Driver Q3 amplifies the output of amplifier Q2 and provides a drive signal for
audio output transformer T2. Emitter resistor R8 provides current stability, and the primary of
audio transformer T2 serves as the collector load. Capacitors C7 and C28 bypass high
frequency signals. A portion of the audio output from Q3 is applied to the AGC (automatic gain
control) network through diode CR5.
The automatic gain control circuit is active at all times and dependent upon the
audio input and output signals of the amplifier. Resistors R6, R10, R11, and R13 form a
voltage divider circuit across the +20-volt DC supply. During the no signal condition, the
voltage drop across resistor R12 is applied to AGC capacitor C10. The voltage present at the
junction of resistor R11 and R13 determines the gating level voltage for the AGC circuit. The
gating level voltage is stabilized by capacitor C11 and diode CR4 and forward biases gating
diode CR5. During the no-signal condition, the collector voltage of Q3 is more positive than
the gating voltage, causing CR5 to be reverse biased. The impedance across the secondary
of transformer T1 is high during no- or low-signal inputs. During high-signal inputs, the
decrease in collector voltage of driver Q3 causes gating diode CR5 to be forward biased and
allows AGC capacitor C10 to charge.
The voltage across C10 and R12 is applied, as forward bias, across diodes CR1
and CR2 to decrease the impedance across the secondary of transformer T1. The lowered
impedance across the transformer causes the input audio signal to amplifier Q1 to be
attenuated. Resistor R9 controls the AGC voltage decay time, and diode CR3 prevents
reverse biasing of diode CR5. Resistor R12 stabilizes the AGC release time.
Single switch installation. If an installation is wired using a single pole, single
throw PUSH-TO-TALK switch, the SELECTOR switch must be in the ICS position for
interphone operation. Operating the PUSH-TO-TALK switch applies a ground through CR13
to the emitter of Q3 in microphone amplifier AR1 to activate the microphone amplifier. The
external ground is applied to the communications control through contact 8 to contact 2 on
Section 1B rear of the SELECTOR switch and the OFF position contacts of HOT MIKE-OFF
switch S2. The audio from microphone amplifier AR1 is applied to the interphone line through
transformer T2 and contacts 5 to 7 of de-energized transmit relay K1. Operation of the TALK
switch grounds the junction of R35 and R31 through diode CR12 and reduces the interphone
sidetone level during talk operation.