For anybody relatively new to vintage radio, as myself, not as technically clued up as some of the more experienced guys, I did not have a clue about AGC, so looked it up with the intention of getting my head around it, thus gaining a working "understanding".
I found the following articles to be not only very interesting and informative, but extremely instructive and easy to understand.
Good reading! Check out the links. You can open these PDF's and save them for later reference!

http://messui.the-chronicles.org/valves/VR199203.pdf
http://messui.the-chronicles.org/valves/SC199103.pdf
http://eddystoneusergroup.org.uk/Restoration%20projects/Tech%20Short%2014%20-%20AGC.pdf
http://eddystoneusergroup.org.uk/Restoration%20projects/Tech%20Short%2012%20-%20Radio%20Fault%20Finding.pdf

Some more really good explanatory links I found:-

http://www.hrsasa.asn.au/docs/collect%20and%20restore%20old%20radios%20stenning.pdf
http://www.justradios.com/safetytips.html
https://books.google.com.au/books?id=g9lr9Yl6Kq8C&pg=PA124&lpg=PA124&dq=,,,
http://www.radiomuseum.org/forum/servicing_old_radios_useful_tips.html
https://books.google.com.au/books?id=cjRv6SSqSDoC&pg=PA171&lpg=PA171&dq=...

This is a very good approach to learning: do the primary research and reading yourself and then ask questions about any stuff that isn't clear.

Thank you GTC, it's much like what I have to do as a mechanic, with automotive technology becoming evermore "electronic" very taxing on the old grey matter, but I have learned a lot with reading those articles! As well as gaining resect and admiration for those who invented these things, absolute genius!

I do wish they'd cover some of these subjects in the vintage radio column in Silicon Chip again, mainly for the benefit of those coming in to the hobby but also to try and attract extra newcomers. Some of these were covered early in the peace in good articles written by John Hill but this was around 20 years ago. Peter Lankshear did the same in a column by the same name in Electronics Australia but again, just over 20 years ago.

The advantage with this is that one can receive local knowledge of local radios coupled with the experience gained on not just the servicing aspect but complete restoration of sets which was optional back in those days but generally considered mandatory now for safety reasons.

Hi,yes those articles are crackers and written by guys who knew what they were doing and hands on especially John Hill.
I reckon you really need to strip a classic superhet say with octal valves down to the bare chassis and then rebuild it stage by stage.
Then you can work out how to test power, audio and speaker, then progress to the harder detector, IF and convertor stages.
When I say rebuild I don't mean a 100 point "pebble beach" resto but just make it work properly again stage by stage.
I did it the hard way by building from scratch both AM and FM tuners and then whole AM sets and then onto rebuilding sets.
I have the advantage over you of a lifetime spent in electronics from black bakelite telephones through computers to modern times and my own workshop is now full of gear and components and stuff from all eras.
.Most of these radio escapades are chronicled in the special projects section of this forum.
I did send a couple of these articles to silicon chip, but in response to the first received a reply that sort of meant they could not fit it into the structure of the magazine and heard nothing about the second. I guess the articles were not professional enough.
If you can, lay your hands on any copy of the "radiotron designers handbook" or a CD of it, later the better.
This bible covers every thing to do with AM valve receivers.
I don't understand 1/2 of what is inside each chapter but everything I have read works out in practice.
Keep studying and experimenting.
Cheers, Fred.

Jamie, given you come from a mechanical background, you can think of AGC like a governor on an engine. Works exactly the same way and it's where the idea came from in radios in the first place.

Also similar to lots of systems used in modern engine management systems like the air flow metering that controls the mixture.

Thank you all for your input. There is a wealth of information in this forum I have hardly touched upon, as I have noticed when googling "things" I regularly find articles from this very forum popping up, from way back, sometimes over five years ago, with contributions from the same brilliant "regulars" who are always helping newcomers solve problems, by providing information gathered over years of experience in easily understood professional and practical fashion. You all know who you are and I cannot thank you enough!