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Vintage Radio

In Pt.1, we looked at the origins of automatic gain control (AGC) systems and described some of the techniques that have been used over the years. This month, we look at simple, delayed, quiet and amplified AGC systems and describe some common faults in these circuits.

by Rodney Champness

Some of the AGC methods described in Pt.1 are still used extensively in domestic radio receivers while others have gone by the wayside. However, although the advantages of AGC were recognised and enthusiastically embraced by some manufacturers during the mid-1930s, many others neglected to use this very useful feature.

In fact, many receivers lacked an AGC system even into the 1950s.

One explanation for this is that some small (and not so small) manufacturers didn’t employ design engineers but just copied the work of others. In addition, some design engineers really didn’t understand how AGC worked or they thought that faults in the AGC system would be difficult to find and so left it out. Certainly, AGC faults were not easy to find in the early days, as will be explained later on.

Another reason for omitting AGC was the cost of the extra parts. This explains why AGC was omitted in so many “economy” receivers, especially the 4-valve types designed for the bottom end of the market.

Simple AGC

As the name implies, simple AGC is easy to implement. In fact, it can involve adding just two inexpensive parts to a receiver.

A common method of reducing the gain in a receiver with no AGC is shown in Fig.1. It involves increasing the cathode bias on the converter and IF valves while simultaneously progressively shunting the antenna input to ground using potentiometer VR1.

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