Thank you Mike. I changed the batteries, uploaded the factory presets in the unit, turned the detune off and tested the unit with a tuner, varying the levels of DCOs 1 and 2.
It turns out the problem is DCO2 which is slightly flat (around -10 cents) and whose pitch fluctuates a bit (-10 to -20 cents). With a very sensitive tuner it can be seen that the DCO2 pitch is quite unstable when that of DCO1 is nice and steady. This happens with detune off, and all modulation turned off. This is enough to make the unit sound weird with double patches, and with single patches, the weird "scale" is the result of different notes being allocated between the two DCOs which are slightly out of tune with each other.
I don't know much about electronics but after downloading the service manual and peering at the schematics it looks like the problem may be somewhere in the detune circuit (22) which affects CLK2 even when detune is off. With detune on the difference between the two DCOs gets worse, never better.
Do you know whether that can be fixed easily without changing the main board?
Thank you
Bruno