TM 1-1510-224-10 signals,  or  the  two  warning  beeps.    The  microprocessor then  waits  until  the  push-to-talk  switch  opens  to  sound the two beeps. To transmit for more than 2 minutes, release the microphone  switch  briefly  and  then  press  it  again.    The 2-minute  timer  resets  and  starts  a  new  count  each  time the microphone switch is pressed. (6) Over-temperature  protection.      The microprocessor regularly monitors the temperature of the transmitter.      If   the   transmitter   gets   too   hot   during   a transmission, the microprocessor will stop the transmission,   and   the   sidetone   will   cease.      When   the microphone switch is released, you will hear two beeps. (Press the TEST switch on the transceiver control unit to observe   the   fault   code.)   As   long   as   the   temperature remains   above   the   limit,   the   microprocessor   will   not respond  to  a  normal  push  of  the  microphone  switch.    If you    must    transmit,    however,    you    can    override    the protection by rapidly keying the microphone switch twice, holding it on the second push. The shutdown temperature is 160°C (320°F). (7) Self-test.        An    extensive    self-test diagnostic  routine  can  be  initiated  in  the  transceiver  by pushing the TEST switch on the transceiver control unit. The   control   unit   will   modulate   the   active   and   preset display intensity from minimum to maximum to annunciate  that  self-test  is  in  progress.    Several  audio tones will be heard from the audio system while the self- test  routine  is  being  executed.    At  the  completion  of  the self-test program, the transceiver control unit will usually display dashes in the active display, and 00 in the preset display.    This  indicates  normal  operation.    If  any  out-of- limit  condition  is  found,  the  transceiver  control  unit  will display  DIAG  (diagnostic)  in  the  active  display  and  a  2- digit  fault  code  in  the  preset  display.    Record  any  fault codes displayed to help the service technician locate the problem.  Refer to table 3-1 for a description of the self- test fault codes that can be displayed on the transceiver control unit.  (The  TEST  switch  must  be  pushed  before any fault code can be displayed.) 3-11.    HF  COMMUNICATIONS  TRANSCEIVER  (KHF- 950). a. Description.          The     HF     communications transceiver (fig. 3-5), provides long-range voice communications  within  the  frequency  range  of  2.0000  to 29.9999  MHz  (280,000  possible  frequencies).    The  unit can  employ  either  amplitude  modulation  (AM)  or  upper sideband   (USB)   modulation.      (Lower   sideband   (LSB) modulation   has   not   been   enabled   in   this   installation.) The HF system consists of a control display unit located on   the   pedestal   extension,   a   receiver/exciter,   a   power amplifier/antenna coupler, a bus adapter, and an antenna.    The  system  is  powered  through  a  25-ampere circuit breaker placarded HF PWR and a 5-ampere  circuit  breaker  placarded  HF  RCV,  located  on the overhead circuit breaker panel (fig.  2-9). b. HF Transceiver Control-Display Unit Controls and Functions. (1) Digital  display.    The  digital  display provides frequency, mode, and operational status information.  The upper area of the display shows a two digit    channel    number    when    in    the    program    mode, followed  by  a  dash  and  the  first  one  or  two  digits  of  the operating   frequency   (with   the   emission   mode   selector switch   set   to   the   USB   or   AM   position).      Transmitter operation  (TX)  is  shown  at  the  right  end  of  this  display. The lower line of the display shows the last four digits of the     operating     frequency     (with     the     emission     mode selector switch set to the  USB or  AM position). (2) Light sensor.  A light sensor located to the right of the display senses ambient light conditions and adjusts display brightness accordingly. (3) Store  switch.      A   momentary   push button switch placarded STO, is used to store in memory the  displayed  data  when  programming  preset  channels. When the  STO  switch  is  pressed  simultaneously  with  a microphone transmit switch, a 1000 Hz operator attention   tone   will   be   transmitted   (required   by   some Canadian radiotelephone stations). (4) Frequency/channel   selector   knob. The   frequency/channel   selector   knob   (inner   concentric with  the  emission  mode  selector  switch)  allows  the  pilot to    set    channels    and    frequencies,    and    serves    as    a clarifier    control    in    sideband    mode.        Depressing    the control knob causes the flashing cursor on the display to move  to  the  digit  that  the  pilot  desires  to  change.    Each time  the  control  is  depressed,  the  cursor  moves  forward to   the   next   digit.      The   digit   at   the   cursor   position   is changed   by   rotating   the   channel/   frequency   selector knob. (5) Emission    mode    selector    switch. The  emission  mode  switch,  placarded   LSB,  USB,  AM, and  TEL (A3J).    (concentric  with  the  frequency/selector knob)  is  used  to  select  the  operating  mode  of  the  HF transceiver. (6) Squelch control.  A knob placarded SQ  (outer  concentric  with  the  off/volume  control  knob) provides    a    variable    squelch    threshold    control.        This control is used to help reduce background noise when a signal is not being received. (7) On, off, volume control.  A knob placarded OFF, VOL (inner  concentric  with  the  squelch  knob)  is  used  to  turn the transceiver on and off, and adjust volume. Clockwise  rotation  from  the  detent  applies  power  to  the 3-11