Roland JX8P - Hearing other waveforms when both DCO's turned off on mixer

Thanks for the suggestion. You’re referring to the electrolytic capacitors correct?
If it were you, would you be looking at recapping just the power supply board, or main board (or both)?

I am not very good at troubleshooting these type of issues on vintage synths but I am very familiar with performing this sort of work so I was hoping to tackle the problem myself.