Many trombones are equipped with an "F-attachment" (sometimes called a thumb trigger), which is activated by a rotary valve. This valve allows greater range and greater agility in playing. The working parts of the rotary valve are connected by a string that sometimes breaks, and the rotary valve needs to be restrung. Although an instrument repair technician can replace the string, most people can do it themselves.
Prepare the Trombone
Find a well-lit, stable, sturdy counter or tabletop that is large enough to hold the entire length of the trombone. Detach the slide and set aside, and place the trombone bell section on the table with the bell toward you. The thumb key extension should be on the left side of the trigger mechanism.
Cut a 10-inch length of the rotary valve string. Tie a knot near one end. Check to make sure that the knot is so large that it won't fit through the hole in the bottom of the thumb key extension.
Thread the string through the hole in the bottom of the thumb key extension, threading from the left side to the right side (toward the trigger mechanism). Loosen the tiny screw on the thumb key extension (a), and the smaller screw (b) to the right of the center cylinder.
Bring the string up and around the center cylinder (the one topped with the larger screw) in a clockwise arc, then loop the string down, under and around the smaller screw (counter-clockwise). Next, tuck the string under itself as you loop it back around the small screw, and continue to loop back up, under the center cylinder. Tighten the small screw (b) slightly.
Thread the un-knotted end of the string through the hole at the top of the thumb key extension, then loop it around the tiny screw (a) at the top. Pull the string tightly enough to remove slack, but still allow the valve to operate. Tighten the tiny screw (a).
Test the valve by operating it with your thumb. If it seems that the throw is too short, loosen the tiny screw (a) and pull the string out to allow a longer throw of the valve lever, then re-tighten the screw. If the string still has too much throw, or too much slack, tighten the string.