Mechanisms which make the connection between the button pressed and the note played are very complex and a novice who takes for the first time a look into his instrument is quickly terrified ... Indeed, various mechanical transmissions allow, for example on bass side, to create an association of 3 or 4 notes from the press of a single button. The adjustments of this mechanical can only be carried out by a professional who will know immediately where to act to improve the performance of such mechanical. We have all of us very specific tools to make very precise adjustments. The concert instruments (chromatic bass) have a second switchable mechanical, which allows you to switch to standard bass, a little further complicating the problem.
The sealing also requires other skills, whether in identification of the leak, valves adjustment or the work of skin. Just the bellows, fundamental piece, includes a dozen different materials and requires a great deal of expertise to respond well depending on the cause of the failure and the state of the instrument. In addition, a high-end instrument will include even more mechanical parts and openings that will create much more difficulties to get a quality instrument.
The renovation of reeds is often the biggest part of the work if the instrument has not suffered harsh treatment. If it has not been revised for over 3 to 4 years or has been exposed to the elements, it is very probable that rust starts to install, requiring cleaning each reed one by one, after getting them all deposited.
Each button is actually related to two reeds: one when pushing the bellows and one when pulling. To avoid interference and air leaks, a skin flap is located behind each reed (see previous photo of a reed). Over time, these skins are relaxed (as seen in the photo of a reed block, which needs a total renovation!) and it is necessary to pick them by giving the nerve expertly, making the compromise between hard skin which will prevent the reed from easily ring and soft skin that will relax quickly. On a very old instrument, we will have to change most of the skins, too old. On low-end instruments, today are used synthetic skin that resist longer but can not be re-tensioned, requiring replacement at first signs of weakness.
The reeds are then resolved one by one for their start on the first stream of air, and to perfect the homogeneity of the instrument. Anti-rust treatment can be applied to delay the onset of this disease.
This list is of course far from being exhaustive, the instruments that come to us have different problems depending on their past and it is impossible to make a repair estimate without being able to see and hear an instrument.