Casio WK-6500 keyboard, one channel dead

Hello,
Bought a used Casio WK-6500 keyboard, with no sound from right speaker (re right hand side of player).
Hoped it was bad speaker connection but found that only right output jack works, so the left/mono output jack is dead too.
I opened it and found nothing unusual on solder side of main board, except a jumper wire to the working right output jack connections.
I connected PC stereo out to the audio input and only get sound again from one speaker on keyboard.
Is it worth fixing? Any help appreciated!
West