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Excessive jack movement on Yamaha U1
#2900998 10/16/19 08:20 PM
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Hi all. Long time lurker, first time poster. Appreciate any advice from the experts on this forum.

I’ve recently acquired a 1985 Yamaha U1. Been in the same family home since new, lightly played and regularly tuned. Plays well. The action is good - regulation distances set correct, everything even and consistent, no excessive friction in action centres, jack springs strong, hammer return spring cords have been replaced and are working right.

However.

On some keys, during soft playing, when a key is held down and the hammer is in check, the jack can be seen to wriggle its way free from its usual stopping position under the butt and continue backwards until it is almost off the end of the butt leather. The jack loses purchase on the butt, the key hangs and will not repeat until it is released fully.

The jack stop rail seems to be doing nothing to stop this behaviour. When the jack stops in the correct/normal position it is at least 5mm away from the stop rail. Even when it misbehaves it is still not quite touching the stop rail. It seems to me that the stop rail is not adjusted right.

Question: Does the jack stop rail adjustment sound like the underlying problem here?

Isn’t this precisely the problem the jack stop rail is supposed to prevent?

What is the recommended method for setting the stop rail position? (i.e. how far should it be from the jack’s normal stopping position.)

Any advice appreciated, thanks.


Yamaha U1. Yamaha P-45. Yamaha RD-250 (a long time ago). smile
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Re: Excessive jack movement on Yamaha U1
scirocco #2901081 10/17/19 04:11 AM
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The jack stop rail purpose is to block the jack from slipping too far from the hammer butt when playing loudly. However, 5 mm from the rail when the hammer is a bit far, but not the problem here.

Quote
regulation distances set correct


Could you be more specific? What did you measure?

It seems an aftertouch problem. You should have a tech look at it when having the piano tuned.


Ulrich Guillerm
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France
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Re: Excessive jack movement on Yamaha U1
scirocco #2901388 10/17/19 06:34 PM
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I occasionally run into a problem with certain pianos where the jack gets...jammed (for lack of a better word) up behind the hammer butt. It doesn't happen often with normal playing but I can often force it to happen by playing the note normally and then instead of releasing the key, holding the lip at the front of the key and jiggling it quickly up and down while slowly raising it, lowering it, and raising it again. The hammer slips out of checking back towards normal rest position, but the jack stays tripped, working its way up the back of the hammer butt.

I wonder if this is related to scirocco's problem. (Also, is the solution to this problem to adjust the jack rail?)


Anthony Willey, RPT
PianoMeter
Willey Piano Tuning
Re: Excessive jack movement on Yamaha U1
AWilley #2901414 10/17/19 08:22 PM
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Originally Posted by AWilley
It doesn't happen often with normal playing but I can often force it to happen by playing the note normally and then instead of releasing the key, holding the lip at the front of the key and jiggling it quickly up and down while slowly raising it, lowering it, and raising it again. The hammer slips out of checking back towards normal rest position, but the jack stays tripped, working its way up the back of the hammer butt.


Yes, that's the behaviour, although a bit more easily triggered. My grand-daughter (who is a beginner) plays very lightly and tentatively and if she doesn't quite hold down a note in the left hand firmly enough before repeating it, the key can sometimes hang. With my adult hands and arm weight I don't see it as much but I occasionally get it.

Originally Posted by ulrichq
Could you be more specific? What did you measure?


Key height, blow distance, let off, key dip and checking distance. All are correct and consistent except checking distance, these are all over the place. After a bit of experimenting I have found that keys where the check distance has been set way in close (under 10mm) do not have the problem. The keys where the check distance is out at the recommended 15mm (from Randy Potter's book) are the only ones that have the problem. I wonder if a previous tech has bent the back-checks in to fix the problem when it's been reported by the previous owner.

I guess I could simply reduce the checking distance on the problem keys but I suspect this would just be covering up whatever the real problem is.

I haven't really measured aftertouch, I guess I don't fully understand its role.


Yamaha U1. Yamaha P-45. Yamaha RD-250 (a long time ago). smile
Re: Excessive jack movement on Yamaha U1
scirocco #2901415 10/17/19 08:29 PM
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Aftertouch is the distance the key travels downward after let-off, so it is an important factor in this.


Semipro Tech
Re: Excessive jack movement on Yamaha U1
scirocco #2901422 10/17/19 09:11 PM
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Thanks yes it certainly makes sense that it would be a factor.


Yamaha U1. Yamaha P-45. Yamaha RD-250 (a long time ago). smile
Re: Excessive jack movement on Yamaha U1
scirocco #2901436 10/17/19 10:33 PM
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I don't have much to add here other than to say that I too have found it to be a puzzling problem. I usually approach it with normal regularion (blow, dip, letoff, lost motion, checking, aftertouch) and that usually reduces the problem. But it still bugs me not knowing if I've actually gotten to the root of it.

Slightly off topic, but this something that would never happen in the Fandrich vertical action.
[Linked Image]


Anthony Willey, RPT
PianoMeter
Willey Piano Tuning
Re: Excessive jack movement on Yamaha U1
scirocco #2901462 10/18/19 02:08 AM
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Anthony, did you check the whippen flange for sluggishness? As the jack recontacts the leather on the return the slight friction might be just enough to stall the whippen. Just a thought.

The OP's problem may be something different.


Chris Leslie
Piano technician
http://www.chrisleslie.com.au

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