Figure 5-16.--Functional diagram of AM-2210/WTC.

The switching circuit is activated when the

Figure 5-16 is a functional diagram of the

amplifier is energized. With power available and neither

AM-2210/WTC. One relay (K1) is used in the switching

local nor remote talk switches closed, the relay (K1) is

circuit.

operated. When operated, the depression of a remote

AUDIO AMPLIFIER.-- The audio amplifier (fig.

talk switch will have no effect upon K1; that is, it will

5-16) consists of a low-level, three-transistor amplifier

remain operated. However, when one of the six local

(Q1 through Q3) and a power amplifier (Q4 and Q5),

talk switches is depressed, the circuit to K1 is changed

with negative feedback employed throughout. The

and K1 restores.

output transformer (T3, not shown) has two

Resistor R31 provides a bias to the base of Q6,

secondaries: the first is used with the loudspeakers and

which normally holds Q6 in a saturated state,

the latter, a tapped winding, is used for as many as six

maintaining K1 in an operated condition. When the local

sound-powered telephone outlets. The amplifier

talk switch is closed, the base of Q6 is connected to

provides a 10-watt output.

ground through the 4.8 ohms of the mouthpiece.

SWITCHING CIRCUIT.-- Figure 5-17 is a

Presently the voltage across Q6 from base to ground

functional representation of the K1 switching circuit,

becomes less than the emitter bias voltage provided by

voltage divider network R32 and R33; therefore, the

showing K1 in an energized and operated condition. The

transistor becomes reverse biased and Q6 becomes

receiver element of the local headset-chestset is in series

nonconductive, de-energizing and restoring K1.

with a dc blocking capacitor, thereby presenting a high

resistance when the talk switch is open. Closing the talk

The incoming and outgoing voice signals are

switch connects the headset across the line, giving the

coupled through capacitor C1 of the amplifier. CR1 is

headset a dc resistance of approximately 4.8 ohms. It is

in the circuit to protect Q6 from surges while it is in the

the function of the switching circuit to sense this change

cutoff state.

from high impedance to low resistance that takes place

with the depression of one of the six headset-chestset

The restoration of K1 will result in normal

communications at sound-powered level between all

talk switches.

Figure 5-16.--Functional diagram of AM-2210/WTC.

5-18