TM 1-1510-218-10 3A-13 Table 3A-2.  Self-test Codes   FAULT CODE DESCRIPTION FAULT CODE DESCRIPTION 00 No fault found 15 Frequency out of range 01 5 Vdc below limit 16   Forward power below limit 02 5 Vdc above limit 17 Transmitter temperature excessive 03 12 Vdc below limit 21 Tuning    voltage    out    of    limit    at    highest    receive frequency 04 12 Vdc above limit 22 Tuning voltage out of limit at 118 MHz 05 Synthesizer not locked 23 Local oscillator output below limit 07 Noise squelch open without signal 24 No-signal AGC voltage too high 08 Noise squelch open without signal 25 Inadequate AGC voltage increase with rf signal 12 BCD frequency code invalid 26 Reflected rf power above limit 13 2/5 frequency code invalid 27 Transmitter timed out 14 Serial message invalid 3A-11.  HF COMMAND SET (718U-5). a.   Description.  The HF command set provides long-range voice communications within the frequency range of 2.000 to 29.999 MHz.  The HF command set employs  standard  Amplitude  Modulation  (AM),  Lower Sideband (LSB) modulation, or Upper Sideband (USB) modulation.    The  distance  range  of  the  HF  command set   is   approximately   2,500   miles,   and   varies   with atmospheric   conditions.      The   HF   command   set   is protected    by    the    3-ampere    HF    RCVR    and    the 25-ampere  HF  PWR  circuit  breakers  located  on  the overhead circuit breaker panel, Figure 2-16, Sheets 1 and  2.    The  associated  antenna  is  shown  in  Figure 2-1. NOTE Keying the HF radio set while operating the ADF    No.    2    set    causes    unreliable    ADF signals.  b.  Controls and Functions, HF Command Set Control Panel.  Refer to Figure 3A-6. (1)   Frequency Selectors. Operating frequency is selected by turning the frequency selector controls    until    the    desired    operating    frequency    is indicated.  The controls may be operated in any order. (2)   MHz    Indicators.          Indicates    operating frequency of set. (3)   SQL   Control.       The   squelch   control   is adjusted    to    mute,    undesired,    background    noise.   Setting the control too  far  counterclockwise  can  result in blocking out weak signals.  There are two different types of squelch control depending on the configuration installed in the aircraft. One configuration is the voice-operated squelch control that functions to mute the receiver output in the absence of a  receive  signal.    The  other  type  combines  with  RF gain   control.      Adjustment   method   of   the   squelch control is different for the two configurations. (4)   RF  TEST  Indicator.    Four  conditions  of the  RF  TEST  lamp  indicate  the  operational  status  of the  system.    The  mode  selector  must  be  in  the    RF TEST  mode  position.    Lamp  illuminated  or  flashing with  system  unkeyed  provides  a  lamp  test  of  the   RF TEST    lamp.        Lamp    not    illuminated,    flashing,    or illuminated    with    the    system    keyed    indicates    the operational status of the rest of the system. (a)  Illuminated. Indicates fault in receiver-exciter portion. (b)  Blinking.        Indicates   fault   in   power amplifier-coupler. (c)  Extinguished. System is operational. (5)   Mode    Selector.        Turns    set    off    and determines operating mode. (a)  OFF.  Turns set off. (b)  USB.  Places set in Upper Sideband mode.