So that’s good that the FET fixed the no signal issue.
Now it sounds like something is wrong with the high frequency signal path. We need to know if the problem is with the digital control (as in the high optocoupler’s led isn’t lighting up) or if the problem is in the audio signal path.
With the depth knob at 0 (fully counter clockwise) what DC voltage do you get on the square pad of the high side optocoupler (opto1)…from ground (black lead on ground, red lead on the square pad)? It should be about 1.8V dc.
-Brach