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
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
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
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
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
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
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