Let's get back to this one.

After thinking more about the sweep part of the circuit, I don't need to change much of my writings in earlier posts. The sweeps are not symmetrical as I first assumed, and the sweep time around Q3 is not that easy to determine because R12 makes Q3 a constant current source which loads up C9. Therefore, the sweep on the left side of R11 is a linear sawtooth sweep and the sweep on the right side is more exponential and it does not go down all the way to 0V because of the voltage divider R19 and R13.

Then the OTA filter. I made a simulation using the following simplified circuit:

For the XOTA I simply used a VCCS subcircuit model:

.SUBCKT OTAS 1 2 3 gm=0.009

RIN 1 2 10000k

Ggm 0 3 1 2 {gm}

.ENDS

and added the transistor buffer from discrete components with reference to the schematic snap posted. There is still some problems in the Y-triggered filter schematic, but I presume that the simplification of the input circuit is close enough. Don't mind the actual gain values I got, because R4 affects to those and I was not sure how to connect that properly due to the problems in the original schematic.

Here are the simulation results as a function of the OTA transconductance parameter:

It seems that it tries to mimic the original wah pedal frequency response with a combination of a high-pass filter (Chp) and a low-pass filter (Clp). The maximum transconductance value (gm=0.009) I calculated from the approximate formula gm = 20*I

_{bias}, and started decreasing it from there. There are two options how it changes the "resonance" frequency. It is either using the fact that the output impedance of the OTA changes with I

_{bias} and this forms a low-pass with the Clp capacitor or then Clp is used as a Miller capacitance in the feedback loop and its value changes with gain (which is controlled by I

_{bias}). I am hoping it would use the former option, but the most obvious is to use the latter mechanism as the original wah pedal uses.

The transistor buffer with the ac feedback through Clp has a transfer function of a first order low-pass filter when it is connected to a voltage source with internal resistance Rs. The cut-off frequency is linearly related (but not directly) to the time constant of Rs and Clp. This would make it possible to alter the cut-off by modifying the source resistance, but I have not been able to verify how much the output resistance of the VCCS block changes with gm. This is why I would opt for the Miller theorem in this case.

Would be nice to know if anyone else has made progress in analysing the complete Y-triggered filter circuit.