<< Have you tried increasing the resistance to more than 6 grams in the low bass? >> No, but what the heck, give it a try. Slow speed checking depends on the hammertail grabbing the backcheck well, otherwise, it isn't going to hit the leather with enough force to stick. Maybe try a light filing to make sure the leather isn't "glazed", and roughing the tail. The jack must have clearance when the hammer is in check, so check the notes to make sure that the jacktender can still be wiggled a slight amount when the hammer is in check. There may be some binding of the balancier against the proximal side of the jack if the dip is more that the blow distance will allow, (this will interfere with checking. Sooner or later, if you play progressively softer and softer, you will reach a point where the hammer just isn't rebounding enough. Aside from that, it is possible for an action to not check at soft play and still feel good. Regards, Ed Foote RPT http://www.uk-piano.org/edfoote/index.html www.uk-piano.org/edfoote/well_tempered_piano.html
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