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Has anyone any experience and/or advice to offer on this problem?

I am regulating grand backchecks. On some notes, when playing softly the hammer does not seem to descend low enough to be caught by the backcheck after rebound. My first diagnosis was that the repetition lever was offering too much resistance. I screwed the tension right down to the point where the hammer sagged as soon as the jack was pulled away from under the knuckle. The problem remained. If I pushed the key down slowly through its cycle and held it down, I felt a marked resistance when I then tried to push the hammer down to be held by the check. Comparing different notes showed wide variations in the tension experienced when doing this. It then occurred to me that the little soft felt pad that cushions the jack after letoff was preventing the jack from properly clearing the knuckle in some cases. I took a whippen that showed the effect badly and cut off about 2 mm from the front of the felt. This cured the problem.

Before I go ahead and reduce the thickness of the jack cushion felts on all the notes that give trouble, can someone with experience reassure me that this is the proper solution? Perhaps I am missing something and there is another way to tackle the problem.

First thing to do is to check the key dip and make sure it's not too deep. If it is, the jack will be pushed into the rebound felt at the end of the keystroke.

Next, check the key height. If the keys are too high and the dip is too deep, the same situation will occur.

Perhaps I should have mentioned that this is the culmination of a full action rebuild: i.e. everything is new.
In fact I want the jack to be pushed into the felt, otherwise it hinders the hammer knuckle as it pushes down the repetition lever, but it seems that the felt is too resilient to allow the jack to go back far enough. I have noticed some improvement when I increase the key dip. Also, if I cause the letoff to happen earlier, I get an improvement. But neither of these things can be a solution, because it prevents my achieving an even touch.

I have carefully checked the regulation on the couple of notes I am experimenting with, but the problem remains. Chopping down the felt is the only thing that has worked so far.

Not to put too fine a point on it, my experience has been that for most piano actions, it is normal to have a certain amount of "wiggle room" between the jack and the cushion felt when the key is fully depressed. The only reason the felt is there is to prevent noise.

In any case, if you have to trim the felt to make it work, then trim away!

What kind of piano are you working on?
If everything is new, then the hammers, shanks/flanges, and wippens have been replaced.

There is possibly a discrepancy in dimensions between the old and the new parts. A 1 mm difference in the distand between the flange screw hole and the center pin, or the distance from center pin to the knuckle, or anywhere else, can lead to serious regulation problems, and worse - touchweight, inertia and playability issues.

I would be concerned that what you are experiencing is only a symptom and not the actual problem. Trimming the felt is not a normal way to regulate a piano, it seems more like a way to address the symptom, not the problem.

Without seeing the action and doing an analysis, it will be hard for anyone to tell you exactly what is going on. At rest, is the jack perpendicular to the hammer shank?

I say take some old parts and carefully compare them to the new ones. Use a magnifying glass to make certain all the dimensions of the new parts are exactly like the the old parts. If you find a discrepacy of 0.5 mm, you have probably found the beginning of where the problem lies.

Usually when there is a problem like this, there is too little aftertouch, i.e. the hammer blow is too long and the key dip is too shallow.

Sorry, I really must learn to express myself more precisely when describing a problem.

This is an 100 year old Ibach. It was little used before I bought it. The problems I had were because of age, not wear. I replaced all the bushings, felts, leathers and reshaped the hammers, although these will have to be replaced soon. The results exceeded my expectations. The piano sounds and feels great. All I have left to do is the fine regulation which will give me an even touch throughout. I have been careful to do everything by the gospel according to Arthur Reblitz. I have quite a bit of experience of regulation of uprights from the past, although I have never gone for perfection before. Getting the backchecks to work consistently over all the notes is my last exercise.
I appreciate that aftertouch could be where the problem lies, but what can I do except compare a note that works properly with one that doesn't. Hammer blow is the same; key dip is the same: let-off is the same; hammer drop is the same; repetition lever tension is the same. All that seems to differ is the amount of space between the jack at rest and the felt behind it. This difference seems to arise from aligning the jack to the hammer knuckle. There were simply differences in how much I had to twiddle the jack screw to line up the jack edge with the back of the knuckle center. This means that some jacks have to compress the cushion felt more than others (all of them do compress the felt - there is no space behind the jack after let-off), and I am guessing that this results in the jack heel being driven further into the cloth button on the let-off dolly rather than pulling the jack far enough back to allow the knuckle - and thus the hammer - to descend freely after rebound.

