator Lights Three lights marked 0,M, and I on both HSI MODE SELECT panels. Lights illuminate when passing over beacons. 3-3-4.  Normal  Operation  -  VHF  Navigation  and  In- strument Landing System.  The following paragraphs provide navigation radio set operating procedures. a.  Starting. (1) Interphone AUX switch - ON Figure  3-3-1.  VHF Navigation and Instrument Landing System Control (AN/ARN-123) (2) NAV VOL switch — ON. b.  VOR/Localizer/ILS Operation. (1) Megahertz and kilohertz selector — Set fre- quency of facility. (2) VOR  SEL  switch  on  HSI  MODE  SELECT panel — Press. Check SEL light lit. (3) VOR ADF switch on HSI MODE SELECT panel — Press, if VOR segment is not lit (4) Volume control — Adjust. Identify facility (5) HSIs — Check for correct indications. c.  Marker Beacon operation. (1) MB VOL switch — ON. (2) MB  SENS  —  HI  for  enroute.  Lo  for  ap- proach. (3) MKR BKN lamps on HSI MODE SELECT panels — Check lit at corresponding station passage. d.  Stopping — NAV VOL and MB VOL switches — OFF 3-3-5.  Direction Finder Set (AN/ARN-89). Direction finder set (AN/ARN-89) is an airborne automat- ic direction finder (ADF) operating within the frequency range of 100 to 3,000 kHz. The equipment provides visu- al and aural facilities for ADF homing and position fixing. It is used as a navigational radio aid to continuously and visually indicate the magnetic bearing of a radio station while providing aural reception. The bearing of a radio station is displayed on the No. 2 bearing pointer on the HSI. Power to operate the direction-finder set is supplied by the No. 1 DC bus and the AC instrument bus through the NAV ADF circuit breakers on the No. 1 PDP. 3-3-6.  Controls and Function, Direction Finder Set (AN/ARN-89).  (fig. 3-3-2) CONTROLS/ INDICATOR FUNCTION Mode Switch OFF OFF position deener- gizes the direction finder set. COMP COMP position selects automatic direction find- ing. ANT ANT position permits re- ception for radio naviga- tion or as a broadcast receiver. LOOP LOOP position, used in conjunction with LOOP switch, permits aural null homing and manual di- rection finding.