DJM 750 mk2 send/ return only playing through one channel

I have a DJM 750 MK 2 and the send and return is only playing through one channel. When I max out volume I can hear cracking from the non working speaker.

Replaced cables, tried diffrent effects units, tested on other mixer. Dosent work on head phone/cue. When I take the out put from effect unit and route though line in it works. 

Any help would be great,