Comparator on an op-amp in a PT2399 delay pedal
Well, this circuit left out the input and feedback resistors, and feedback capacitor. 

Try Rin of 100k, in front of pin 16. 

Between pins 15 and 16, try 150k and 47pF. Actually, use a 500k pot for this, so you can adjust it. After you found a sensitivity that you like, replace the pot with a fixed resistor.