I know the dangers of jumping to conclusions where piano regulation is concerned, which is why I seek the advice of experts.

Thanks to all who have helped so far.

The amount of felt in the rep groove should all be the same thickness.

The rep spring should be set so the hammer will slowly rise back toward the strings when released from check. The hammer should drop only a wink from let off. There should be 1/2" of rise from check.

The best for you be to find a tech that would make an hour call and regulate a note in every octave for you and let you know if you really are that close with the regulation you have done. Make sure the keyframe is bedded properly before you start anything.

Sorry, I really must learn to express myself more precisely when describing a problem.

This is an 100 year old Ibach. It was little used before I bought it. The problems I had were because of age, not wear. I replaced all the bushings, felts, leathers and reshaped the hammers, although these will have to be replaced soon. The results exceeded my expectations. The piano sounds and feels great. All I have left to do is the fine regulation which will give me an even touch throughout. I have been careful to do everything by the gospel according to Arthur Reblitz. I have quite a bit of experience of regulation of uprights from the past, although I have never gone for perfection before. Getting the backchecks to work consistently over all the notes is my last exercise.
I appreciate that aftertouch could be where the problem lies, but what can I do except compare a note that works properly with one that doesn't. Hammer blow is the same; key dip is the same: let-off is the same; hammer drop is the same; repetition lever tension is the same. All that seems to differ is the amount of space between the jack at rest and the felt behind it. This difference seems to arise from aligning the jack to the hammer knuckle. There were simply differences in how much I had to twiddle the jack screw to line up the jack edge with the back of the knuckle center. This means that some jacks have to compress the cushion felt more than others (all of them do compress the felt - there is no space behind the jack after let-off), and I am guessing that this results in the jack heel being driven further into the cloth button on the let-off dolly rather than pulling the jack far enough back to allow the knuckle - and thus the hammer - to descend freely after rebound.

I know the dangers of jumping to conclusions where piano regulation is concerned, which is why I seek the advice of experts.

Thanks to all who have helped so far.

The hammers do not have to check after every blow. As long as the hammers do not bounce back and hit the strings you are fine, so if the hammers do not check on light blows, there is no problem.

Listen to what I am saying. If the rep spring is set too weak, the rep lever and knuckle will drop too far and the space will not be enough. The hammer checks at no more than 3/4 inch below the string. This is where the jack should barely clear the knuckle. Let off is 1/16 inch and drop is another 1/8. If the rep spring is set properly and so is checking, the hammer cannot take the rep lever down to the point you talk about.
If you don't have the butterfly springs and have the Schwander style rep springs with the screw adjustment, there is an extra step. Otherwise the hammer bouncing on the rep lever will confuse the drop setting and you won't see the rise after check.

I do appreciate this and there is no blubber. What I am striving for is consistency. There is a slight, but detectable difference in touch between a non-checked and a checked note. In the first case, the rebound energy is dissipated on the rep. lever directly after the hammer blow. In the second case the rep. lever does its little dance (with attendant action noise) after the key is fully released. The repetition mechanism obviously relies on the rep. lever storing the energy from the rebound in order to lift the hammer when the key is partly released. If that energy is not used, it has to be dissipated somehow.

I mentioned earlier that, after all the work I have put in, I am going for perfection this time but, if you tell me that pefection is nowhere near attainable, I will have to be satisfied.

I don't quite understand this. The tension on my rep. levers is adjusted by a crude, but ingenious, mechanism which relies on a silken thread wrapped round a tiny metal peg. Age has taken its toll on these threads too, but I find that a length of polyester thread from my wife's sewing box is an adequate replacement for the ones that tear.

Ahhh. The springs will need to be replaced if you want perfection. That will be the only way to get everything even.

