Resistor R31 is connected to the junction of R32 and R33. This resistor is grounded during
interphone talk operation to provide interphone sidetone reduction. The sidetone reduction
makes the operator talk loud to ensure loud and clear reception of his message at all stations.
Diode CR14 isolates the keying circuit from the protective device and filter VR1. Diodes CR13
and CR14 are protected from damage by spike voltages by the inclusion of protective diodes
CR9 and CR15 as part of VR1.
Dual switch installation. An installation may be wired using a single-pole, double
throw (ICS-OFF-TRANSMIT) switch, instead of the single-pole, single throw type. Circuit
paths are the same as explained above. The microphone amplifier output is applied to the
above except that ground is applied from either contact 2 or 8 of S1B (rear). ICS operation is
possible for all positions of the SELECTOR switch.
Hot microphone operation. Hot microphone operation provides hands-free
intercommunication of the interphone line. When the HOT MIKE-OFF switch S2 is set to HOT
MIKE, the emitter of Q3 in microphone amplifier AR1 is continuously grounded through the
HOT MIKE switch, and the microphone amplifier is activated. Audio from microphone amplifier
AR1 is then applied to the interphone line through transformer T2 and contacts 5 and 7 of
transmit relay K1.
Radio transmission circuit. In the transmit mode, SELECTOR switch S1 is set
to position 1, 2, 3, 4, or 5. Operation of the PUSH-TO-TALK switch applies a ground to the
emitter of transistor Q3 in microphone amplifier AR1 through switch S1B rear to activate the
microphone amplifier. A ground is also applied through switch S1B to terminal, 6 (coil) of relay
K1 energizing relay K1. Energizing relay K1 connects resistor R30 across the transformer T2
to maintain the proper load on amplifier AR1. The path is through relay K1 contacts 2 and 8.
Energizing relay K1 also applies the microphone audio output line through relay contacts 7 to
1, level adjust control R29, dropping resistor R28, and selected contacts of SELECTOR switch
S1A front. The level adjust control allows for presetting the amplifier output for 0 dBm (.4-volt)
into the 150-ohm transmitter audio input lines. Transmitter audio is applied through the
associated receiver to the communications control as a sidetone signal. Operation of the
TALK switch also applies a ground signal to the selected transmitter control line keying the
transmitter. Ground is applied from the PUSH-TO-TALK switch, and through SELECTOR
switch S1B rear to the transmitter control line. Diode CR9 is a blocking diode and prevents
ground signals from transmitter control lines from operating relay K1.
THE LEVEL ADJUST CONTROL R29 MUST NOT BE ADJUSTED BY
UNAUTHORIZED PERSONNEL, OR FLIGHT SAFETY WILL BE AFFECTED.