Quick Answer: Wasted movement between the interaction of certain piano parts.
Quirk Answer: Check your butts, they may be dragging you down.
Long Answer: Technicians may tell you that your piano has too much lost motion. Lost motion is a condition where you have movement but no action. Generally there are two places in the piano where you will need some lost motion; the pedals & the keys. Without the proper amount of lost motion in your pedals (especially the sustain pedal) and the keys will ring after being played. In keys that have no lost motion you may have difficulties repeating the note after it is played.
How do we create lost motion?
By creating a slight delay between the primary part (key or pedal) and its secondary part. While watching the video notice the almost imperceptible motion of the 3rd back check (count from the left) then a few seconds later you will see the 5th back check. Notice how much further the 5th moves before the butt check starts moving?
This adjustment is critical in the playability of an instrument. Too much lost motion and the parts move at the wrong time and could cause a bobble sound (a hammer hitting the string multiple times with a single key stroke).
The goal is to have as little lost motion yet still allow the mechanism to reset and cycle.
Now you're likely asking, what is a butt check and a backcheck... I'm mean really aren't you checking the back when doing a butt check? Moving on...
In all modern vertical pianos (spinet, console, studio, upright, upright grand) what you see in the video will be similar to your piano. You will notice a block attached to a wire, this block in the video is yellow with a pad of green felt glued to it. This block is called a back check, it catches or "checks" the hammer approximately 5/8" from the string after being played when the key is held down.
The next block you see is the butt check if you play your key fully at mezzo-forte the leather covered block will come back and be caught by the backcheck. When setting the "lost motion" of the keys you want to see a slight movement of the backcheck before the butt check moves. If you can feel the lost motion in the key you will need to have an adjustment made.
Can you make this adjustment on your own? Likely.
The tool you will need depends on your instrument. Generally a long tapered thin steel rod will work.
Remove the case parts from the piano to allow access to the keys, you should be able to slide the keys off of the guide pins (balance & front rails). You NEVER should remove all the keys at the same time unless the keys are clearly numbered. They keys fit into the piano in a specific order and do not generally fit well when placed in the incorrect order.
If you remove a key from the piano at the end of the key, closest to the strings there will be an adjustable part, either a brass fixture or a dowel screwed onto a threaded metal rod, this is the capstan. Your long tapered steel rod should fit in the hole provided in the capstan, if you're capstan is hexagonal or octagonal you will need a tool to fit this shape. This adjustment is most easily made with the piano keys in place as you can test your progress as you make adjustments. Note: When turning the capstan your tool should not contact neighboring action parts, you could damage the mechanism if you're not careful.
Insert the tool into the hole and turn to the treble(higher notes) to reduce the lost motion making the key more efficient. If you release the key slowly until it comes to rest as you able to play the note again? If not you likely don't have enough lost motion. This is a subtle adjustment that will take some practice to perfect, the good news is you have 88 chances to get it right, the bad news... you have 88 notes to get perfect.
This adjustment normally will take an experienced technician less than an hour to complete, and in most circumstances can be done at the time of service, provided they are notified prior to the appointment to allow the proper amount of time for the adjustment.
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