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I have been searching far and wide because I’ve seen them before but don’t know what to call them or where to find them. I’m looking for a machine that your place on top of the key tops and these little dowel things press down on the keys and simulate playing the piano for hours. It’s supposed to help condition new strings so that they stay in tune better. Can anybody direct me to where I can find one of these machines?
I think most people who have them just cobble them together.
It's called a "pounder" . (See Steinway pounding room on YouTube)

WN&G now sells a portable version of what they use at the Mason & Hamlin factory -- made from PianoDisc parts. I have one and use it after installing new sets of hammers. I get much better results now than previously telling the customer to "play it for 30 hours". I run it for 8-20 hours depending on the situation.

But, a pounder has relatively little to do with strings. It's purpose is to work the action to "wear it in" and particularly to get some playing time on hammers. The hammers I use respond particularly well to pounding. Before pounding they might seem like they need a little reinforcement but after pounding they are fine.

Feel free to PM me if you have further questions.
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