TM 1-1510-224-10 be  displayed  in  the  lower  window.    When  this  switch  is held to the MEM position, one of the six stacked memory frequencies    will    be    loaded    into    the    preset    display. Successive pushes will cycle the six memory frequencies through the display (   2,3,4,5,6,1,2,3   ). (3) Store switch. This switch, placarded STO, allows up to six preset frequencies to be selected   and   entered   into   the   control   unit's   memory. After   presetting   the   frequency   to   be   stored,   push   the STO  switch.    The  upper  window  displays  the  channel number  of  available  memory  (CH  1  through  CH  6)  while the  lower  window  continues  to  display  the  frequency  to be stored.  For approximately 5 seconds,   the   MEM   switch   may   be   used   to   advance through    the    channel    numbers    without    changing    the preset  display.    Push  the  STO  switch  a  second  time  to commit  the  preset  frequency  to  memory  in  the  selected location.    After  approximately  5  seconds,  the  control  will return to normal operation. (4) Tuning    knobs.        Two    concentric tuning   knobs   control   the   preset   or   active   frequency displays.  The larger knob changes the three digits to the left  of  the  decimal  point  in  1-MHz  steps.    The  smaller knob  changes  the  two  digits  to  the  right  of  the  decimal point in 50-kHz steps (or in 25-kHz steps for the first two steps after the direction of  rotation  has  been  reversed).    The  numbers  will  roll over at the upper and lower frequency limits. (5) Active  switch.    The  active  switch, placarded ACT, enables the tuning knobs to directly tune the  VHF  transceiver,  when  depressed  and  held  for  two seconds.    The  bottom  window  will  display  dashes  and the   upper   window   will   continue   to   display   the   active frequency.  Pushing the   ACT  switch  a  second  time  will return the control unit to the normal two-display mode. (6) Test switch.    This  switch  placarded TEST     initiates     the     transceiver     self-test     diagnostic routine.  (Selftest is active only when the  TEST switch is pressed.) (7) Light    sensor.        This    built-in    light sensor automatically controls display brightness. (8) Volume control.  The volume control is concentric with the power and mode switch. (9) Power    and    mode    switch.        The power     and     mode     switch     contains     three     detented positions.      The   ON   and   OFF   positions   switch   system power.  SQ OFF position disables the receiver squelch circuits. (10) Annunciators. The transceiver control    unit    contains    a    MEM    (memory)    and    a    TX (transmit) annunciator. The MEM annunciator illuminates whenever a preset 1. Active Frequency Display 2. Transfer/Memory Switch 3. Store Switch 4. Tuning Knobs 5. Active Switch 6. Test Switch 7. Light Sensor 8. Volume Control 9. Power and Mode Switch 10. Annunciators (MEM, TX) 11. Preset Frequency Display 12. Compare Annunciator A93FD00D0098 C Figure 3-4.  VHF Communications Transceiver Control Unit 3-8