Hi, I have had something rattling around in my Flextone II head for a while and opened it up just enough to see what it was and found the capacitor you see in the photo. If you can't tell from the pic it says that it is 1000 micro farads and uses a voltage of 200V. I don't know how long it has been broken off, but my tone doesn't seem to be affected at all, which leads me to believe that it is part of an input or something else that doesn't effect the tone directly. I can't seem to find a wiring diagram, so I'm asking if you know where it goes in order to, either help me fix it, or help someone else do it since I know it can be dangerous to work on these.
That cap looks like it may have fallen off the power amp section. You should definitely take it to a service center so they can re-solder a new on back on or else you could risk blowing up your power amp.