TM 55-1510-219-10 either    standard    amplitude    modulation    (AM),    lower sideband  (LSB),  or  upper  sideband  (USB)  modulation. The  distance  range  of  the  set  is  approximately  2,500 miles and varies with atmospheric conditions.  The unit is   protected   by   a   3-ampere   HF   RCVR   and   the   25- ampere HF PWR circuit breaker on the overhead circuit breaker panel (fig.  2-26).  The control panel is located on the pedestal extension (fig.  2-10). b.  Controls and Indicators. (1)  HF control panel fig.  3-7). CONTROL/ FUNCTION INDICATOR Frequency Selects the desired operating fre- selector quency. Frequency Displays the selected frequency. indicator SQL control Adjusts squelch level. RF TEST Indicates  operational status of indicator set when RT  TEST is selected with the function selector. Function selector Turns set off and determines op- erating mode. OFF Turns set off. USB Selects upper sideband modula- tion. LSB Selects lower sideband modula- tion. AM Selects amplitude modulation. CW Not used in this installation. SVU Not used in this installation. SVL Not used in this installation. RF TEST Check operational status of the system. c.  Normal Operation. (1)  Turn-on. 1.  Insure aircraft power is on. NOTE It    is    presumed    the    AVIONICS MASTER POWER switch is on and that  normally  used  avionic  circuit breakers remain depressed. NOTE Aircraft    can    be    configured    for either HF or VOW on position 4 of the audio control panels. 2.      AVIONICS   MASTER   POWER   switch (overhead panel) - ON. (2)  Receive. 1.  HF audio switch (#4, audio panel) - ON. 2.    Function  selector  (HF  panel,  pedestal extension) -Set to USB, LSB, or AM. 3.      Volume   knob   (audio   switch   #4,   audio panel) - Set to mid position. 4.  VOL knob (audio panel) - Adjust. 5.    SQL  knob  (HF  panel)  -  Adjust  to  just quiet   noise   when   no   signal   is   being received. 6.    Tuning  knob  (HF  panel)  -  Set  desired frequency. (3)  Transmit. 1.      Transmitter-interphone   selector   (audio panel) -No.  4 position. 2.    Function  selector  (HF  panel)  -  Set  to USB, LSB, or AM. 3.  MIC  HEADSET/OXYGEN MASK switch (instrument panel) -As desired. 4.    SQL  knob  (HF  panel)  -  Adjust  to  just quiet   noise   when   no   signal   is   being received. 5.  MIC switch - Depress to transmit. (4)  Shutdown. 1.  Mode selector knob (HF panel) - OFF. d.  RF Test. (1)  Receive check. 1.  Set SQL control fully clockwise and tune to WWV.  Check receive operation. NOTE Radio station WWV operates continuously ton 2.500 Mhz, 5.000Mhz, 10.000 Mhz, 15.000 Mhz, and    20.000    Mhz.        Select    the frequency  that  provides  the  best received signal strength. 3-14