Yamaha PSS-170 keyboard, no sound

This keyboard lights but does not play.  Clicks when turned on, appears to function when different numbers are entered, flashes tempo when playing the demo, etc, but no sound either through speaker or headphones.  A board with numbers XB967, M1796, and ELNA M1 94HB has a scorched component, looks like a small diode, maybe a zener diode.  Anyone have a schematic for this board?  I know it's a long shot.