Aleph Null Cremulator Troubleshooting Low Frequency Oscillation

[ThomasAvenue]

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...

 

[jwin615]

Keen to wonder if C8 and C4 may be working as a ghetto relaxation oscillator via R7

 

[jessenator]

Now I’m glad I socketed Q1!
…but, naturally, I have no 2N7000s…

It seems that Q1 (and maybe Q2) vlaues/specs are also a factor, because when I replaced the subbed MOSFETs back to all BS170s, I had zero oscillation on every power setup.

 

[Aleph Null]

I'm updating the build doc to recommend using 2N7000 instead of BS170. There are already notes about the effect of C4. If anyone else finds another fix or interesting mod, please let me know.

Thank you all for your help! Part of what makes this forum a great place to be is how generous the members are with their time and knowledge.

 

[Cybercow]

After futzing with the things I said last, I found the best results with simply changing the value of C4 to 1.5µF and omitting C3. During my auditing I found C3 does nothing that my ears can detect - in or out of circuit. So I left C3 out.

As a high gain circuit, with C4 at 1.5µF, I find the somewhat cyclic 'thrumbing' goes away. Even when everything is dimed, all I detect is a greatly increased noise floor. Something to be expected from a high gain circuit.

 

[ThomasAvenue]

Alas, the 2N7000 hasn’t solved the issue for me. I get some 1n5s Monday, after that I’m kinda at a loss…er, even more of a…

 

[Aleph Null]

Alas, the 2N7000 hasn’t solved the issue for me. I get some 1n5s Monday, after that I’m kinda at a loss…er, even more of a…

Dang...just Q1 or all three transistors?

 

[Ginsly]

As a high gain circuit, with C4 at 1.5µF

Longtime lurker firsttime caller - just helping to keep things clear for everyone, do you mean 1.5nf?

 

[jessenator]

Alephnull can speak to the specificy of C3, but I found it to be more of a polishing agent in the stock configuration. Might help keep the frequency shift on the tilt control. It's subtle, but it's there.

Alas, the 2N7000 hasn’t solved the issue for me. I get some 1n5s Monday, after that I’m kinda at a loss…er, even more of a…

Forgive me for asking, but did you make sure to flip it 180°?



BS170 is S-G-D in that same orientation

 

[Aleph Null]

Alephnull can speak to the specificy of C3, but I found it to be more of a polishing agent in the stock configuration. Might help keep the frequency shift on the tilt control. It's subtle, but it's there.

C3 is a treble bleed to help keep things from getting too muddy with the Fuzz control turned down. If you are reducing C4 significantly, it'll do little to nothing.

 

[ThomasAvenue]

Well, changed them all out, flipped 180 each…seems I must’ve damaged a trace or something this time because now the whole thing isn’t working. Worked “fine,” albeit with the same issue, with Q1 replaced with a properly oriented 2N7000 but with all transistors replaced I’ve got almost no sound with the volume and fuzz dimed.

 

[Aleph Null]

Well, changed them all out, flipped 180 each…seems I must’ve damaged a trace or something this time because now the whole thing isn’t working. Worked “fine,” albeit with the same issue, with Q1 replaced with a properly oriented 2N7000 but with all transistors replaced I’ve got almost no sound with the volume and fuzz dimed.

Let me know if you can't get it working in the next couple of days. I'm happy to send you another board as a "thank you" for beta testing for me.