TM 1-1510-218-10 3B-8 active frequency display while the  XF / MEM switch is held in the   MEM  position.    All  six  stored  frequencies can be displayed one at a time in the preset display by repeatedly   positioning   the   XF   /   MEM   switch   to   the MEM  position.  After the desired stored frequency has been    recalled    to    the    preset    display,    it    can    be transferred    to    the    active    display    by    momentarily positioning the XF / MEM switch to the XF position. During normal operation, all frequency selections and revisions are done in the preset frequency display.   However, the active frequency can be selected directly as described in the following paragraph. (3)   Direct  Active  Frequency  Selection.    The active   frequency   can   be   selected   directly   with   the frequency select knobs by pushing the  ACT button for about   2   seconds.      The   active   frequency   selection mode  is  indicated  by  dashes  appearing  in  the  preset display.    Also,  the   ACT  annunciator  will  flash  as  the frequency select knobs are turned to indicate that the transceiver is being returned. NOTE The  ACT  annunciator  continuing  to  flash after    the    frequency    has    been    selected indicates  that  the  transceiver  is  not  tuned to  the  frequency  displayed  in  the    active display. To    return    to    the    preset    frequency    selection mode,     push     the     ACT     button     again     for     about 2 seconds.  As a safety feature, the transceiver control unit  switches  to  the  active  frequency  selection  mode when  a  frequency  select  knob  is  operated  while the STO   and   TEST  buttons,  or    XF   /   MEM  switch  are actuated. (4)   Frequency    Storage.        To    program    the memory, select the frequency in the preset frequency display using the frequency select knobs and push the STO   button   once.      One   of   the   channel   numbers (CHAN 1  through  CHAN  6)  will  appear  in  the  active display for approximately 5 seconds.  During this time the channel number can be changed without changing the  preset  frequency  by  momentarily  positioning  the XF/MEM switch to the MEM position.  After the desired channel   number   has   been   selected,   push   the   STO button again to store the frequency. NOTE When    storing    a    frequency,    the    second actuation of the STO button must be done within     5     seconds     after     selecting     the channel number or the first actuation of the STO   button.        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 button.    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  pressed,  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 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)   Overtemperature 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  button  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    button.        The    control    unit    will