Aleph Null Cremulator Troubleshooting Low Frequency Oscillation

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ThomasAvenue

Member
Got it built up, everything seemed fine at first but when I take the fuzz knob to the last 1/4 or so of its travel I get a strong low frequency oscillation. Worked with @Aleph Null via DM and tried swapping several components to no avail, so it was suggested to come here and seek the wisdom of the forum sages...
 

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I'm hoping that getting more eyes on this will help us find the issue. Here's the schematic for reference:



Things we've tried already (in no particular order):
  • Increase C4. Pitch changes, but still sounds like a foghorn.
  • Replace the electrolytics. No change.
  • Increase R3 and/or R5 to reduce gain. No oscilation when guitar is disconnected. Oscilation comes back with the guitar is plugged in.
I don't see any obvious solder bridges, so that's good. It's probably still worthwhile to reflow the joints just in case. Also, do you have an audio probe?

It's probably also worthwhile to check the voltages at the legs of each transistor. Be sure to install stock values for R3 and R5 before checking. Here are the voltages on my working copy:

Q1Q2Q3
Pin 11.704.074.46
Pin 21.661.944.06
Pin 30.250.021.95

(I powered the pedal using a One Spot with 9.46V output.)
 
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ThomasAvenue said:
That’s only superficial as it was lightly marred during component replacement; the issue was present before the capacitor was swapped and any surface damage done.
Click to expand...
My bad, I should have specified that it looks like the solder connection for the ground plane is bad. Probably not the source of the issue, but less power filtering could possibly exacerbate the issue
 
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