The centre voltage is decided by IC7 which produces a ‘half-rail’ point. So there is no way of setting the negative rail separately. If the voltages are wrong, it’s most likely that IC7 has failed in some way.
I suggest that you replace IC7, and make sure that the 2N5401 is good…. the regulation looks to be slightly off 
It’s very unusual to see this sort of fault; it suggests that the unit has been hit with 240VAC with the fuse selector in the 120V position!