[sdiy] Troubleshooting the Morph-Lag

[appliancide]

Hello All,

I built this circuit to use for as part of a demonstration at my job, and I have run into a little problem. Certain settings of the "shape" knob (close to fully linear I believe) are causing the CV out to hit the positive rail. I am using a TI TL072IN if it makes a difference. 

Has anyone ever experienced that with this circuit? I found some suggestions from David Dixon in the archives and I may try those to see if the problem goes away, but I wanted to rule any other possible causes out first.

One of my coworkers made the suggestion that the shape potentiometer could be bad. Sound plausible?

Also...is there an "academic" write up of this circuit (or one that is similar) available anywhere? 

Thanks,
Paul