Yamaha PSR 620 keyboard shifts tone randomly

by Dave
(Springville, NY)

My PSR 620 is behaving strangely. It is unusable because the tone sweeps up or down randomly for the entire keyboard,the same way tones shift when you use the tone wheel. I don't know if this is a component failure, a loose connection, or some integrated circuit keyboard defect. Can anyone help me out? I am electronically inclined and have done minor repairs, such as changing keypads. Dave