[Arran]

I think that it should be pointed out that motorboating is basically low frequency feedback, and most of the time when I run into it it's because of an open bypass capacitor, or in the last instance a missing one because the guy who "restored" it left one out. In these old schematics it isn't called AGC, it's called AVC "automatic volume control" different terminology for the same thing. Another thing that can cause motorboating is having the primary and secondary leads of an IF transformer crossed.
Regards
Arran