Bell System Practices Plant Series Service page 2

SECTION 512-620-100
circuit. This avoids a singing condition while
receiving.
(3) When speech is transmitted, the gain of
the transmitter circuit increases to normal.
Simultaneously, the gain of the loudspeaker circuit
lowers to avoid singing as a result of the
increased transmitter gain. The amount of the
gain change depends upon the setting of the
receiver volume control. The gain change is
smaller at the lower (counterclockwise) settings
and greater at the higher (clockwise) settings.
(4) A circuit, referred to as a switch guard,
utilizes the voltage across the loudspeaker
to reduce the possibility that surrounding room
noise will cause false operation of the switching
circuit while receiving speech.
(5) A predetermined voice level is necessary to
switch from the receiving to the transmitting
condition. In the presence of steady room noise,
a special circuit automatically raises the required
threshold level to prevent operation of the
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Fig. 1—Typical 3-Type Speakerphone System, Basic Components
switching control circuit by the noise. Talkers
will still switch satisfactorily because they increase
their levels under noisy conditions.
Loudspeakers
2.07 External—760A equipped with R2FK mounting
cord. Consist of permanent magnet, dynamic
loudspeaker mounted on a metal base with plastic
housing.
2.08 Internal—762A used with 752-, 1752-, and
2752-type panel sets. Consists of a P-21F246
speaker, neoprene rubber acoustic shield, plastic
cover, and 13-inch spade-tipped leads.
Transmitters
2.09 External 666-type transmitters consists of
an AC1 transmitter unit, transistorized
amplifier, etc. The 666A (MD) is equipped with a
T11A mounting cord. The 666B has a T7A cord.