WHY the hammer of the Upright Piano is Bobbling there? Fix it’s yourself.

[themaximillyan]

• In this video the note plays twice when hit and the green part (the green felt) called 'the backcheck of the wippen' upright piano contacting the hammer rebounds off there. 'The the backcheck of the wippen' must catching 'the butt catcher of the hammer' after it's the hammer hited to the string there.
  Here's a step-by-step guide to help you fix the issue:
  
  1 Align the keyboard: Ensure that every key (white and black) has a standard height. Sometimes keys may have different heights and appear wavy. To fix this, use a cardboard or paper shim. Place it on 'the balance rail key pin' in the keyboard to adjust the key height.
  
  2 Adjust hammer-to-string distance: Reduce the distance between 'the head of the hamme'r and the string. The typical distance should be about 45-47 mm for modern upright piano.
  
  3 Minimize the jack-to-hammer butt distance: Minimize the distance between 'the jack of the whippen' (upright) and 'the hammer butt'. If the bobbling persists and 'the whippen's jack' does not drop down far enough from 'the hammer butt' after being lifted, adjust the position of 'the whippen's jack'.
  
  4 Remove the lost motion: Remove any lost motion by raising 'the capstan of the key' (the barrel on the key's pin) by unscrewing it.
  
  5 Increase the key dip: Raise the hammer height by shimming 'the balance rail key', similar to the process in step 1. This will result in a higher and more responsive key when pressed.
  
  6 Bend 'the backchecks' of 'the wippen' closer: To ensure proper hammer return, slightly bend the rod with the green felt (called 'the backcheck') closer to the hammer assembly by pressing it and using your fingers only. This process is known as adjusting the "butt catcher."
  
  Remember to work slowly and carefully when adjusting these components, as they are delicate and can be damaged if too much force is applied. If you are uncertain or do not feel comfortable performing these adjustments, seeking the help of a professional piano technician is recommended.