TM 55-1510-220-10 (6) Manual   frequency   selector   (hundredths   and thousandths).    Selects  hundredths  and  thousandths  digits  of frequency (00, 25, 50, or 75) in MHz. (7) Preset  channel  selector.    Selects  one  of  20 preset channel frequencies. (8) Mode  selector.    Selects  operating  mode  and method of frequency selection. (a) MANUAL. Enables the manual selection of any one of 7,000 frequencies. (b) PRESET.  Enables selection of any one of 20 preset channels. (c) GUARD.  Selection automatically tunes the main receiver and transmitter to the guard frequency and the guard receiver is enabled. (9) SQUELCH    switch.        Turns    main    receiver squelch on or off. (10) VOL control.  Adjusts volume. (11) TONE pushbutton.  When pressed, transmits a 1,020 Hz tone on the selected frequency. (12) Function selector.  Selects operating function. (a) OFF.  Turns set off. (b) MAIN.      Selects   normal   transmission with reception on main receiver. (c) BOTH.      Selects   normal   transmission with   reception   on   both   the   main   receiver   and   the   guard frequency receiver. (d) ADF.        Activates    ADF    or    homing system (if installed) and main receiver. c. Normal Operation. (1) Turn on procedure: 1. Insure that aircraft power is on. NOTE It  is  presumed  that  the  avionics  master power   switch   is   on,   and   that   normally used    avionic    circuit    breakers    remain depressed. 2. Avionics   master   power   switch   (over- head control panel, fig. 2-12) - ON. 3. Function   switch   (UHF   control   panel, fig. 3-3) - MAIN or BOTH position, as required. NOTE If  function  selector  is  at  MAIN  setting, only   the   normal   UHF   communications will  be  received.    If  selector  is  at  BOTH position,   emergency   communications   on the    guard    channel    and    normal    UHF communications will both be received. (2) Receiver operating procedure: 1. UHF    audio    monitor    switch    (No.3,    audio control panel) - ON, or transmitter-interphone selector  switch  (audio  control  panel)  -  No.3 position. 2. Volume  control  (UHF  control  panel)  -  Mid position. (3) To use preset frequency (UHF control panel): 1. Mode selector switch - PRESET position. 2. Preset    channel    selector    switch    -Rotate    to desired channel. (4) To use non-preset frequency (UHF control panel): 1. Mode selector switch - MANUAL position. 2. Manual  frequency  selectors  (5)  -  Rotate  each knob to set desired frequency digits. NOTE The     PRESET     channel     selector     and manual frequency selectors are inoperative     when     the     mode     selector switch is set to the GUARD position. 3. Volume - Adjust. 3-7