Figure 6-4.--Simplified schematic diagram of the transmission network.

normally closed dial (impulse spring) contacts 1 and

SLIDE ASSEMBLY RELEASE CON-

2 and dc paths 1, 2, and 3.

TROL.-- The slide assembly release control is a

mechanical control that consists of a button that must

Dc path 1 is through resistor R1 and varistor RV1.

be depressed to unlock the handset retainer slide.

Resistor R1 limits the current through varistor RV1 to

When the handset retainer slide is unlocked, the

protect it from high line-voltage surges. Varistor RV1,

handset can be removed from the handset holder.

with varistor RV2, acts as an automatic gain control

to maintain a constant input and output level over the

TRANSMISSION NETWORK.-- The trans-

range of loop resistance encountered when used with

mission network is the control circuit for the telephone

any specific automatic dial telephone switchboard.

set. The transmitter output and all inputs to the

Dc path 2 is through inductor L1, the transmitter,

receiver are extended to and from the associated

resistor R2, and inductor L2. This path prepares the

automatic dial telephone switchboard under the

transmitter for audio excitation. Inductors L1 and L2

control of this circuit. This circuit also produces and

balance the line for varying levels of outgoing and

controls receiver sidetone (transmitter sounds

incoming audio signals. Resistor R2 limits the current

reproduced in the receiver of the same handset).

through the transmitter to protect it from high

Figure 6-4 is a simplified schematic diagram of

line-voltage surges.

the transmission network circuit. The handset

Dc path 3 is through inductor L1, varistor RV2,

transmitter and receiver, the hookswitch contacts, and

and inductors L3 and L2. This is a balancing and gain

the dial contacts are also shown to simplify the

control path, with varistor RV2 acting with varistor

description of the circuit.

RV1 as described in path 1. Inductors L1 and L2

When the handset is removed from the handset

function as described in path 2. Inductor L3 aids in

balancing the circuit and, by mutual inductance,

holder, hookswitch contacts 1 and 2 close the dc loop

couples audio signals from the transmitter circuit to

between terminal 1 (L1) and terminal 2 (L2) by

Figure 6-4.--Simplified schematic diagram of the transmission network.

6-4