I wanted to update and note that I successfully identified the problem and I was able to repair it!
Taking the Q-link pcb out, unscrewing it and lifting it out, I found that the solder joint connecting the slider to the board had come loose. Pushing it to one side made the connection to the board while remaining untouched it was completely off. I removed the old solder and reattached it with a new joint (I did this to both prongs on the loose side) - now it works perfectly!
I think it became detached because of its age,
obviously, but also - I have been getting into using the Q-links as "live performance tools". Bandpass filter on a distorted kick drum for example, and I've played it rather,
enthusiastically, lets say... I believe this must have helped it loosen up quite a bit.
It is a case like this that reminds me
this is an aging piece of gear, but also how easy it is to work on. And it def was easy. I was high too. Cheers er'body !