Yamaha YPG 625 playing extra notes

by Ben
(Boynton Beach, FL)

My Yamaha YPG 625 has developed an interesting problem recently. When each key is depressed individually they sound fine. However, whenever C and F are played simultaneously (with C in the bass) the keyboard plays C, F, and F#. Similarly, if F# and B are played together (with F# in the bass), it sounds F#, B, and C. I'm wondering if anyone else has dealt with a problem like this, and whether it is something that I can fix myself or if I should contact a technician. I've had the keyboard for many years and it has always served me well. I would like to get it working again if possible.