Casio MT-70 repair

Hi everybody

I just bought a Casiotone MT-70, that yesterday was fully working. After just a couple of hours of owning it and playing it a problem has occured :S

The C# and G on all octaves is not making sound and I can’t switch to the “piano” and “funky” tones. Here comes the weird part. When I switch the vibrato button from “delayed” to “on” they all work, but now the C and the F# on all octaves do not work, as well as the “flute” and “wah brass” tones. When I turn the vibrato off, everything works.

All of it was working 1 hour ago and the keyboard didn’t move or get damaged in that time.

Does anybody have an idea on where to start with this issue?

Thanks in advance.

Looking at the MT-68 schematic which is very similar to the MT-70:
My first guess would be a bad connection on any of the cables that go to the keyboard and the panel. C#'s and G’s are all controlled by the same KI4 scan line, C’s and F#'s are controlled by the KI5 scan line AND the KIn lines also scan the switches on the panel. Hence … if you have bad contacts then bad reads are happening. Alternatively there could be a shorted diode pertaining to any of the switches, panel and/or keyboard.

So - I would start by just taking apart, clenaing re-assembling to start with

Thank you so much, I will try to do that.