TM 55-1510-220-10 (5) RF  TEST  indicator.    Tests  operational  status of set.  Unkeyed Tests the light.  Keyed Indicates operational status of set. (a) Burning  steadily.    Indicates  a  fault  in the receiver exciter. (b) Blinking.  Indicates a fault in the power amplifier-coupler. (c) Extinguished. Indicates normal operation. (6) Mode selector.  Turns set off and determines operating mode. (a) OFF.  Turns set off. (b) USB. Selects Upper Side-board modulation. (c) LSB. Selects Lower Side-board modulation. (d) AM.  Selects Amplitude Modulation. (e) CW.  Not used in this installation. (f) SVU.  Not used in this installation. (g) SVL.  Not used in this installation. (7) RF  TEST.    Checks  operational  status  of  the system. c. Normal Operation. (1) Turn-on procedure: 1. Insure that aircraft DC power is on. NOTE It   is   presumed   the   avionics   master power  switch  is  on  and  that  normally used  avionic  circuit  breakers  remain depressed. NOTE The   aircraft   can   be   configured   for either HF on position 4, or VHF No.  2 and  VOW,  on  position  5  of  the  audio control panel. 2. Avionics master power switch (overhead   control   panel,   fig.      2-12) ON. (2) Receiver operating procedure: 1. Transmitter-interphone selector switch (audio  control  panel,  fig.    3-1)  No.4 position. 2. HF  function  selector  switch  (fig.    37) Set to USB, LSB, or AM. 3. No.4    radio    monitor    control    (audio control  panel,  fig.    3-1)  Set  to  mid position. 4. Volume  control  (audio  control  panel) Adjust. 5. Squelch control (HF control panel) As required. 6. Frequency  control  (HF  control  panel) Set desired frequency. (3) Transmitter operating procedure: 1. Transmitter-interphone selector switch (audio control panel) No.4 Position. 2. Function   selector   (HF   panel)   Set   to USB, LSB, or AM. 3. MIC     HEADSET-OXYGEN     MASK switch (instrument panel) As desired. 4. SQL  knob  (HF  panel)  Adjust  to  just quiet  noise  when  no  signal  is  being received. 5. MIC switch Depress to transmit. (4) Shutdown procedure: 1. Mode selector knob (HF panel) OFF. d. HF Command Set Emergency Operation. Not applicable. 3-15.   VOICE SECURITY SYSTEM (TSEC/KY75). a. Description.    The  KY-75  voice  security  system  is used  in  conjunction  with  the  HF  command  set  to  provide secure (ciphered, two-way voice communications.  The voice security  control  panel  is  located  in  the  INS  equipment  rack, behind the copilot (fig.  3-8). Change 2 3-16