Plaid Preamp Only Works when Bass knob is dimed

[pantsco]

This is only the 2nd pedal that I’ve built so my troubleshooting skills are weak, and I would appreciate any insight.

The Plaid I built (when engaged) only passes audio when the bass (A1m pot) is between 95-100% and then the pedal works normally. It does crackle when about 90% or so. I tried a replacement pot to same effect.

I’m a little stumped and still learning to read schematics etc so I’m sure it’s something I’m overlooking. Thank you!

 

[pantsco]

So I made an audio probe, and I still have signal going on Pin 5 of the bottom opamp, but when I touch pin 6, there is an louder signal for a moment before it fades away to silence. Pin 7, which I believe is the out, is silent. I feel like I'm pretty close but I need help connecting the dots.

I swapped the opamps and same issue. Again, if I turn the bass pot up above 90% I get output, so I've been testing with it turned to 0.

 

[jimilee]

Basic troubleshooting stuff: reflow all your solder joints, check component values to make sure they are correct. A small build like this is easier to mix up components than larger ones. Then check voltages. Hope this helps.

 

[Feral Feline]

Visual inspection is key, too.

Look for solder bridges — even a whisp of solder bridging components not meant to be connected can cause havoc

The behaviour you describe seems similar to something shorting to a capacitor to ground.
While the cap is full you've got signal but as the cap drains away to ground, so goes with it your signal.

I see you've got pot-condoms on so it's not the back of the bass-pot touching a nib on the back of the board.

Caps near the bass pot are C13 and C100, the former being the output cap not going to ground and the latter a power filter going to ground. Another close cap is C12, which is a coupler.


Look for bridges from your bass-pot's pins to C100; R11 may be slowing down the loss to C100.

This suggestion is a total stab in the dark, I'm not known for trouble-shooting prowess (or any prowess).
Schematically speaking the bass-pot is isolated from any caps that would bleed signal to ground, and yet pin-5 of the IC1.2 is still getting signal so...

 

[pantsco]

I appreciate your input I’ll give it a shot. I had to walk away as I was getting lost in the sauce a bit.