The rep spring is a shock absorber. It has enough of it own energy to propel the hammer back toward the string. The energy is released when you release the key. A fast repetition would be terribble to feel without a spring.

I have some difficulty accepting this statement. After all, I can still adjust the rep. lever tension over the full range from sloppy to very springy. That being the case, I fail to see how the age of the spring has any bearing on the problem. The springs are brass-plated. There is no rust. When a note is working to my satisfaction, the repetition rate seems as it should be.

If you are right, or there is any doubt at all, I am more than willing to replace the springs, but then there is the big question: where on earth could I source springs of the same size and shape?

The range of the spring. Sloppy is not good. With that tpye of spring the range or number of degrees the spring has to move is limited. Either yo don't have the spring in the proper range or the spring is weak. In other words it doesn't have consistant power throughout the range. You have to set it so the rise is snappy or it bobbles on the lever when it goes into drop.
Really you need to take my suggestion and call a tech who knows regulation and have him "grade" your reg job and do a few notes right for you. You say you have some notes regulated correctly because it plays ok but that may not be true. I base this on your confusion. When some notes work fine and some don't, you are missing something.

Good technicians are a bit thin on the ground around here. If I knew one I could trust, I would have him over straight away. The last one I had failed to diagnose a noise problem: I eventually found the cause myself.

When you say I am missing something, you could well be right but, going back to my original post, I still have to say that the notes that give me trouble are the ones where the hammer meets resistance as it travels downwards after rebound. If I look carefully, it seems to me as if the jack is being gripped between the hammer knuckle and the cushion felt, thus increasing friction on the knuckle. Earlier let-off, increased key dip, and making the felt thinner all show an improvement. Trimming the felt on the one note on which I tried it cured the problem . Is it really beyond the bounds of possibility that the felt pads I bought are all too thick or too resilient? I seem to remember the old ones were softer, but I put this down to age. Stupidly, I didn't save any to make a comparision.

What also supports my diagnosis is that there were a couple of notes where I had mistakenly stuck the felt on the wrong way round: against the grain as it were. This makes the felt so much stiffer that the jack is unable to compress it, resulting in a total block as the jack cannot let off properly. Even the key is unable to descend fully.

I appreciate your help but, if I have any confusion, it's because you guys are telling me I am missing something, but I still don't know what.

Yes, it is possible that the felt is not right.

When you start describing a problem like this, it would be good if you list everything that has been done or replaced since the piano was new. I automatically assume everything is original. Anything else is just a guess. You are misleading us if you do not tell us these things.

If you replaced the felt, the felt may be the wrong size, it may be glued in a slightly different position, or the resilience may be wrong. If trimming works, then by all means go ahead and do it.

You shouldn't need to rely on the spring to compress the felt in order for the jack to clear the knuckle. If there is not ample room for the jack between the knuckle and the felt then the felt you used is too thick.
Did you replace the shanks and/or knuckles too? If so you might compare them to see if they are identical in size and knuckle position.

Earlier let off and more key dip. Exactly. You are narrowing the window of the rep spring. The check distance gets closer as you add key dip. The window of movement of the rep lever (and spring) is from the check position to drop. If you have too much check distance when you set the spring for proper rise, the spring quits as it gets to the top. Then when you try and set drop, the hammer bobbles and you can't get that adjustment right and this messes the rep adjustment, etc.

I suspect your checking should be set as close as possible. 1/2 inch.
Try this; Choose the keydip you like. With the rep spring set slightly strong, set the letoff at 1mm, really tight. Find where you think the hammer is blocking against the string and back off just a mm. Then watching where the jack toe contacts the letoff button, adjust the drop screw so it contacts the rep lever at the same time. Now set the checking at 1/2". Readjust the rep spring so it has the proper slow smooth rise. Check rep lever height after setting rep spring. Refine the drop by feel. There should be a platform feel when the drop and letoff are working together. Refine the aftertouch with the hammer blow distance.

The real reason I wrote all this is for the other techs on the list. The spring type with the short arm and tail like on a Schwander action can be confusing because what happens when you make adjustments doesn't yield the same results as the renner style.