Posted by John Swensen on February 12, 1999 at 14:47:10:
In Reply to: over-rotating rotary valve posted by Pete Jenkin on February 12, 1999 at 13:23:41:
Sounds like either the cork bumpers are worn, or the stop plate doesn't fit well and is shifting slightly with hard key presses. Probably the valves are worn in the casing, which allows them to leak unless the alignment is close to perfect.
If the shaft on the back side of the valve has alignment marks, trimming the corks is easy; if not, you may need to try to adjust for best sound with the valve both open and closed.
If the stop plate is moving (try loosening the attachment screws slightly if you're not sure) you could either insert a bushing in the hole (a piece of brass tubing from a hobby shop might fit) or shave a piece of a toothpick and put a shaving next to each attachment screw to take up the slack, then tighten the attachment screws.
After fixing the stop plate in place, you probably need to insert new corks (or neoprene - an O-ring with an appropriate cross section, cut up, works very well) and trim them for best alignment with a razor blade. Reportedly, wine-bottle corks can be cut to fit.
Except for ruining the screw heads or breaking the stop plate while forcing the cork or neoprene into the stop plate hole, it is pretty hard to permanently damage your tuba doing this. Screws can be replaced, and a new stop plate could be filed out of brass, worst case.
If you are the kind of person who is not afraid of changing the oil in a car, you can probably do this job yourself. If not, ask your repairman if the stop plate could be shifting, throwing off the alignment.