I just completed the Norcal 40a AGC circuit. I wrote something about it a month back. Since then, I have been reading "The electronics of Radio" on and off, pretty much the same set of pages again and again, trying to understand what is going on.

I even prototyped the AGC circuit on breadboard.

But I did not get much insight from it. The bias voltages were checked, V-gs and V-ds were measured to verify that the JFETs were operating in the variable resistance region of JFET. But that was not the case. The insight was when I realized that the circuit in the book assumes that the previous part of the circuit (i.e. the product detector) was already populated because at the point AF2, a voltage comes out via pin 4 of U2 (internally, Pin 4 is routed to 8v via a 1.5k resistor).

With that failed experiment, I decided to go ahead and populate JFET and the biasing scheme and the two schottky diodes into the board. The arrangement needs matched resistors. I was using 2.2M 5% tolerance resistors. Also my board does not have any via so those parts whose legs act as a via also needs to be installed. This is the test setup. The input signal from the function generator to JFET is via a 300k resistor.

I had a missing via and once that was found and the component installed and the U2 also installed, the bias measurements on JFET's legs were all as expected. The problem 32 measurements were performed.

V-c (DC) V-out (rms) 
7.09     1 
7.1      1 
5    1 
4.6      1 
2.57     0.23 
3.11     0.98 
1.6      0 
2.49     0.086 
2.7      0.62 
2.89     0.956

This is a nice curve that switches from an audio voltage of 0v to Vcc pretty quickly at a control voltage around 2.5v.

I then went ahead and installed the two capacitors C29 and C30. These capacitors are the ones that puts the "A" (automatic) in AGC. I did a handwavy test by increasing the input signal's amplitude and observing the audio output connected to a speaker by ears. At low input signal magnitude, increasing the input increased the output volume. However after a point, increasing the input signal magnitude did not significantly increase the audio volume, it more or less remained a constant.

On to the next phase..