That's why you do the first measurements without powering the keyboard. There is no need
to put power on it to check the switches, wiring and visual inspection. Those have to be
OK before you put power on it! If you can measure or see that there is something wrong why
take a chance to make it worse? Put your multimeter in "continuity check" for that. It's
when it makes an audible beep when you make the leads touch.
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Expert in non-working solutions