After trying several more elaborate AGC circuits, I came up with the very simple circuit
above. On a quiet frequency the output to the IF amplifiers is about 6 volts. When the
receiver is tuned to a strong signal, the signal from the audio pre-amp is amplified by
the first BC109 transistor, then converted to a slowly changing DC voltage by a voltage
doubling rectifier. This DC voltage is inverted by the second BC109. The voltage at
the collector of the second BC109 will decrease as the received signal gets stronger.
This voltage is applied to gate 2 of the IF amplifiers. The output is also used to
drive the signal meter. I used a 50 microamp moving coil meter. The 10K pot is
used to zero the meter. The 100K pot sets the meter deflection.
I set mine so that very strong 100KW broadcast stations read full scale on the meter.
If you have an accurate signal generator, you can calibrate the meter to read S9
for 50 microvolts input. I will calibrate mine properly, one of these days......
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