TM 1-1510-223-10 To   return   to   the   preset   frequency   selection   mode, push  the  ACT  switch  again  for  2  seconds.    As  a  safety feature,    the    ADF    receiver    control    unit    automatically switches  to  the  active  frequency  selection  mode  when  a frequency select knob is operated while the STO,  TEST, or XFR/MEM switches are actuated, or the memory recall inputs to the control are grounded. (4)   Frequency    storage.        Up    to    four    preset frequencies   can   be   stored   in   the   memory   in   the   ADF receiver  control  unit  for  future  recall.    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   4)   will   appear   in   the   active   display   for approximately five seconds.  During this time the channel number   can   be   changed   without   changing   the   preset frequency    by    momentarily    positioning    the    XFR/MEM switch  to  the  MEM  position.    After  the  desired  channel number has been selected, push the STO switch again to store the frequency. NOTE When storing a frequency, the second actuation of   the   STO   switch   must   be   done   within   5 seconds  after  selecting  the  channel  number  or the  first  actuation  of  the  STO  switch.    If  more than 5 seconds elapse, the control will revert to the  normal  modes  of  operation  and  the  second store  command  will  be  interpreted  as  the  first store command. After  a  frequency  has  been  stored  in  memory,  it  will remain   there   until   changed   by   using   the   STO   switch Memory is retained even when the unit is turned off for an extended period of time. d. Normal Operation. 1. Power and mode switch - ANT, ADF, or TONE (BFO). 2. Tuning knobs - Set desired frequency. 3. ANT function - Position power and mode switch to ANT.  Select ADF on audio system and adjust volume. 4. ADF function - Position power and mode switch to ADF.  Bearing pointer will indicate relative bearing to tuned station. NOTE When the ADF system is not receiving a reliable signal,   bearing   pointer   will   be   removed   from view. The ADF bearing pointer may momentarily    blank    during    station    crossings because of signal loss. 5. TONE function - Position power and mode switch to TONE (BFO).  A 1000-Hz tone will identify keyed CW stations. (1)   Self-test 1. Power and mode switch - ADF. 2. Tuning knobs - Tune a nearby NDB, compass locator, or broadcast station. 3. TEST switch - Depress.  Bearing pointer will rotate 90 degrees from the previous valid indication.  Release TEST switch and verify that the bearing pointer returns to previous valid indication. NOTE If the signal received is weak or of poor quality, bearing pointer rotation will be slow. ADF receiver fault codes are listed in table 3-8. 3-26. TACAN/DME SYSTEM (AN/ARN-154). a. Description.   The TACAN/DME system (fig.  3-26) operates in conjunction with TACAN and VORTAC ground stations to provide distance, groundspeed, time to station, and    bearing    to    station.        The    TACAN/DME    system electronically    converts    elapsed    time    to    distance    by measuring the length of time between the transmission of a radio signal to a selected DME/TACAN station and the reception  of  the  reply  signal.    The  distance  is  measured on  a  slant  from  the  aircraft  to  the  ground  station  and  is commonly   referred   to   as   slant-range   distance.      Slant range     approximates     actual     ground     distance.          The difference    between    slant-range    distance    and    ground distance  is  smallest  at  low  altitude  and  long  range,  and greatest  when  in  close  proximity  to  the  selected  ground station.  The system is capable of tracking the range of up to     four     stations     and     the     bearing     of     two     stations simultaneously.    The  system  will  operate  on  any  one  of 252   preselected   frequencies   (126   X   mode   and   126   Y mode  channels).    Course  deviation,  bearing  to  station, and    distance    to    TACAN    or    VORTAC    stations    are displayed on the EHSI.  Distance, time to station, bearing, station  identifier,  and  groundspeed  may  be  displayed  on the    TACAN    digital    display.        The    TACAN/DME    also provides   distance   information   to   the   inertial   navigation system  (INS).    The  system  is  protected  by  a  2-ampere circuit     breaker     placarded     TACAN,     located     on     the overhead circuit breaker panel (fig.  2-7). b. TACAN  Control  Unit  Controls  and  Functions  (fig. 3-26). (1)   TACAN  channel  display.    Indicates  selected channel. 3-63