The fix for a broken tuning scroll on a guitar can only be replacement. Tuning scrolls, more commonly known as tuning keys, tuning pegs or machine heads, are geared mechanisms which adjust and hold tension on the guitar string to enable tuning. Gears can wear out, the knob can break or the shaft can bend, causing the tuning key to not hold tension or become inoperable. Replacements are found at most music stores, and can be installed in minutes with common hand tools.
Determine the configuration of tuning keys by looking at the guitar head. Tuning keys are single units, three to a side or six in-line.
Remove the string from the broken tuning key. If the key will not turn, cut the string with wire cutters and remove. If single-unit tuning keys are installed, you will only need to remove the string attached. If three-on-a-side or six in-line tuning keys are installed, you will need to remove all strings attached to the tuning key set.
Remove the tuning key or set by unscrewing the small screws anchoring the unit to the back of the guitar's headstock. If your tuning key will not push out easily, see Step 5.
Remove the bushing --- the washer-type unit on the top of the headstock that centres the tuning key post, also called a ferule --- by pushing it out from the underside of the headstock with a screwdriver. If the bushing is difficult to remove, leave it alone and reuse it for the new tuner. You'll only need to remove them if a direct replacement cannot be located. Classical (nylon string) guitars will have no bushings at all.
Remove the hex nut bushing if installed on your guitar tuner model by unscrewing with a pair of pliers or an adjustable wrench. When the hex screw bushing is removed, the tuning key will slide easily out of its place in the headstock.
Take the tuning key to a music store to match it with a direct replacement. Avoid purchasing replacements through the mail, as they may not fit properly and are often only available in full sets.
Replace the tuning key by pushing it through the vacated headstock hole and lining up the screw hole.
Replace the set screw by tightening it with a screwdriver, and replace the bushing by pushing it in place. Tighten hex screw bushings with pliers or an adjustable wrench.
It's a good idea to also take the guitar to the music store when you take the tuning key. Direct replacements may not be available, and you may need to find a close match that fits the headstock holes. Music stores often have "spare parts bins" so you may not have to purchase a full set. If screw holes in the headstock wood are stripped or loose, dip a toothpick in wood glue to fill the hole, snap it off flush with the surface and install the tuning key and set screw.
Do not drill into the headstock to accommodate different tuning keys, as severe damage may occur. A professional can do the job for a reasonable price. When removing and replacing hex nut bushings, be very careful with the pliers or wrench to avoid marring the headstock and stripping the hex nut. You may wish to place a strip of masking tape around the work area to help protect it.