Discussion about the Korg PolySix synthesizer

Hi, I have just repaired my polysix due to the batery leakage problem.
It all functions ok now but the only problem is that the last 2 voices are distorted when cycling through (digital distortion type sound). 
The first 4 voices are working perfectly. Can this problem be caused by the board KLM367 that I have repaired or is it a new fault located on the voice board (KLM366) as the first 4 voices are working ok. thanks