Loud sounding keys on Yamaha PSR-520 61 Key Music Keyboard

by Jesse
(Oakland)

Loud sounding keys on my Yamaha PSR-520 61 Key Music Keyboard!

4 of my keys on my Yamaha PSR-520 keyboard sound-off louder than the other keys.

What is the problem and how can it be fixed? I love the keyboard. I've made professional recordings using my Yamaha PSR-520 61 Key Music Keyboard.