keff Hi Keff, thank you for sharing your thoughts.
The photo was taken by pressing the two keys not released so yes the dampers are lifted. Actually the right hammer is OK as it arrives the correct position after it strikes the strings (holding the key), while the left one is wrong as I use the same strength to press the key and hold it but it turned out in a higher position.
I took out the action earlier and observed the hammer:
1) The problematic one bounces on the repetition lever even after using the same strength to press the key, which cause the hammer shank/hammer much closer to the strings than the OK one.
2) For comparison, the OK one rests on the back-check and NO bounce is observed.

The flanges are the first thing I checked when pulling out the action, they are OK

    Is the hammer bobbling? Is the jack disengaging from the knuckle like its neighbors? Does the dip of the key seem to be appropriate compared to its neighbors? (Any chance there is a coin that got dropped down between the keys and is sitting there underneath the front part of the key altering the keydip for that key? Just an out there idea that came to mind.)

    Kinda sounds like a hunt to figure out "what's different".

      Bellyman Is the jack disengaging from the knuckle like its neighbors?

      That would be my guess based on the symptoms. If not properly regulated, the jack will not disengage at slower velocity attacks but would on faster attacks.

          The reality is it could be any one of several things, from jack adjustment/alignment, repetition spring strength, back-check adjustment, hammer tail surface, after touch etc, etc.

          Sydney Australia
          Retired part-time piano technician

            Bellyman
            Hey thanks for replying. I like your way of thinking about finding the difference and that is also what I did earlier. I pulled out the action and even switch the OK hammer to the problematic one and the problem still exists. So I can confirm the hammer/hammer shank is ok and unrelated to the issue.
            the knuckle does bounce per my inspection.The key dip is good as its neighbors
            see below video shoot earlier.
            1) the first part of the video shows the problematic hammer. its knuckle bounces after striking strings.
            2) the OK hammer in latter part of the video stops after striking strings when using similar velocity as the problematic hammer.
            [

              Rubens Dear Rubens, thank you for your reply. I attached a video showing the knuckle movement in my reply to Bellyman. Please help to have a check when you have a chance. Thanks!

                JohnCW Dear John, many thanks for your reply. I attached a video showing the knuckle movement in my reply to Bellyman. Please help to have a check when you have a chance. Thanks!

                  Make another video, action in the piano, filming the hammer through the strings. Hit the key hard enough to put the problematic hammer into check, and hold it there briefly with the key fully depressed. Then just every so slightly release the key allowing the hammer to bob back up .... Do it a couple times.
                  Clearly I'm interested to see the repetition spring tension.

                  Sydney Australia
                  Retired part-time piano technician

                    JohnCW Hi John, noted. What you descrip is the way to check the spring tension. I did tried so but no video was filmed. I will make the video per your instruction within this week and let you know then.

                        AlvinC I understand the drop appears before hammer hits the strings.

                        I thought the drop is what happens after hitting the strings. But maybe I'm wrong. I thought what happens before is called let off.

                            Josephine I thought the drop is what happens after hitting the strings. But maybe I'm wrong. I thought what happens before is called let off.

                            Correct. "Let off" is the jack being pushed out from underneath the knuckle (the round leather cylinder on the hammer shank) just prior to the hammer striking the strings as it moves upward (around 1.5-2 mm from the strings). At the very same time (in a well regulated grand piano) the repetition lever (the other bit sitting under the knuckle) contacts its stop adjustment. As the piano key continues to be pushed down past these 2 contact points the repetition lever 'drops' (typically about the same distance again as let-off). Now the hammer is sitting around 4 mm from the strings (let-off plus drop). Then as the key is pushed to the very end of its travel the hammer again starts to rise. But it doesn't rise sufficiently to 'bobble' against the strings (assuming let-off, drop, and after-touch are correctly adjusted).

                            Clear as mud.


                            Sydney Australia
                            Retired part-time piano technician

                                AlvinC Hi John, noted. What you descrip is the way to check the spring tension. I did tried so but no video was filmed. I will make the video per your instruction within this week and let you know then.

                                Hi Alvin,
                                Just be aware I'm 'on the road' for an indefinite time so may be slow to respond to your video (hate using my mobile phone for this sort of stuff). What you are looking for is the speed the hammer rises from check when released by taking pressure of the key. It should rise as what is often said "at the speed a normal person rises from sitting in a chair".

                                If the repetition spring is set too strong it fights the hammer rebounding back into check. If it leaps up, it is set to strong. Also check the hammer tail and see if it has become polished and smooth where it contacts the back check leather. Taking any shine off the timber tail with just an ever so light rub with a modest grit sandpaper can make a big difference. Just remove the shine, no more.

                                Sydney Australia
                                Retired part-time piano technician

                                    JohnCW

                                    Do you think it's a good idea to turn the drop screw a little bit to see if this fixes the problem of AlvinC's piano? (I'm asking because I want to learn from this, if the first video is made after playing the note the hammer looks too close to the strings).

                                        Josephine Do you think it's a good idea to turn the drop screw a little bit to see if this fixes the problem of AlvinC's piano?

                                        No. That's just adjusting things willy-nilly. To do the job right, everything needs to be adjusted correctly. But the staring point is having a somewhat correct repetition spring tension. And as an overly strong repetition spring is one of the main reasons for the hammer not checking on a soft blow, the best place is to start there.

                                        I'm assuming the back-check itself is set close to correct as in the video the two hammers checked in a near identical place on a firmer blow.

                                        Sydney Australia
                                        Retired part-time piano technician

                                            JohnCW

                                            Thanks for explaining!

                                              JohnCW Hey John, no problem as I am not in urgent need to restore it correctly. I guess the hammer tail is fine as I switched the OK hammer to this problematic key for test and sadly the problem does not disappear.
                                              Happy and safe trip!

                                                Josephine Hi Josephine, correct me if I am wrong since I am not a technician and only a piano fan. But I am indeed interested in learning regulation as I can understand much better of the piano I play.

                                                    AlvinC

                                                    I'm not a technician either, I just like to learn about pianos 🙂

                                                      JohnCW
                                                      Hi John, when you are available could you help to check the spring of below? the first hammer is the problematic one and it needs really heavy velocity to set on check
                                                      [

                                                        Repetition spring is to strong. It needs to be weakened to remove that bounce at the top of the hammers travel (which I assume you should be able to feel through the key).

                                                        I believe you said you had previously adjusted it so I assume you know how to do this and have the tool to hook the butterfly spring in and out.

                                                        Sydney Australia
                                                        Retired part-time piano technician