TM 1-1510-224-10 frequency  is  being  displayed  in  the  lower  window.    The TX  annunciator  illuminates  whenever  the  transceiver  is transmitting. (11) Preset frequency display.  Displays the preset (inactive) frequency and diagnostic messages. (12) Compare annunciator. An annunciator    placarded    ACT    momentarily    illuminates when    frequencies    are    being    changed.        The     ACT annunciator flashes if the actual radio frequency to which the  transceiver  is  tuned  is  not  identical  to  the  frequency shown in the active frequency display. c. Operating   Procedures.      The   frequencies displayed  on  the  transceiver  control  unit  show  only  five of  the  six  digits.    The  sixth  digit  is  always  a  zero  (when the fifth digit is a 0 or 5) or a 5 (when the fifth digit is a 2 or   7),   therefore,   the   sixth   digit   does   not   need   to   be displayed. NOTE 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  continually  assessing the   reasonableness   of   operation   as displayed on the associated transceiver   control   unit   and   by   the quality of received signals and transmissions. (1) Equipment turn-on.  The transceiver and  the  control  unit  are  turned  on  by  rotating  the  power and  mode  switch  on  the  transceiver  control  unit  to  the ON  position.    When  the  transceiver  is  first  turned  on  it sounds  a  brief  tone  while  the  microprocessor  checks  its own   memory.      If   there   is   a   memory   defect   the   tone continues,   indicating   that   the   transceiver   can   neither receive   nor   transmit.      After   the   memory   check,   the transceiver  control  unit  will  display  the  same  active  and preset frequencies that were present when the equipment was last turned off. NOTE If  two  short  800-Hz  tones  are  heard, the     transceiver     has     detected     an internal  fault.    Push  the  TEST  switch on    the    transceiver    control    unit    to initiate  self-test  and  display  the  fault code.  Adjust the volume and perform a   quick   squelch   test   by   setting   the power    and    mode    switch    on    the transceiver   control   unit   to   SQ   OFF and  adjusting  the  volume  level  with background noise. After a comfortable  listening  level  has  been established,   return   the   power   and mode  switch  to  the  ON  position.    All background   noise   should   disappear unless a station or aircraft is transmitting on the active frequency. (2) Frequency   selection.      Frequency selection   is   made   using   either   the   frequency   select knobs, or the XFR/MEM  (transfer/memory  recall)  switch.    Rotation  of either frequency select knob increases or decreases the frequency  in  the  preset  frequency  display.    The  larger, outer  knob  changes  the  frequency  in  I-MHz  increments (the  number  to  the  left  of  decimal  point).    The  smaller, inner knob changes the frequency in 50-kHz increments when  first  rotated,  or  in  25-kHz  increments  for  the  first two increments after the direction of rotation is reversed. After the desired frequency is set into the preset frequency   display,   it   can   be   transferred   to   the   active frequency display by momentarily setting the   XFR/MEM switch   to   XFR.      At   the   same   time   that   the   preset frequency    is    transferred    to    the    active    display,    the previously  active  frequency  is  transferred  to  the  preset display.      A   short   audio   tone   is   applied   to   the   audio system  to  indicate  that  the  active  frequency  has  been changed,    and    the    active    (ACT)    annunciator    on    the control  will  flash  while  the  transceiver  is  tuning  to  the new frequency. NOTE The active (ACT) annunciator continuing  to  flash  indicates  that  the transceiver     is     not     tuned     to     the frequency    displayed    in    the    active display. The transceiver control unit's memory permits storing up to six preset frequencies. Once stored, these frequencies   can   be   recalled   to   the   preset   display   by positioning  the  XFR/MEM  switch  to  the    MEM  position. The   storage   location   ( CH   1   through    CH   6 )    for    the recalled  frequency  is  displayed  in  the  active  frequency display while the  XFR/ MEM  switch  is  held  in  the  MEM position.  All six stored frequencies can be displayed one at  a  time  in  the  preset  display  by  repeatedly  positioning the  XFR/MEM  switch  to  the  MEM  position.    After  the desired stored frequency has been recalled to the preset display,  it  can  be  transferred  to  the  active  display  by momentarily   positioning   the   XFR/   MEM   switch   to   the XFR position. During normal operation, all frequency selections     and     revisions     are     done     in     the     preset frequency  display.    How-ever,  the  active  frequency  can be    selected    directly    as    described    in    the    following paragraph. (3) Direct   active   frequency   selection. The  active  frequency  can  be'  selected  directly  with  the frequency  select  knobs  by  pushing  the  ACT  switch  for about  2  seconds.    The  active  frequency  selection  mode is  indicated  by  dashes  appearing  in  the  preset  display. Also,  the  ACT  annunciator  will  flash  as  the  frequency select knobs are turned to indicate that the transceiver is being returned. 3-9