Aion Quadratron Screeching...

[smsnkaty]

Hello, all. I've been pulling my hair out with this Lovetone Doppelgänger clone. I feel like it's so close to working. Some extremely helpful users over at the Reddit DIYPedals Sub have been helping me troubleshoot here, but we're coming up dry. So, I wanted to throw it over here.

Here's the Build Doc

I initially posted b/c my LFOs weren't vibing. I was reminded that the expression pads needed to be shorted. They were jumped, and the LFOs are sailing smoothly.

Here are the basics for the current issue:

- The problem I'm left with seems to hover around the Vibrato toggle, the Color pot and the Blend pot.
- If I push the Vibrato toggle up (which connects Lugs 2-3, and breaks the signal between 1-2) and turn the Blend pot to 100%, I get lots of screeching and static as I dial through the Color pot spectrum
- Video demonstration, here: https://www.dropbox.com/s/3cb4uuqry6nq6wk/quadratron.mov?dl=0
- The static/screeching doesn't feel related to the LFO rate or span. It's only adjusting Blend and Color, and flipping the Vibrato toggle, that cause the problem
- LFOs and rate pots work, LF/HF Span and pots work, Slow toggle works, Square toggle works, LFO Cancel toggle works, Anti-phase toggle works, Dual LFO 3pdt works, Bypass 3pdt seems to work
- jumping the Dry Out pads made no difference, Connecting the NC pad on the breakout board to ground made no difference (the build instructions didn't say to do this, but we tried)
- I reflowed every single solder joint
- I have audio probed most of the circuit, and things start sounding ugly when I probe Pin 7 on IC3. Pin 5 going in sounds clean, Pin 7 coming out sounds distorted. I replaced C8/R13 just in case, no change.
- I have swapped the all of the TL072s with no change
- I have replaced C8/R13/R16/R22 with no change
- These also confirmed with probe:

   
• right leg of R20 is ground silence in both Vibrato toggle modes
• left leg of R20 is clean with Vibrato on (down), silent with Vibrato up (off)
• right leg of R16 is silent is both toggle positions
• left leg of R16, with toggle up or down:
• with 100% Blend the signal is either static or almost completely silent...
• with Blend at 99% or less, there is a clean Wah-Wah fluctuation that is in rhythm with LFO 1 LEDs, and Color acts as a volume control for the Wah effect

- The signal going through R15/C9 sounds clean
- I have checked Voltages for VA/VB/VC/VD and got 8.27/4.12/8.5/4.24
- Voltage check on Lug 1 of Color & Blend pots both indicated 4.11V (VB)
- Voltage on C27: + side reads 4.11, - side reads 0
- Voltage on C13: side connected to + reads 4.13 side connected to - reads 4.09
- Resistance check on the Color and Blend pots seemed fine
- Continuity check was fine on the Vibrato toggle switch
- Open Elec Caps C10/C12/C14/C16 all simply omitted, replaced with the Film alternate offered in the build doc
- Note: when both my LFO and Bypass 3pdt lights are on, and I dial Color and Blend to the noisiest screeching positions, there is a sound difference when I slide between True Bypass and Buffered Bypass. The difference isn't there when dialed to a non-screeching position with the pots
- It was suggested that this screeching might be the sound of voltage leaking through to the signal

Not really sure where to turn. I'm happy to test anything else on this board, if someone has a suspicion as to what might be going on. Thank in advance!

[LaceSensor]

did you try replacing the pots..?