Basic Nonrestricted Set

inductor L4 in the receiver circuit. The coupling

In the off-hook position, hookswitch contacts 1

and 2 close to connect the telephone set to the line.

between inductors L3 and L4 results in the receiver

Contacts 3 and 4 open to remove the short circuit from

producing a comfortable level of sidetone.

across the receiver, thereby preparing the receiver for

When contacts 1 and 2 close the dc loop, the

response to audio inputs or sidetone.

automatic dial telephone switchboard produces dial

When you remove the handset from its holder to

tone for outgoing calls and cuts off ringing voltage on

place a call, the dial and transmission network

incoming calls.

assemblies send an off-hook supervisory signal to the

At the same time that the dc loop is closed, the

automatic dial telephone switchboard. This signal

incoming and outgoing audio loops are also closed.

informs the switchboard to extend dial tone (100

The incoming audio loop is closed by hookswitch

millivolts [mV] peak-to-peak, 600 Hz modulated with

contacts 1 and 2, dial contacts 1 and 2, inductor L1,

120 Hz) to the telephone set when it is ready to accept

the receiver, inductor L4, capacitor C2, and inductors

dialing.

L2 and L3. Audio signals appearing at terminals 1 (L1)

If you do not start dialing within 30 seconds, the

and 2 (L2) cause the receiver to reproduce the audio

supervisory signal informs the switchboard to extend

signals generated at the calling station (speech, busy

the howler tone (600 Hz at 8 impulses per second) to

tone, howler tone, and so on).

the telephone set. To regain the dial tone, you must

The outgoing audio loop is the same as that

hang up.

described in dc path 2. When speech or other audio

DIAL.-- The dial on the basic nonrestricted

patterns excite the transmitter, the direct current

telephone set operates in the same manner as a rotary

flowing through the transmitter varies around its

dial on commercial telephones.

nominal value at the audio source frequency. This

When the dial is at rest (no digits being dialed),

variation extends to terminals 1 (L1) and 2 (L2) and,

dial contacts 1 and 2 close and contacts 3 and 4 open.

subsequently, to the distant end (receiving telephone)

When the dial is rotated, dial contacts 3 and 4 close to

by the loop. The loop consists of hookswitch contacts

short the receiver. Shorting the receiver prevents dial

1 and 2, dial contacts 1 and 2, inductor L1, the

pulses from entering the receiver. If dial pulses enter

transmitter, resistor R2, and inductor L2.

the receiver, the receiver could become demagnetized.

Capacitors C2 and C3 and resistor R3 function

Contacts 3 and 4 remain closed as long as the dial is

with varistors RV1 and RV2 as an impedance

rotating.

balancing network for line impedance. Capacitors C2

When the dial is released, contacts 1 and 2 open

and C3 also compensate for the line capacitance. In

and close according to the digit dialed. For example,

addition, capacitor C2 prevents dc voltages from being

if the digit dialed is a 4, contacts 1 and 2 will open the

extended to the receiver. Capacitor C4 is in the ringer

dc loop four times as the dial returns to the rest

circuit.

position. These four dc impulses inform the associated

HOOKSWITCH.-- The hookswitch automati-

automatic dial telephone switchboard that digit 4 has

been dialed. This process will be repeated for each

cally operates when the handset is removed from or

digit dialed. Capacitor C1 and resistor R1 in the

replaced in the handset holder. When you remove the

transmission network are bridged across dial contacts

handset from the handset holder, the switch shifts to

1 and 2 to suppress contact arcing.

the off-hook position. When you replace the handset

in the handset holder, the switch shifts to the on-hook

If the called number is busy or the automatic dial

position.

telephone switchboard is operating at full capacity, the

switchboard will extend a busy tone (100 mV

In the on-hook position, as shown in figure 6-4,

peak-to-peak, 600 Hz interrupted at 60 impulses per

hookswitch contacts 1 and 2 open to prevent any line

minute) to the telephone set.

currents from entering the transmission network

circuit or the receiver and transmitter when the

DIAL ILLUMINATION LAMP AND LAMP

telephone set is not in use. Contacts 3 and 4 close,

DIMMER CONTROL.-- When the handset is

placing a short circuit across the receiver to bypass any

removed from the handset holder, a lamp mounted

internal flow of current around the receiver. This

above the dial illuminates the dial. You can vary the

guards against demagnetizing the receiver.

brightness of the dial lamp with the lamp dimmer

6-5