Filter behaving as oscillator

I've built a state-variable filter, to this schematic: R6 and R7 are a dual gang potentiometer (with the 3rd terminal unconnected, so variable resistor), and R2 is a single gang potentiometer (again as a variable resistor). R6/7 controls the filter cutoff and R2 the Q-factor.

To a certain extent it works as expected (as my analysis expected), however, when I bring the potentiometer to the position such that R6 and R7 are zero, the circuit starts behaving as a square wave oscillator, with R2 controlling the frequency.

I understand that my analysis breaks down when R6/7 = 0 - I end up with the cutoff frequency being infinite because I'm dividing by zero, but can anybody explain why the circuit starts oscillating? Why is it a square wave, and can I mathematically work out the frequency?

Also, simulating the circuit doesn't (at least for me) end up with it acting as an oscillator

• Because the phase shift is not an integral multiple of 360°. – Ignacio Vazquez-Abrams Aug 14 '15 at 17:23
• Check out Barkhausen criteria – JIm Dearden Aug 14 '15 at 17:26
• @IgnacioVazquez-Abrams Barkhausen criteria states it will oscillate when it IS a multiple of 360? – ACarter Aug 14 '15 at 17:31