I pointed out in the past that we really do not set the jack height, as the jack is not variable in its height. Rather, we set the height of the repetition lever relative to the top of the jack. However, I have always called this regulation setting the jack height, and I intend to continue to do so. The specification in regulating is to have just a fraction of space between the top of the jack and the bottom of the knuckle with the whippen at rest. This can be compared to setting the lost motion in a vertical piano. However, in a grand the amount of space needed is even less than in the vertical.
Before taking the action completely out of the piano to adjust all keys for the jack height, I like to set sample keys for blow, let-off and drop, as well as jack height. Doing this first and proving these sample keys with aftertouch insures that after all of the steps in Section IV are regulated, the piano will play properly.
In order for the jack height regulation to be stable, the repetition springs must be strong enough to support the hammers. We have all seen hammers that do not return to the same height after each time they are played. This can be caused by either too much lost motion between the top of the jack and the knuckle, or by repetition springs that are too weak.
Assuming that the repetition springs are strong enough, the culprit is the jack height. As well as causing an uneven hammer line, too much lost motion also causes a loss of power. On the other hand, too little space will not allow the jack to return under the knuckle as fast as it should (or not at all) and hence cause faulty repetition. Some technicians set this measurement by feeling the top of the jack versus the side of the balancier window where the jack sits. The specs call for a 0.2mm (or a paper’s thickness) space that the jack should be below the window.
This is fine if the piano being regulated is brand new or if the knuckles have just been replaced. Under these conditions, the surface of the knuckle is very even across the face where the jack contacts it. If the knuckle is not new, chances are that the leather has begun to get grooved in the middle from the force of the jack hitting it. To obtain the proper space between the jack and a grooved knuckle will require that less than a 0.2mm measurement be used. In some cases the top of the jack may have to be even with or higher then the balancier window. With a grooved knuckle, there is only one way to regulate. That is by using a process called “winking the jack”.
With the whippen at rest, gently slip your finger in between the top of the jack tender and the bottom of the let off bottom. Slowly depress the jack tender as if it were going through the process of let-off. Be careful not to push down on the whippen while tripping the jack. Depending upon the type of action you are working upon, you may see the hammer “wink” or not! By “wink” I mean that the hammer will fall slightly lower than its neighbours. Sufficent space must be allowed for the hammer to fall, so the hammer rest rail must be set a shanks diameter below the hammershank at rest. If the action has hammershank rest felt instead of a rest rail, either raise the shank or remove some of the rest felt.
For some unknown reason, some makes always give a perfect little “wink” for this test, but others are not as reliable. Therefore, do not keep adjusting the jack height until the hammer “winks”. It may never show. Experience is the rule here. Furthermore, just because the hammer “winks” does not mean that the jack height is set correctly. It can fall too far. The proper distance is about 1/32”, give or take a little. Adjust all 88 whippens so that the amount of “wink” is the same for all keys.
On actions that do not “wink”, or to recheck one that does, feel the jack tender as it is released to slide back under the knuckle. If possible, look to what is happening. The jack should not hang up on the knuckle, but rather go all the way back to its rest position (or just a fraction of an inch away from the spoon). This can be felt by the regulator. Again, all 88 jacks should feel the same. Just the slightest amount of resistance should be felt. Too much means that there is not enough lost motion. No resistance means that there is too much lost motion.
Remember when turning the repetition lever regulating screw to lift up slightly and get the felt of the repetition stop button on the bottom of the regulating screw away from the repetition stop felt. This will eliminate tearing up the fibres of the felt while adjusting the jack height, and will make for a more stable regulation. Otherwise, these felt fibres will compact later and change the height. Keep in mind that the knuckle will flatten and groove a little on a brand new piano, so plan on coming later and change the height. Keep in mind that the knuckle will flatten and groove a little on a brand new piano, so plan on coming back to make minor adjustments in the blow and jack height after the action has been played a while.
Likewise on the Schwander style actions. The felt that is under the adjusting screw next to the balancier will compact and take the shape of the top of the balancier. Turning the regulating screw to adjust the jack height often turns this felt to a new position. It will compact again as the piano is played, changing the jack height.
As a help in “winking the jacks”, lift the adjacent hammers and watch if the jack being regulated returns to line up with the adjacent jacks. This assumes that the adjacent jacks are resting properly against their spoons. For a final check, put the anmer into check and upon a very slow release of pressure on the key observe if the jack returns to its rest position. If while “winking” an undue amount of resistance is felt, raise the hammer and observe the jack in the window. The jack may be rubbing on the side of the window. If so, space the jack before continuing. If not, check for a tight jack centre.
As a final note, on pianos with the hammer rest rail type action, sometimes the rest rail must be adjusted very close under the hammer-shanks to keep the shanks from impacting upon the top of the repetition spring or upon the top of the repetition lever regulating screw. This is a source of a very audible click. It is possible, if the jack height is way off, which it can be on a new piano, that as the regulating screw is turned up to eliminate too must lost motion, the shank will lower itself to rest upon this rest rail! Further adjustments at the regulating screw are fruitless as the hammer will not “wink”, being now supported by the rest rail. This little thorn in your side has to be dealt with on a one-to-one basis. Once the hammer lowers to this point, the technician must raise it back by turning up the capstan. Proceed “winking” as usual.
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