Dynamic stack repairs, initial thoughts
Posted: Mon Jul 20, 2026 10:39 pm
So one of the reasons the piano was dead was because it could barely play anything. Taking a look at the Duo Art Dynamics box it wasn't hard to understand why....
This is the theme dynamics accordion. It's obviously gone, and that looks like extra thin pneumatic leather. Given that it is always under some tension from the dynamics knife (the lever that controls the loudness valve to the piano key pneumatics) my guess is it dry-rotted and ripped apart over the years.
Now, the problem is this: The dynamics work by moving a lever a specific distance to reduce the flow to the keys. The largest bellows on there is supposed to collapse exactly 1/2 inch when vacuum is applied. That should cut the volume in half. The second will move 1/4 inch, third 1/8 and the top one only moves 1/16. Thus you can get 16 levels of volume because of course this is a 1925 digital piano.....
Anyway, there is a problem though: The documentation specifically says that the stack collapses 15/16 of an inch when compressed. Although this thing is torn up, there is enough remaining to measure the height before and after compression. And the distance is interesting.
Fully expanded this thing is 84mm high from bottom to top. Fully compressed to the stops (there are three adjustable screws on each one) it is 72mm. Or in other words the max compression in 12mm..
But 15/16 of an inch is 24mm. Did Dad measure this thing wrong so it was exactly moving 50% as much as it should have?
Not sure why he would do that, but it's an interesting issue: It means that if I cover this it needs to be 1/2 inch or so taller.
I'm going to need to make some gauges using the 3d printer....
This is the theme dynamics accordion. It's obviously gone, and that looks like extra thin pneumatic leather. Given that it is always under some tension from the dynamics knife (the lever that controls the loudness valve to the piano key pneumatics) my guess is it dry-rotted and ripped apart over the years.
Now, the problem is this: The dynamics work by moving a lever a specific distance to reduce the flow to the keys. The largest bellows on there is supposed to collapse exactly 1/2 inch when vacuum is applied. That should cut the volume in half. The second will move 1/4 inch, third 1/8 and the top one only moves 1/16. Thus you can get 16 levels of volume because of course this is a 1925 digital piano.....
Anyway, there is a problem though: The documentation specifically says that the stack collapses 15/16 of an inch when compressed. Although this thing is torn up, there is enough remaining to measure the height before and after compression. And the distance is interesting.
Fully expanded this thing is 84mm high from bottom to top. Fully compressed to the stops (there are three adjustable screws on each one) it is 72mm. Or in other words the max compression in 12mm..
But 15/16 of an inch is 24mm. Did Dad measure this thing wrong so it was exactly moving 50% as much as it should have?
Not sure why he would do that, but it's an interesting issue: It means that if I cover this it needs to be 1/2 inch or so taller.
I'm going to need to make some gauges using the 3d printer....