I have a King Super 20 tenor with the underslung octave key. The horn is otherwise marvelous, but the octave key is giving me quite some trouble. The problem is that despite considerable spring tension, it won't stay down properly. If I do slap tongues on the lower notes, I can see the octave key moving (ever so slightly, but still), and loud lower tones tend to step up an octave. I've identified the problem in the octave mechanism, because if I close the key permanently, it disappears completely.
I've tried to get my head around it, but the problem seems to be beyond me. As I said, the spring tension is quite high as it is. If this rings a bell to anyone, I'd be extremely grateful for further tips. Thanks!