Repairing a spring inside a music box cylinder depends on the type of problem the music box may have. The spring powers the music box. When winding up a music box, the spring is compressed, and the energy is stored in the spring. As the spring unwinds, the energy is released, turning the cylinder and causing the music box to sound.
Determine the exact nature of the problem with the music box spring. If it is too tightly wound, then proceed carefully because trying to remove the spring can result the music box suddenly releasing all of its components as the tension is very high. If the spring is old or rusted, then the spring will not be able to turn at all, whereas if it is just too tight, it will turn slightly, but not enough to make the music box sound.
Add lubricant to the spring if the spring is rusted or too old. Using a light lubricant such as WD-40 will be less damaging to the other components of the music box. Add a little bit at a time, and allow two minutes between additions to let the lubricant work its way through the mechanism.
Take apart the case of the music box very carefully if the spring is too tightly wound. Unscrew the base and the key of the music box using a screwdriver. Gently pry out the cylinder and spring using the tip of the screwdriver.
Remove the spring by holding down the metal music box cylinder and carefully and slowly unscrewing the side screws of the spring. Add a few drops of WD-40 if the screws are at all rusted or stuck. Carefully and slowly pry off the spring until it is separated from the music box cylinder.
Measure the length and width of the old music box spring and purchase a replacement of the same size. Reattach the new spring to the music box cylinder by compressing the ends with your fingers and then placing it in the holder beside the cylinder. Screw the holding screws back into place, while holding the spring in place so that it does not pop out. Replace the cylinder and spring component into the music box case, and reattach the base and the key.