Christian,
Roel is correct; however, if his suggestion fails, chances are that you have a problem with the motherboard. In any case, if you purchased the SD1 less than one year ago, you are entitled to a free repair with complete replacement of the defective part at any authorized Ketron center.
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.