[sgroup] Roland S-50 output overload issue

Hello to all. I have Roland S-50 sampler in nearly mint condition. Just one minor issue: periodic overload (overdrive) of output signal. I've tested power/filter board and it works fine. S-50 is equipped with multivoltage power supply and I've tried 117 and 220V wirings but always is the same: every 7-8th notes from S-50 are played distorted. The Visually sampler is in perfect condition - all seems to work fine, there are no sights of burnt components inside. This problem exists on all outputs: individual, phones, mix. I've seen few mentions of similar issues in the internet: http://www.vintagesynth.com/roland/s50.php and http://jimatwood.wordpress.com/2011/09/16/free-roland-s-50-sampler-found/. Does anybody know how to cope with this problem? Thanks, Alex Mylnikov