While the schematic you posted does have problems, the oscillator part is OK. There in the bottom right is U3 with a 33K resistor going from pin 2 to pin 7. Change that resistor to a fixed 10K hooked to a 50K pot, and that should allow you to change the frequency. That sucker is in the red box of the attached gif. Hopefully that's what you meant by "Ring knob".