An accordion is usually trouble-free for many years. Most of the fixes it might need are fairly easy for an amateur who has had some experience in building model trains or airplanes, or is handy around the house. Since I first wrote this, I have discovered a new book, "The Ins and Outs of the Accordion" by Thierry Benetoux (see links). It covers the inner workings very well, including reed work.
After a button accordion has been played a while, the bass strap may begin to stretch. Usually it's fastened by a couple of screws at each end. It can be shortened by removing the screws at one end, punching a couple of fresh holes, and refastening.
A better bass strap fix is to make a new two-piece strap with Velcro adjustment. Adhesive-backed Velcro is sold at some of the big office supply stores. A skilled leather worker could make a new strap from fresh leather, but pieces from an old belt will be serviceable. If the new strap is thicker than the old one, and it probably will be, the screws will not reset well when put back in their holes. A good grade of rubber cement like Shoe Goo will fix this. Coat the screw threads with rubber cement and put a tiny bit underneath the strap end so the strap won't want to wiggle. If possible, thin the end of the strap a little first.
The home made adjustable strap is also a good solution for a strap that's too heavy and stiff. Sometimes a strap that looks really great will make the instrument hard to play.
Sometimes the adhesive backing will come off the Velcro. This has happened to me. Replacing the Velcro is just a temporary fix. I take a little duct tape of the appropriate color and wrap it around the straps. This keeps the adhesive in sufficient contact with the Velcro to make it secure. But the more I think about it, the more I think it might be better to skip the Velcro and use double-sided tape.
Some small boxes come without provisions for shoulder straps. Shoe repairmen have D-rings with eyelets which they rivet onto heavy shoes to hold the laces. Some of these are big enough to hold accordion straps. Installation requires drilling a hole for each one in the box. This is NOT a job for a power drill. The drill bit should be held in a hand vise and slowly rotated by hand. A great deal of care is required to avoid damaging the outer surface of the box. Also look inside before drilling to make sure you are clear of the inner workings. Then use a good machine screw and nut with washers on both ends plus a lock washer under the nut. Opening the box is covered under bellows repair.
The bellows gets a real workout and an inexpensive one may show some wear. My Hohner HA-114 began to show some bellows wear after I had played it a while. I had carelessly let the bellows rub against my belt buckle. This instrument does not have fabric tape covering the bellows folds. My solution was some clear Mylar wrapping tape. 3-M makes it under its Scotch brand. It makes a lesser tape, which will not hold up, under its Highlander brand. Avoid cheap tape because it will either peel off or get gummy after a while. My HA-114 repair is still good after 10 years.
Loose-fitting bellows frames can cause a lot of air loss. Look for leaks by squeezing without playing and hold your face next to the bellows. You can feel any escaping air with your cheeks.
The fix for air escaping around a bellows frame is to reduce the size of the opening in the box end into which it fits. This means taking the bellows off the box end.
A bellows is typically held to a box end with four round-headed pins. Take a good grip with a pliers and pull each pin straight out. Be careful not to wiggle it in a way which will enlarge the hole.
When the bellows is loose, look carefully at the inside of the box end. Note that there's a sliding fit. This fit can be tightened with duct tape. Cut a piece to the right length and very carefully cover the surface against which the bellows frame slides. Don't let it get on a surface which the frame butts against. Take a real sharp knife and carefully trim off the excess duct tape. Then try the fit of the frame into the box. It may be necessary to tape all the way around, or it may work out to tape one or two edges. When the bellows goes back on the box, replace the round-headed pins very carefully, making sure they go back into the holes in the frame. They will have to be pushed through the duct tape. The tape will make them less inclined to work loose, as they do on some instruments. When the instrument is reassembled, there should not be any duct tape visible.
A leak in the bellows itself is much harder to fix. If it's in a fold, the Mylar tape should take care of it. A leak in the soft flexible material in the corners is a little tougher to fix. A patch made from an old kid glove may work, but the material might be too thick. Airtight nylon taffeta like is used in small tents is much thinner than leather and should work well. Whatever is used, it needs to be held in place with a strong and flexible adhesive. The best adhesive is Goodyear Pliobond. Some hardware stores sell it. Pliobond is brown, very smelly, and hard to clean up. Use it only where it will not show. The bellows will have to be kept stretched open until the Pliobond is completely dry, otherwise it will stick the folds together. WARNING--Never use Pliobond on screws or bellows pins, or you will never get them out again. Never use it on any joint which may have to come apart in the future.
Sometimes the round-headed pins that hold the bellows to the box end will work loose. One fix that seems to work is to moisten the holes in the bellows frame and put in some powdered chalk. Never use glue or rosin--the pins may not come out when you want them to.
