I've run into this problem in Yamahas and other pianos with the spring / cord hammer return system. I've always been able to make things work by getting creative with the regulation (deeper dip, less blow distance, wider let off, experiment with back check position and angle). But after giving it a little more thought it may be that there isn't enough friction in the system. You might first check the for loose pinning at the hammer flange. Sincerely, Mitch Staples Ohio State University
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