[Boatanchors] Homebrew Receiver AGC Circuit Question

[whitebear1122 at comcast.net]

I am building a W1DX HB-67 double conversion superheterodyne receiver from
the 1967 and 69 ARRL Handbook. I'm nearly done with the 80-meter base
receiver, powered it up yesterday and it's hearing signals.

The audio derived AGC is not working though, and I'm at a disadvantage
because I don't understand how that specific circuit works.  I've seen this
circuit used a couple of times in the ARRL Handbooks and ARRL SSB Handbook,
so it's not an untried circuit.

While I get the premise of AGC and generating a negative voltage which is
applied to the IF amp grids.

Notes: the IF gain pot is connected to a -15 volt DC bias supply. Tube pin 2
is capacitor coupled to the audio output of the product detector, before any
amplificiation or volume control. The diode common anodes go to the IF amp
grids or 'bottom side' of IF transformer secondary, through 10K resistors.
Tube = 6CG7.

 

When I poke around the circuit with the oscilloscope, I see the AGC voltage
sitting at the minus bias coming from the IF potentiometer, as expected. I
get that. What I don't see is any change in AGC voltage - due to the
rectified audio.  I think the rectified audio is supposed to put a larger
negative bias on the AGC line, more negative than the voltage provided by
the IF gain bias. That actually makes sense to me as well.  I don't think
there is enough voltage being coupled to the secondary, which would be
rectified into a larger negative voltage, more negative than the IF gain
bias. Maybe the amp tube grid needs to be driven harder. I did modify the
detector portion of the radio, replacing the diode detector with a tube
product detector. Maybe the output from the PD isn't high enough.

Then I'm not sure how the RC time constant factors into all of that. I
understand time constants, but I don't understand how this circuit provides
a slow recovery following a fast attack. I am assuming the rectified audio
provides the fast attack part, makes sense. Is the RC being discharged (more
negative) during the initial attack, and then slowly ramping back up to
increase the IF amp gains?  I think I have some of it right, and some of it
wrong.

 

If you'd like to take a stab at it, email me and I'll email back the
schematic of that circuit.  Yes I would have attached it if the server
allowed attachments.

 

Any guidance here is greatly appreciated. Thanks. 

 

73 Scott WA9WFA