TM 55-1510-219-10 Microphone Controls  activation  of micro- select switches phones. HOT MIC Admits  speech  to  interphone system without need to key se- lected microphone. NORM Blocks speech from interphone system unless selected micro- phone is keyed. ICS OFF Deactivates microphones (3)  Transmitter select switch (fig.  3-1). CONTROL FUNCTION Transmit Connects microphone and head- interphone set to selected radio transmitter selector switch or interphone line routing re- ceived audio to headset.  Bypass- es control of respective receiver audio switch. PVT Position not used. ICS Activates pilot-to-copilot  inter- com. 1 Permits audio reception from VHF-AM No. 1 transceiver. Routes key and mic signals to VHF-AM No.  1 transceiver. 2 Permits audio reception from VHF/AM/FM transceiver. Routes key and mic signals to VHF/AM/FM transceiver. 3 Permits audio reception  from UHF transceiver.  Routes key and mic signals to UHF trans- ceiver. 4 Permits audio reception  from HF or VOW transceiver.  Routes key or mic signals to transceiver. 5 Permits audio  reception  from backup VOW.  Routes key and mic signals to transceiver. (4)  CIPHER light - not used. c.  Normal Operation. (1)    Turn-on.    Both  Audio  Control  Panels  are activated when electrical power is applied to aircraft. NOTE It    is    presumed    the    AVIONICS MASTER   POWER   switch   is   on, and   that   normally   used   avionic circuit breakers remain depressed.    The  circuit  breakers of routinely used avionic systems are normally left depressed. (2)  Receive. 1.  Receiver audio switches (Audio panel) - As required. 2.    Master  VOL  control  (Audio  panel)  -  As required,   in   combination   with   volume control of system being utilized. NOTE Audio select switches and volume controls     are     routinely     left     in positions of normal use. 3.    Move  each  receiver  audio  switch  ON then   OFF,   separately,   to   verify   audio presence in headphones for each system.  Adjust volume. 4.  Mic select switch - As desired. (3)  Transmit. 1.      Transmitter-interphone   selector   (audio panel) - Set for transceiver desired. 2.  MIC  HEADSET/OXYGEN MASK switch (instrument panel) -As desired. 3.  MIC/INTPH/XMIT  switch (control wheel) - XMIT. 4. MIC switch (headset/oxygen mask/floorboards) -Depress  to transmit. (4)  Intercommunication. 1.    Transmitter-interphone      selector  (audio panel) -ICS. 2.  MIC  HEADSET/OXYGEN MASK switch (instrument panel) -As desired. 3.      Mic   select   switch   (audio   panel   -   As desired (NORM/HOT  MIC). a.    If  HOT  MIC  selected  -  Talk  when ready. b.    If  NORM  selected  -  Depress  MIC switch and transmit. c.          Master     VOL     control     (selected transceiver) -Set for comfort. 3-3