TM 1-1510-218-10   3C-13 panel to the left of the tuning knobs is used to store in memory  the  displayed  data  when  programming  preset channels. (8)   OFF    /    VOLUME    Control.        A    knob, placarded  OFF / VOLUME,  located  on  the  HF  control panel turns the system off and on and controls volume.   Clockwise  rotation  from  the  detent  applies  power  to the   system.      Further   clockwise   rotation   increases audio output level. (9)   SQUELCH   Control.      A  knob  placarded SQUELCH  on  the  HF  control  unit  provides  a  variable squelch threshold control. (10)   CLARIFIER  Control.    A  knob  placarded CLARIFIER PULL ON located on the lower left corner of  the  HF  control  unit  is  used  to  eliminate  unnatural audio  quality  when  operating  in  the   Single Sideband (SSB) mode by slightly shifting the receiver generated frequency to match the frequency of the signal being received.    To  operate  the  clarifier,  the  knob  is  pulled out and rotated in either direction from the index mark (which is located directly above the clarifier knob) until the audio quality is optimized.  When the voice quality is   natural   the   clarifier   knob   should   be   pushed   in.   Turning   the   clarifier   knob   has   no   effect   when   it   is pushed in. c.   Frequency  Selection.    The  HF  system  has two  methods  of  frequency  selection,  frequency  mode and     channel     mode.          In     the     frequency     mode, frequencies are tuned directly on the display using the tuning  knobs.    In  the  channel  mode,  preset  channels are programmed with an assigned channel number (1 through 99) and stored in memory. (1)   Frequency    Mode.        In    the    frequency mode,  the  desired  frequency  is  set  into  the  display using   the   tuning   knobs.      Only   simplex   operation   is allowed while operating in the frequency mode. (2)   Channel Mode.  When the HF control unit is in the channel mode, channels and their respective frequencies  are  changed  using  the  tuning  knobs.    To place   the   unit   into   the   channel   mode,   press   the FREQ/CHAN  switch.    This  allows  access  to  existing programmed  channels.    Frequencies  in  the  channel mode are stored with channel number, emission mode (USB,    LSB,    or    AM),    and    transmit    and    receive frequency. (3)   Program Mode.  When the HF control unit is  in  the  program  mode,  channel  numbers,  emission mode  (USB,  LSB,  or  AM),  and  transmit  and  receive frequency   are   set   up   and  stored  in  memory.    The program  mode  is  entered  from  the  channel  mode  by pressing the PGM switch with a pencil or other pointed object.    When  in  the  program  mode,  channel  number, emission    mode,    and    transmit    frequency    are    all displayed.      Transmitter   operation  is  inhibited.    The transmit  frequency  may  be  examined  by  keying  the microphone.    Channels  may  be  programmed  for  the following types of operation. (a)   Receive Only. The operator programs a frequency in the receive portion of transmit frequency.      The   transmitter   is   locked   out   when   a channel    has    been    programmed    for    receive    only operation.      The   receive-only   function   is   used   for listening  to  weather,  time,  omega  status,  frequency standard, and geophysical alert broadcasts. (b)   Simplex   Operation.       The   operator programs the same frequency in receive and transmit, and  assigns  an  operating  mode  (USB,  LSB,  or  AM).   The   simplex   function   is   used   by   air   traffic   control, ARINC, and others. (c)   Semi-Duplex. In semi-duplex operation the operator programs two different frequencies,   one   for   transmit   and   one   for   receive.    The     semi-duplex     function     is     used     by     maritime radio/telephone network (public correspondence) stations. d.  HF Communications Transceiver Operation. (1)   Direct Frequency Tuning Operation (Simplex Only). (a)   Frequency/Channel  Switch  –  FREQ (out position). (b)   Mode Selector Switch – Set emission mode (USB, LSB, or AM). (c)   Tuning Knobs – Set desired frequency, then stow cursor. (d)   Antenna    Coupler     –    Tune,    press microphone button. (2)   Programming Preset Channels. (a)   Receive Only. 1.   Cursor  –  Stow  if  a  frequency digit is flashing. 2.   Tuning Knobs  – Select channel to be preset and desired frequency.