Marcel Carey is correct. I saw that Kawai high-speed film too, and it is possible to have a jack so loose, and cushion felts so bouncy, that at just the right repetition speed the jack will bounce and end up in exactly the wrong place at the wrong time, causing repetition to fail. On the other hand, not long ago I was filling in for another technician, and I ended up tuning for two different concerts in two different places on the same evening. I had just finished tuning one piano when I got a frantic call from the manager of the other orchestra, which was having final rehearsal before performance. Several notes were failing to play at all. The other location was only 20 miles away, and I got there ASAP. The jacks were too tight. The relative humidity had increased, and the jacks got even tighter. They were failing to return at any speed. A careful application of shrinking solution followed by some heat from a neat little device I had made that heats only the spots that need to be heated, and all was well. The pianist wanted me to do a stand-by during performance. I told her why I couldn't, but I assured her that the problem was fixed, and she would have no problem. WARNING: Do NOT try the heat thing on jack centers using a heat gun, or you may cause the jack tenders to come loose. Jim Ellis
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