Technical description
4-8 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
The demodulated audio output from IC203 is first amplified by IC204/A then
passed via a resistive attenuator (R263 and R264) to a full wave active peak
rectifier circuit V210 and V211.
Under no signal conditions, IC204/A output, V210 and V211 bases and emitters
are all at the same potential of 5 V. When the audio swing at the output of IC204
exceeds approximately ±0.75 V (from 5 V), it causes V210 to conduct during the
positive peaks and V211 during the negative peaks. The common collector output
charges capacitor C237 during the transistor’s conducting period. The resulting
DC is applied to inverting amplifier IC205/A causing the AGC output to drop to a
level determined by the strength of the received signal. This lowers the voltage
applied to gate 2 of both FETs V207 and V208, reducing the overall gain of the
455 kHz IF amplifier.
AGC is applied to a second inverting amplifier IC204/B to extend the dynamic
range of the receiver. Its output produces an inverted AGC, nominally 0 V under a
no signal condition, but starts to rise once the input to IC204/B falls below 3.2 V
(set by R249 and R250).
The inverted AGC from IC204/B is divided into two paths:
• one path is connected via R223 to gate 1 of V207, improving the gain control
of the 455 kHz IF amplifier
• the second path is via a delay and damping network (R221/V206, R220, C215,
D203, and R41) to the base of V4 (see Sheet 1)
The pin diodes D19 across the first mixer output act as variable resistors. The
resistance falls as V4 collector current increases with a rise in AGC, thus reducing
the mixer output gain at high signal levels.
Another AGC circuit is incorporated at the output of the second mixer. It is used
for signals that are out of the 2.5 kHz sideband frequency, but within the 15 kHz
BPF bandwidth (see Sheet 1). V15 is connected between one side of the balanced
output and the "B" rail. When the output swing from the mixer IC7 exceeds 0.7 V
between the base and the rail, V15 conducts and reduces the gain of the first mixer
by increasing the current through the pin diodes D19.
The AGC fast attack time is set by R262, R263 and R264 in the emitter circuit of
the peak rectifiers V210 and V211 (see Sheet 2). Capacitor C237 charges and the
resultant slow decay time of C237 is through the resistor network R260 and R259.
The attack and decay time will vary at different signal strengths, but nominally are
about 2 ms and 0.5 s respectively. Resistor/diode networks R276/D207 and
R277/D208 are connected to outputs on IC209 (pins 4 and 5) and can be
grounded, under software control. This reduces the AGC decay time constant
when required for special applications.
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