Roland R-70 low output from right main channel?

[synthman]

Basically. What could cause one main output channel in this case the right one, to be lower then the left. All other outs, the individual outs & the headphone are fine. Dirty plug socket, volume slider, loose wire/connection. All suggestions wellcome

Edit: the seller has done a factory reset and has said that the unit hasn't been used for a while?