TM 1-1510-224-10 (7)   Light  sensor.    This  built-in  light  sensor automatically controls display brightness. (8)   Volume  control.    The  volume  control  is concentric with the power and mode switch. (9)   Power   and   mode   switch.      The   power and    mode    switch    contains    three    detented    positions placarded   OFF,   ON,   and   HLD.      The   ON   and    OFF position    switches    system    power.        The    hold    ( HLD) position  allows  the  navigation  receiver  frequency  to  be changed while holding the DME in its present frequency. (10)Annunciators.    The  receiver  control  unit contains MEM (memory) and RMT (remote) annunciators. The MEM annunciator illuminates whenever  a  preset  frequency  is  being  displayed  in  the lower window.  The  RMT annunciator is not used in this installation. (11)Preset   frequency   display.      The   preset (inactive)        frequency    and    diagnostic    messages    are displayed in the lower window. (12)Compare  annunciator.    An  annunciator placarded ACT momentarily illuminates when frequencies  are  being  changed.    The   ACT   annunciator flashes if the actual radio frequency which the receiver is tuned  to  is  not  identical  to  the  frequency  shown  in  the active frequency display. c. Operating Procedures. WARNING It is not practical to provide monitoring for all conceivable system failures and, it is possible    that    erroneous    operation    could occur   without   a   fault   indication.      It   is   the responsibility  of  the  pilot  to  detect  such  an occurrence  by  means  of  cross-checks  with redundant or correlated information available in the cockpit. (1)   Equipment turn-on. The VIR-32 receiver  and  the  CTL-32  NAV  control  are  turned  on  as follows: 1. BATTERY  switch  (overhead  control panel, fig. 2-9)  - ON. 2. AVIONICS MASTER POWER switch  (overhead  control  panel,  fig. 2-9) - ON or  EXT PWR as required. 3. EFIS   power   switches   (2,   overhead control panel, fig. 2-9)  -  ON. 1. Active Frequency Display 2. Transfer/Memory Switch 3. Store Switch 4. Tuning Knobs 5. Active Switch 6. Test Switch 7. Light Sensor 8. Volume Control 9. Power and Mode Switch 10. Annunciators (MEM, RMT, HLD) 11. Preset Frequency Display 12. Compare Annunciator A93FE00D0107 C Figure 3-24.  VHF Navigation Receiver Control Unit 3-58