TM 1-1510-223-10 is a 0 or 5) or a 5 (when the fifth digit is a 2 or 7), there- fore, 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.  AU 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 XFRI  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 1-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  XFHRMEM  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  XFRMEM  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. However,  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. NOTE The ACT annunciator continuing to flash after the frequency has been selected indicates that the  transceiver  is  not  tuned  to  the  frequency displayed in the active display. To   return   to   the   preset   frequency   selection   mode, push  the  ACT  switch  again  for  about  2  seconds.    As  a safety  feature,  the  transceiver  control  unit  switches  to the  active  frequency  selection  mode  when  a  frequency select   knob   is   operated   while   the   STO,   TEST,   or XFR/MEM switches are actuated. (4) Frequency     storage.          To     program     the memory,  select  the  frequency  in  the  preset  frequency display  using  the  frequency  select  knobs  and  push  the STO  switch  once.    One  of  the  channel  numbers  (CH  1 through   CH   6)   will;   appear   in   the   active   display   for approximately 5 seconds.  During this time the channel number  can  be  changed  with-  out  changing  the  preset frequency   by   momentarily   positioning   the   XFR/MEM switch to the MEM position.  After 3-9