If the bellows frame is split where a pin passes through it, the fix is a little more complicated. Get some liquid wood glue, such as Tite-Bond or carpenter's glue. Put a drop or two of the glue on a clean surface--a spoon is good for this. Then add a drop of water and mix with the head of a pin or the end of a straightened paper clip. Get it reasonably runny, and trickle it into the split. Be careful not to get it into the hole where the pin goes. Then clamp the wood with a small clamp to close the split until the glue is dry. Next cut a thin piece of soft wood to fit inside the frame. Make a hole in it to accomodate the end of the pin and glue it on the inside of the frame so the holes line up. Clamp it until the glue is dry. Then, if necessary, trim it with a sharp knife to remove excess glue and any of the wood patch that sticks out beyond the side of the frame.
Sometimes pins will work loose because the bellows frames fit too loosely in the box ends. There will probably be air leaks, too. Use the duct tape treatment as in bellows repairs above.
Sometimes an older instrument will have one or more leaky valves. Frequently this is because the valve body does not rest squarely against the holes in the sound board which it is supposed to stop up. In most instruments, the treble valves are behind a grill which is held on by screws. Take the grill off and locate the leaking valve--it won't be hard to find. It may be to one side of its correct location, or it may not seat flat on the board. Either of these problems can usually be fixed by very gently bending the rod on which the valve head is mounted. Pliers with smooth jaws will work for this. They are commonly used by jewelers and model builders.
Reeds are very tricky and best left to the professionals. A dead reed can sometimes be restored by cleaning. A reed that is out of tune can be fixed, but it's an expert job.
There's a good video on accordion repair which covers reed work. It's "How to Repair Your Diatonic Accordion or Concertina" by John Townley and Jehan Paul. It's sold by Lark in the Morning (see links). The instruments they demonstrate on have reeds which are easy to get at. Bigger, more complex instruments, have some rather interesting reed housings which amateurs should stay clear of.
Reeds are usually held in place with wax. The video gives a recipe for making your own. Reed wax is also available from Reyes Accordions (see links). An accordion left inside a closed car in the summer with the sun on it may overheat and have a wax meltdown. This is very expensive to repair. Weltmeister advertises that the reeds in its instruments are nailed as well as waxed, so the reeds can't come loose. The Acadian accordion has screws to help secure the reeds.
Some accordions do not use wax. The reed plates rest on a layer of soft leather and are held in place by a groove in the reed stick on one end and an L hook on the other. This makes it very easy to work on the reeds.
The pitch of the reeds is easily checked with an electonic tuning instrument. Most music stores sell them. The best ones for an accordion give a read-out of the nearest note and the percentage of sharpness or flatness. When two or more reeds are played, the frequency of the higher-pitched reed will display. A typical read-out for a Hohner Corona III, which has three reeds per note, would be about 20 percent sharp. This is required for the tremolo or wet sound. This is easy to demonstrate on the Hohner HA-114. It has two middle reed sets which can be independently selected with the pull-out stops. One set should read right on the money. The other will be about 20 percent sharp. The high and low reed sets should be true to pitch. The Oellerer web site (see links) has some 10-second audio clips demonstrating various degrees of tremolo.
Many of the reeds are fitted with leather flap valves which keep air from going through the wrong way. Usually about the lower two-thirds are so fitted. When the leather gets old and curls it no longer does its job and the instrument does not play as well. Replacement leathers are available from vendors who have web sites, but replacing a lot of leathers can get kind of expensive.
I noticed that the "leathers" in my little Weltmeister are plastic. Then I noticed that Mylar "leathers" are available on the market. Then I realized I had some Mylar drafting film which I use for making patterns for wood carvings. I cut some new "leathers" from the Mylar film and installed them in an old instrument. At least initially, they work fine. I glued them in with Shoe Goo, putting the adhesive on the frosted side. I even replaced a couple of helikon bass leathers with Mylar film. Some of the leathers I replaced had booster springs, but I didn't put springs on the Mylar.
Damage to the finish of an instrument can often be made to disappear with judicious use of a waterproof felt marker. Choose the correct color, apply it to the nick or scratch, and immediately wipe the spot with a bare finger. The finger wipe will make the slop-over less obvious.
Some old Hohner instruments have been played and handled so much that the black finish is worn or cracked off the edges of the keyboard. This exposes a light cream-colored plastic. This plastic absorbs waterproof felt marker ink very nicely. It usually takes several applications, leaving a few minutes to dry after each one.
For a lot of advice on accordion care and maintenance, check out Ike's Accordion Hospital (see links).