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Automatic
Volume Control Page 3
3-Time
Factor- When a radio program stops for an instant after
a burst of static, we say the a-v-c action has "time-lag."
This is caused by resistor R2 and/or condensers C2, C3 and
C4 being so large that it takes an appreciable time for
the a-v-c voltage to charge condensers C2, C3 and C4 through
resistor R2. For this reason, this resistor and the associated
condensers must be considered together. When R2 is 1 megohm,
C2, C3 and C4 may be as high as .05 mfd., but when R2 is
2 megohnis or more, C2, C3 and C4 should not exceed .01
mfd.
4-Cost- In low-priced sets where the number of parts
must be kept to a minimum, condenser C2 and resistor R3
may be eliminated if a 1000 ohm cathode resistor is used,
by returning the grid circuit of the i-f tube to ground
and thus leaving this tube out of the a-v-c system. If an
attempt is made to eliminate more parts by connecting the
cathodes of the r-f and i-f tubes together and omitting
R7 and C7, it will be found that a strong signal will so
reduce the plate current in the r-f tube that insufficient
bias will be developed for the i-f tube and over-loading
will result.
When the a-v-c system of a receiver is broken down into
its two parts, as has been done in this article, it is possible
to understand the functions of each part and to see exactly
the relation each part bears to the other. It is also easy
to see why certain precautions must be taken in order to
prevent certain difficulties which will otherwise occur
in service. It is hoped that this article will assist-in
clarifying the functioning of an automatic volume control
system in an average radio receiver.
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