Parasit Active Baxandall layout issue

[mdcmdcmdc]

Hi folks,

I'm having an odd issue with a build of Fredrik Lyxzén's active baxandall layout.
I built the circuit based on the layout on his Parasit site:
https://www.parasitstudio.se/stripboard-layouts/active-baxandall-tonestack

But there's a discussion and a few variations in a thread here (the schematic for the design is also on this page):
http://guitar-fx-layouts.42897.x6.nabble.com/Bax-in-a-box-active-baxandall-tonestack-td7264.html

The circuit works and sounds really nice aside from one odd issue that I can't figure out.

- When the TREBLE knob gets to about 85% CCW (say, 7:30-ish) there's an audible 'pop' and the circuit cuts out completely from 85% CCW all the way to fully CCW (ie, treble completely cut).

- If I bypass the circuit and then click the footswitch again **with the treble pot in the problem range** the sound returns and i'm able to sweep up the full range of the pot. When I then sweep down and hit that same spot, the pop returns and the circuit cuts out.

- Just for the sake of repetition, that's treble knob at full CW= sound; sweep down to 85% CCW=pop and no sound; 85-100% CCW= no sound; cycle the 3PDT footswitch= sound; sweep up to 100% CW = sound; sweep down to 85% CCW=pop and no sound.

I've tried:
- replacing the treble pot (no change)
- checking for obvious cold solders or bridged stripboard tracks (nothing obvious)
- replacing the TL072 (tried 3 different ones, no change)
- my very sophisticated debugging technique of just randomly poking parts of the circuit to see if anything happens...

What I found is that if I bridge all three lugs of the treble pot with my finger, everything works perfectly fine. Would this suggest a grounding issue that might disappear when the circuit is properly boxed up? I'm currently trying it out through through my test box so it's sitting out in the open.

FWIW, the test box seems to work without issue in general, and I've used it without problems for a bunch of stripboard builds.


Here are some photos:





And the voltage readings from the TL072 with BASS knob fully CW and TREBLE knob fully CCW:
1  8
2  7
3  6
4  5

DMM reads 9.30 at the source.

1 - 4.65
2 - 3.70
3 - 4.65
4 - 0.00
5 - 4.65
6 - 4.65
7 - 4.65
8 - 9.30


Thanks very much in advance — apologies for the lengthy post, just want to provide as much info as possible and sort this out.

EDIT - oops, just noticed I labeled the pinout for the TL072 incorrectly, it's been fixed.