Discussion about the Korg PolySix synthesizer

Hi again. This time I have a Polysix whose VCOs are always modulated by a very slow sine or triangle wave. Of course all modulation controls in the panel and mod wheel are at zero value. If I change the battery board using a good one, the modulation disappears, so it is clearly a problem of the KLM-367A. Do you know what parts should I check?. Thank you for your help.

Kind regards

Manuel