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......
