TM 1-1510-218-10 3A-9 d.  Emergency Operation. NOTE Transmission    on    emergency    frequency, 121.500 MHz, will be restricted to emergencies only. An emergency frequency  of  243.000  MHz,  guard  channel, is   also   available   on   the   UHF   command radio set. 1.   Transmitter selector switch – VHF 1 or VHF 2. 2.   Frequency selector, VHF control panel – 121.500 MHz, emergency frequency. 3.   Microphone switch – Press. 3A-10.  VHF   COMMUNICATIONS   TRANSCEIVERS (VHF-22D). a.   Introduction.        The    VHF    communications transceivers  are  an  8.33  kHz  channel  spacing  cable system.      The   operation   between   the   25   kHz   and 8.33 kHz   modes   is   seamless.      Within   the   space occupied    by    each   2 5    kHz    frequency    exist    three 8.33 kHz   channels.      The   8.33   kHz   "frequency"   is referred  to  as  a  "channel,"  not  the  actual  frequency tuned.  The channel scheme avoids possible confusion associated    with    shared    25    kHz    and    8.33    kHz frequencies.  This method ensures that the equipment will  always  be  operating  in  the  proper  spacing  mode and    occurs    automatically    as    the    user    tunes    the controller.      Provides   airborne   VHF   communications from 118.000 through 151.975 MHz and is operated by two CTL-22C transceiver control units.  Refer to Figure 3A-5. The    solid-state    transceiver    includes    capture- effect automatic squelch to help prevent missed radio calls,   plus   audio   leveling   and   response   shaping   to ensure   audio   quality.      Transmitter   sidetone   comes from  detected  transmitter  signal,  and  is  therefore  a reliable   check   of   transmission   quality.      Each   VHF transceiver  is  powered  through  its  respective  circuit breaker, placarded VHF # 1 or VHF # 2, located on the circuit breaker panel. b.  Operating  Controls.    All  operating  controls for   the    transceivers    are    located    on    the    CTL -22C transceiver control units.  Refer to Figure 3A-5. (1)   Active  Frequency  Display.     Displays  the active frequency (frequency to which the transceiver is tuned) and diagnostic messages. (2)   Transfer / Memory Switch.  This switch is a three-position spring-loaded toggle switch placarded XF   /   MEM,   which,   when   held   to   the   XF  position, causes  the  preset  frequency  to  be  transferred  up  to the active display and the transceiver to be returned.   The  previously  active  frequency  will  become  the  new preset  frequency  and  will  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...). 1.     Active Frequency Display 2.     Transfer/Memory Switch 3.     Store Button 4.     Frequency Select Knobs 5.     Active Button 6.     Test Button 7.     Light Sensor 8.     Volume Control 9.     Power and Mode Switch 10.     MEM TX Annunciators 11.     Preset Frequency Display 12.     Compare Annunciator Figure 3A-5.  VHF Communications Transceiver Control Unit  – CTL-22C