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

    To pinpoint the location of the problem, would it be a good idea to just swap hammers or wippens with the neighbor key and see if the problem has shifted to the new key?

      Rubens Hi Rubens, I did swap the hammers earlier and the problem still exists in the problematic key.

          AlvinC Ah, so it's almost certainly something in the wippen.

              JohnCW Hi John, thank you for the prompt response. what about the spring of the other two keys shown in the video? it's not easy for me to tell which one is too strong by watching its speed

                  AlvinC Just assess the rise speed of each hammers when released from check. As I mentioned in a previous post they should rise about the speed someone gets up from a chair, with no kick/bounce at the top of their travel, and no kick should be felt through the keys.

                  Sydney Australia
                  Retired part-time piano technician

                      Rubens Yes, now I just need to test whether adjust on the repetition spring can help. Thanks.

                        JohnCW Thanks, John. I will try to adjust and keep all of you posted later.

                            AlvinC If you're going to try adjusting the repetition spring do yourself a favour and buy the Hart Spring Tool. It is considered the best going for hooking out those pesky butterfly springs.

                            Now you might think it's overkill just to adjust one spring. But look at it this way, there are bound to be more among the other 87 that need adjusting.

                            Sydney Australia
                            Retired part-time piano technician

                                JohnCW Dear John, thank you so much for making the regulation look like more desperately. Just kidding. I do respect every technician, the regulation is really time-comsuming and tedious. I am new to this forum and regulation and so grateful to have all your generous and friendly replies.
                                BTW, I did try to lower the tension of the spring with tool in the weekend. It improved but still the problematic key takes harder blow to be checked by the backcheck (see below video). So I just want to know what is the standard velocity for the hammer to be checked? I always worry to set the spring strength too low or damage the spring.
                                [

                                    AlvinC Repetition spring is still to strong. If you excessively weaken it by bending it simply won't push the hammer up out of check when you release pressure on the key.
                                    It needs to rise without that clear bounce/kick at the end of its upward travel when released from check.

                                    Sydney Australia
                                    Retired part-time piano technician

                                        JohnCW OK. What about the hammer right next to it？I guess it is with the correct tension.