Troubleshooting a Roland JX-3P with bad rumbly sound

This JX-3P came to me unplayable but is getting much better under my loving care and expert repairs (or hacks, depending on your point of view). In any case, it has a bad noise that shows up in patches using DCO 1. Using both thickens a sound, so it is nice to have both working. This video discusses test mode, making patches to test for certain problems, and using an oscilloscope to view the CV signals. Great fun and I found a smoking gun IC.