What Causes Squealing?...

[Coda]

Working on a Univox Square-Wave Fuzz (Pulse Wave Fuzz PCB) and I am getting a lot of oscillation. I get fuzz when the guitar is played, but as soon as it is stopped the squealing starts. I think it might be a grounding issue, as the oscillation changes when I touch the enclosure/input/output. I also know that transistor gain can also cause oscillation.

Is there anything else that could be the cause of the issue?...

 

[Chuck D. Bones]

It squealed with the other two JFETs, right?

It's not Q2's fault that the bias numbers are off. Q2's source & drain voltages are dependent on Q1's Vbe & hFE, R3, R4, R6, R9 & R10. The current in R8 is so small that we can ignore R7 & R8.

As long as the circuit is oscillating, the bias will be unstable and difficult to measure. I just wanted to see if it was even in the ballpark and it is. I'm pretty sure Q1 & Q2 are OK. Have you tried other transistors for Q1? Any Si NPN will work. Most, but not all, will have the same pinout as 2N5088.

I still think the most likely culprit is stray coupling caused by the combination of the board layout and dirt or debris on the board.

 

[Coda]

I have not tried anything else for Q1, and I will...as soon as I unsolder it from the board. An inspection of the back side of the board shows no issues, that I can see. It’s a bit smudgy, but not much stray solder that I can see.

 

[Chuck D. Bones]

Make sure you remove all of the flux residue with IPA. Don't let any dirty IPA get into the pots.

 

[Coda]

Make sure you remove all of the flux residue with IPA. Don't let any dirty IPA get into the pots.

The only place IPA will get into is my belly...unless we are talking about a different IPA

 

[Barry]

The only place IPA will get into is my belly...unless we are talking about a different IPA

Isopropyl Alchohol

 

[Coda]

Isopropyl Alchohol

I know, it was a joke.

 

[Coda]

Ok. Today I thoroughly cleaned the PCB, soldered Q2 in place, and checked all solder joints. I still get the squeal. I also notice that it definitely changes pitch with the fuzz control.
Next, I guess I’ll desolder Q1 and start trying other transistors. I have a feeling that if the gain of Q1 is the issue, a 2N2222 might solve the issue...

 

[Untro]

Just chiming in, I think Nostrodomus was the one to suggest (in a different but similar thread) using shielded cable for the in/out wires from switch to jack, and using the cold to ‘double down’ on the grounding connections, making the jacks grounded to the pcb, the enclosure, and the 3dpt switch. Also ive had the impression that this is why some choose to house high gain circuits in larger enclosures, so to better physically separate the physical wires from eachother?
But thats all i got, my sabbath pedal is stuck in end of year limbo still, yet to actually do this process. Lmk if ive misunderstood these concepts