Hi
I am in the process of repairing a Polysix that has a similar issue. I lost all use of the MG section adjustment before tracing the issue to leftover damage from the battery leakage on KLM367 that affected the KLM370 board. Although the MG section adjustment now works the selector switch still doesn't behave as I would have expected. Mine behaves differently from above in that I get some VCO modulation regardless of the switch position. VCO > LFO modulates VCO, VCF > LFO modulates VCF but still some VCO, VCA > LFO modulates VCA and still some VCO. In some Googled forums I have seen it as described as the way Polysix's normally operate, and in another that it is a problem that needs fixing. I too keenly wait to hear from someone who has already gone down this road. Synthhacker blog has some useful information on it but no fix.