Ok, so how good are you at taking things apart...!!! ????
I've been inside these things a number of times, fixing this, that, the other....
I would start by first pulling off the keyboard connector that mates with the
main circuit board.....these just pull off....straight up steady pull....by doing so
you discover several things ....one is if there are any broken wires...they don't always
show themselves until disturbed..this of course goes for both sides of the circuit
each end can have loose connections..
Then there is just plain connectivity due to typical corrosion of mechanical contacts that are only pressed together, not soldered. This is the single most common failure mode of all electronics (I've been doing repair for 30+ yrs) Just the very act of pulling connectors
apart then reconnecting them causes enough friction to 'clean' the metallic surfaces
In fact do this a number of times, and it usually cures such problems.....
Then there are actual component failures, which occur, but less often than the above...
you'll notice quite a number of DIODES wired to the back of the Keyboard circuit (
I assume you are having trouble with the Playing Keys, not the control keys...!)
Well one of more of these could be internally open-or-shorted....from your description
its more likely open, since several keys evenly spaced by 8, no longer function.
So open it up, take a look ....tell me what you find !
Dan