Next to the cylinder, one of the most important parts of the musical box is the key-board. We will first see how all accidents happening to a key-board can be remedied.
It is well known that the number of vibrations of a pendulum in a given time, is regulated by the weight of the pendulum-ball. The heavier it is, the more slowly will it vibrate, and the lighter it is, the more quickly it will go. The same is to be found with the key of a key-board, which is nothing but the half of a tuning-fork.
The lower tones giving a less number of vibrations in a second than the higher ones, it will suffice to load the end of the key to lower the tone, and to lighten it to have a higher tone. It will also be easily understood that a thick key or a short one will vibrate more quickly than a thin or long one. After these suggestions, it will be very easy for any one to put any number of keys to the right tone.
Any person having had a key-board in hand, will have noticed that there are two kinds of keys; some having lead at the end, and others that have none. For those having lead, it will be sufficient to cut some of it to elevate the tone, and to file the key between the lead and the brass plot, to lower it. For those without lead, the same must be done to lower the tone, but having no lead, must be filed near the end underneath, to elevate it. As you must avoid having any thin keys (these not possessing good sound), instead of filing a key to lower it, it will be often preferable to change the lead for a heavier one, or supply the deficiency by solder.
We have now to see in what manner a missing tooth may be replaced. Take a piece of steel and make a key of the same shape as the missing one, or the adjacent ones, but on the under part a heel must be devised, as indicated in Fig. 203. In the steel block of the keyboard, with a file of the width of the key, make a notch as indicated by Fig. 202. Hammer the new key in its place, so that the heel will exactly fill the hollow space, and so that the key will be placed as much as possible in the right direction and right level. In making the new key, the point must be made a little longer and a little wider than the adjacent ones. Then temper the new key, draw it to a dark blue, so that it will vibrate like a good spring, and at the same time so that it can be filed. Whiten the heel of the key, put it in place, and solder it. This must be done with a soldering bit, which weighs at least 6-8 lb., so as to retain sufficient heat. Lay the copper pretty hot on the key when in its place, and after a few moments' delay the solder will run. The solder and acid are the same as used by tinmen. The key, well fixed, must then be finished, filed on the top to a level with the other keys, and tuned by filing it underneath.
It is necessary here to say in what way the under part of a key can be easily filed. Put in the vice a small block of steel or brass, a little thicker than the key is wide, about 3/4 in. long, with a small elevation, lengthwise. Place the key to be filed on this block, the whole comb being held in the hand under side up, and with a certain pressure the key will rise above the others, and will be easily filed with a square file 3/16 in. wide, and 6 in. long. When the key to be filed is in the middle of a long key-board, it will be advantageous to make an appropriate handle to the file, as indicated in Fig. 205.
The point of the key must then be finished, that is, filed to its proper width (to correspond to the other points), and, at the same time, brought as nearly as possible to the same distance from the two adjacent points. For putting the point to its exact length, it would be well to hold the key-board with the keys perpendicularly on a piece of flat window-glass, and by reflection it will be easily seen when it is brought to the same length as the others.
Place the point of the key, when it is filed to the right width, as nearly as possible to its level, and proper distance from the adjacent ones. Sometimes it may be found necessary, however, to change the place of the point of a key; to lower it so as to put it on a level with the other ones, or to shift it to the right or left. In this case, a small anvil must be made, well tempered, of about the same shape as the one used for filing the keys, but quite flat on top, with no elevation. The hammer used must have one end tempered, with the end a little rounded and not too sharp. If a key is forged on the left angle, it will move to the right, and vice versa. The key must be forged on the under side. Here a certain practice is quite necessary; the key must be well placed on the anvil, the spot to be reforged resting well on it, and 2 or 3 strokes of the hammer will make the key move a little.
To lower or elevate a key, another anvil of the same size as the preceding one is necessary, tempered, but notched on the top (Fig. 206). The key is laid lengthwise and quite flat on this anvil, and by striking the key with the other end of the hammer (Fig. 207), which is flat and not tempered, the key will bend upwards. In both these cases much care must be taken, as it is very easy to break a key in using this hammer.
In case only a point of a key is broken, it is not necessary to replace the whole key. With the blowpipe, the end of the key must be untempered, but care must be taken that the flexible part of the key be not beaten and untempered (the sound would be lost); a small notch is made with a narrow file, and a small piece of spring is filed and pressed in. It will be easily soldered with a small soldering bit. Then the point must be finished as already indicated.