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Trying to get Isaac Oleg to clarify his broken English is almost impossible, at least as difficult as figuring out what is wrong when you will not answer my question.

Stack on - if you raise the key to the normal height where you say it should be - is there a gap between the top of the key capstan and the whippen heal?

To clarify... at rest, the key seems slightly depressed (about 2 mm). The jack is slightly up as if it were being played. If I remove the action and move the hammer out of the way, the jack is far above where it should be. If I move the key up from the front to the proper height, the jack and whippen are properly aligned. If I switch the whippen, it is the same problem.

Gene, if I raise the key to proper height, then all things look aligned.

To imagine the problem, it is as though a ghost is playing the note. There is no bushing problem or backfelt problem.

By "does not play": I mean that because at rest the key gets depressed, the jack slips in front of the knuckle. That way, if one tries to play the note, the jack slips-- misses the knuckle, and the note does not play.

I should also say, that it is not an issue of having correct amount of balance rail punchings. Its not a key height issue, it is that the key is actually slightly depressed and engaging the jack at rest.

Once you have made sure that nothing is obstructing the key, you need to regulate that note. Is the hammer shank resting on the shank rest? It sounds like the capstan has slipped way low. Crank it up so the hammer is above the rest. If it is really loose, you will need to tighten it. Check it when you remove the key to make sure there are no obstructions.

Sorry I mean taking off the stack from the key frame to have access to the keys.

Could be a key rubbing lightly, but what strikes me is that the original whippen was repaired, there should be a reason why. My hypothesis is accident, you will know what happens when you will have looked at the keys (even taking out the key up stop rail would have been enough probably)

You better give the brand and age of the instrument/action, some have specific trouble.

Take a regulation manual with you when you go back and go over everything...everything.

Check all of what has gone before in this thread and make a list.. the adjusting screw in the middle of the jack if there is one, back it off a turn (or two, temporarily) also check the repetition lever height that it allows the jack to get under the knuckle. swapping parts without re-regulating won't always help.

Make sure the jack is free completely, sometimes they stick at a crucial spot and also check that the spring is strong enough.

Consider that the two main presenting symptoms may not be related, especially if the note has had some kind of attention before.

Will do, working on a top to bottom repair/regulation so will be back to piano on Sunday and Monday. Will report what problem was!

Just remembered another highly unusual case.

The repetition spring is seated through a small hole in the jack. In one case, the spring was touching the opposite side of the hole as if the spring wire was too long and making the jack stop rotating past a certain point.
The spring wasn't too long but lying to flat. I reintroduced a curve in it, effectively shortening it so that it no longer reached the end of the hole. Then re regulate the spring strength using the top wing of the butterfly spring.

Question, anybody. How do you think that situation arose?

Edit,

Come to think of it, that situation under some circumstances could produce similar symptoms.

If you think about it, the two symptoms you describe....the key being depressed at the front and the jack remaining in a forward position, unable to return to "resting position" under the knuckle, are indicative of something pushing up on the section of the key distal to the balance rail pivot point. Since you have already checked the back end of the key for debris and found nothing, the next logical place would be either on the balance rail itself or the underside of the keyshoe. Forget the keylead theory. That would mean that the piano would have been doing this since it left the factory whenever it was manufactured

You don't have to think anything. That's self limiting. Your reasoning is incomplete. Although, a key with a pronounced dogleg (d. o. g. l. e. g, pronounced "dogleg") and a worn bushing can fail to come up to level.

Are the rollers on the hammer shanks all in line?

CC2: But why won't the jack reset when the key is only down 2mm? I think it is likely there is more than one problem.

Perhaps the let off button is set too low, or the rep spring has come out of the notch in the rep lever, or the whippen is hanging up on the one next to it.

Then there is the hammer rebound rail being too low and interfering with the whippen flanges, so:

YES, Yes, yes, yes, yes, y.... But considering the key also, probably not just ONE problem with ONE yes.

Questions like this almost drive me insane. Only because if I were there, I could probably figure it out in less than a minute

Outcome?

10 € on the broken key

A picture could save a thousand words. Or better yet, a cell phone video of the problem.

I wouldn't be a bit surprised if you are correct, Isaac.

More than once, I've had a cracked key/split button drive me a little crazy before noticing the problem was right there in front of me.