The problem has gone away. I’m not sure exactly why but after examining the Polysix schematic and realizing that the VCA ENV signals don’t go to IC16 I probed around the ENV outputs at R218 to check voltages an soon after the VCA started working correctly. I can only think there must be a bad connection at one of the tap points for the ENV to go to the Polysex board. For now it’s working!
The mod is amazing. It really adds another depth to this synth.