Well, I took my flatmate's X911 to my usual tech because it followed the pitch only down to a certain note. Below that point it stayed at the same pitch.
The tech guy says he can't figure out what half of the circuit does at all, it seems to be such a strange design. (And he's a reliable, experienced guy, I can tell you.) So he borrowed another X911 from a friend and turned out to have the same problem. And a third from a collector, that hadn't been turned on for years, and can you guess it?
I would understand that the X911 won't follow extremely low pitches, but the critical point on all three was more or less in the middle of the guitar range, making them useless. (The CV control was perfect, though.)
Has anyone experienced the same problem? Is there any component that dies inside old X911s? Do all X911s share this flaw? Is it only a string of bad luck?