Casio SK-5 sound selection error

[americantourist]

Hello friends.  I've modified my Sk5 with a pachbay, and she has been very solid for a couple years now.  yesterday the sound selection buttons stopped functioning.  Here is the scene, i power the sk5 on (all mods off) and press a note on the keyboard, it produces no sound.  subsequent note presses sound fine and all four voices sound.  while the pads also sound, none of the other buttons seem to do anything.  i thought i must have a short on the keyboard matrix, but everything appears clean.  has anyone else seen an sk which stops responding to button presses but still produces notes?  It seems like one of the bits on the decoding matrix is faulting, but i don't see any problem.  i wonder if i could have fried one of those decoding lines.  any thoughts.  thanks.

sam