TM 1-1510-223-10 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. (5) Stuck  microphone  switch.    Each  time  the push-to-talk  switch  is  depressed,  the  microprocessor  in the    transceiver    starts    a    2-minute    timer.        (The    TX annunciator    on    the    transceiver    control    unit    will    be illuminated  whenever  the  transmitter  is  transmitting.)  If the   transmitter   is   still   transmitting   at   the   end   of   2 minutes,     the     microprocessor     turns     it     off.          Most intentional transmissions last much less than 1 minute;, a  2-minute  transmission  is  most  likely  the  result  of  a stuck  microphone  switch.    This  timing  feature  protects the ATC channel from long-term interference. When  it  turns  off  the  transmitter,  the  microprocessor switches  the  transceiver  to  receive  operation.    A  stuck microphone    switch    will    prevent    you    from    hearing received   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 micro- phone switch briefly and then press it again.  The 2-minute timer resets and starts a new count each time the micro- phone 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,   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. 2-7). 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  (X)  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 3-10