Several keys don't make any sound unless they are pressed together

Hi,

I have a problem with my Casio Celviano AP 38. I appreciate any kind of feedback.

Several keys (4 white keys in a row and 3 black keys in between) don't make any sound unless any four of them are pressed together.

There is one other black key at the other side of the keyboard, which plays well until one of the broken keys mentioned before is pressed. When pressed simultaneously, black key doesn't make any sound.

I wonder, if these symptoms could isolate the problem and show which element of the piano is broken. I tried to clean the keys and the electronic board under them, checked cables connections, but keys still don't work. Will replacement of the board under the keys help or this could be the problem of other elements? If yes, I will start looking for the new board in the internet.

Thank you!