As I expected: The problem was a broken trace due to a leaked cap. Cap exchanged, trace repaired, presets work. Looking through ebay it seems that many of the MicroPresets sold during the last few months had this very problem. There's one on ebay UK with this fault right now. There needs to be continuity from the middle pin of S6 to TR24. This is a very long trace that goes all over the PCB. In the vicinty of C50 this trace was almost completely missing.
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Update: more info about Korg Micropreset M500 problems
2015-02-20 by Malte Rogacki