Today I tuned a piano where one key activated two hammers.

Upon closer look, I could see that the jack was crooked and positioned a bit under the neighbouring hammer butt.

90% of the time, this is caused by the jack becoming unglued from the whippen.

I decided to remove the whippen and investigate.

First, I removed the bridle strap.

Then removed the whippen screw.

Once I removed the whippen, I saw that it was not an unglued jack, but a slipped jack pin.

I just used the tip of my screw driver to reinsert the pin and then I replaced the whippen and reattached the bridle strap. Job done.

If I had looked more closely at the jack with the whippen in the piano, I may have seen the pin sticking out. Then I could have just inserted my screwdriver between the whippen and carefully pushed the pin back in, making sure the jack was aligned under the butt so that the pin had a good chance of lining up with the felt bushing.

There’s a chance of pushing the felt bushing out, but I’ve never had that happen with a vintage piano (<1930). I would be hesitant to try this on a newer piano  (>1960) because typically the felt is a poorer quality

If the jack was unglued, I would have just added a little wood glue, pressed the pieces together, waited a few minutes and reassembled the whippen.