Andrea,
I had to replace the filter board plus the motherboard and a few days before my SD1 too started to behave in a strange way (change of volumes, display not working correctly, etc). Since the warranty expires after one year, I suggest that you take your SD1 to the nearest Ketron center (the list is on the Ketron website). They are really kind and they usually change whole parts without being too fussy.
Ciao
Andrea
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Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.