4.9
AVC SYSTEM
The AVC threshold is established by the setting of R111, the R.F. gain
control.
R111 is normally operated fully clockwise . This applies
approx-
imately +2 volts to the AVC line through R104, R100 and CR15 with no
received signal.
Signals from T16 drive Q25 into base current on the pos-
itive R
.F
. driving cycle which pulls the
collector
negative because the
emitter
is returned to the
-
5 volt
line.
The negative potential on the
col-
lector of Q25 is applied to the AVC line through CR18 and to C 171
through CR17.
The rate that
C162
is
charged
determines the AVC attack
time.
The AVC release time is determined by the rate that C 171
dis-
charges through
R100
allowing Cl62 to discharge through
CR15.
The
at-
tack and release times for Q4 are lengthened by R22 and C70 to eliminate
undesirable transients.
Large signals are kept from overloading
Q4 by the application of a posi-
tive voltage on Pin 3, the signal
gate.
This voltage is derived from the
S-meter
and Q-18 and the delaying circuit of CR10, R47, R52, and
C180.
On AM , S 14R shorts
out
CR10
to obtain a symmetrical attack and release
response.
4.9.1 MUTING
Muting is accomplished by placing an
open
circuit at J9 which removes
base current from Q26, lowering its
collector
voltage to -5 volts. The
cathode of CR16
will
also be at -5 volts causing it to
conduct,
applying
cut-off voltage on the AVC line which turns off Q4, Q10, Q23, and Q24
and mutes the receiver.
4.10
S
-METER
The
S-meter
is a voltmeter that measures the AVC voltage and is
calibrat-
ed in
”
S
”
units .
The source follower Q14, is connected to the AVC line
and drives IC- 1 which is a differential amplifier that operates the
S-
meter.
The zero adjustment is
affected
by R64 and the S-meter full
scale
sensitivity is set by R73.
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16 -
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