I can't read diagrams much but I don't see one. If you can't see one when you open it up, there probably isn't one..they would hardly be hidden anywhere, at the most in a casing in the lead like in car stereos etc.

Check the tweeter with a weak/almost dead battery, like a used up 9V, is it completely silent? DO NOT use a fresh 9V it's too much.

Otherwise Duffy pretty much explained all there is to know and do :-)