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Various models of General Music digital piano. These are high quality products, well worth spending considerable time and effort repairing.

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Disposition of keyboard contacts

I’m trying to diagnose a problem with a Gem digital piano, by email, on account of the Accursed Pestilence currently stalking the world.

Like all touch-sensitive keyboards, there are 2 contacts under each key, the speed of travel from one to the other determining the loudness of the note. So with 2 switches (or a 3-position switch) and 2 diodes I shoud be able to make a gadget to simulate one key, to give to the owner in order to test the keyboard scanning logic.

But it’s not clear from the schematic whether both contacts are normally open and both close one after the other as a key is depressed, or, as might be implied by the signal names on the schematic, the first is normally closed and opens, and the second, normally open, closes a fraction of a second later. Or something else.

This may vary from manufacturer to manufacturer, but the keyboard itself seems to be a generic Denon part so it could be that there’s a standard way all touch sensitive keyboards work.

Can anyone enlighten me with actual knowledge or exterience?

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From research elsewhere it appears that on domestic devices at least, beneath each key there are one or two contacts each cosisting of a conductive rubber pad which makes a connection between two circuit board traces, like the buttons on many feature phones, calculators and other small devices. So they are necessarily normally open. I would guess that high end devices such as an electrically operated church organ keyboard would have a more sophisticated switch mechanism, as conductive rubber pads tend to wear out.

For the record, the fault on this devices was that one of two ribbon cables, each connecting half the keyboard to a keyboard scanning board, had been inadvertently connected the wrong way round.

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Philip Le Riche sarà eternamente grato.
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