If I have the schematic for a radio in front of me, what do I look for to see if the radio has an AVC circuit?

It's a bit of a mystery to me so I thought that I would ask!

Thank you for your help.

Joseph

You will see a feedback loop, normally a high value resistor from the 2nd IF back to the 1st IF and the RF stage (like the antenna/antenna coil).

This is a very basic explanation, as there are several similar ways to do it.

Look at the circuitry feeding the control grid of any RF or IF stage. If the DC voltage is being controlled from somewhere else, that's probably AGC (AVC).

Example: If a stage has a grid resistor to ground + a capacitor to the previous stage, then it does NOT have AGC.

If you can post the radio model, or better yet, a link to the schematic on Nostalgia Air, I am sure someone
can walk you through the AGC path

...AGC/AVC is usually derived from the detector stage, is it not? If you look at your detector stage, you will usually see a recovered audio path leading to the high side of your volume control. Usually there will be a fairly small capacitor to ground on this line to strip away the IF frequency and leave the audio. If you see another path branching off this audio path (usually through a resistor) and leading back into the RF and/or IF section, that's probably going to be an AVC or AGC line. The AVC line will also be more heavily filtered (larger cap value; you'll typically see an electrolytic on it) than the audio line (on the audio line, you only want enough filtering to remove the residual IF frequency and leave the audio frequency; on the AGC line, you want to filter it completely to derive a DC voltage that is directly related to the amplitude of the received signal. This voltage is fed back to the RF and/or IF stages and varies the gain in those stages by 'tweaking' the bias voltages such that the stronger the signal, the lower the gain)...

Chapter 12 of Elements of Radio Servicing, Marcus & Levy, on detectors and AVC would be a good start. The book
is online and can be downloaded. It is an easy read.

The other way is to read up on variable mu tubes like the 35, 6D6, and 6BA6 because they came
into being because of the need for AVC.

Some sets also have QAVC.

Here's an example for a transistorised circuit:

http://www.electronics-tutorials.com/re ... ivers2.htm

On the schematic of a typical AA5 or AA6 the AVC circuit ends at the terminal on the inside winding of the loop antenna. You can start from this point and follow it's path.