What Causes Squealing?...

Coda

Coda

Well-known member
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?...
 
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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.
 
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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.
 
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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...
 
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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
 
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Coda said:
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...
Click to expand...
Did you manage to resolve the squeal?

I just built a Megalith/Box-of-Rock and it does squeal a bit with high gain and all turned on. Alone with guitar and amp only it is quite fine, so I suspect the circuit is one that prefers to sit early in the chain. The Gain and Boost pots do scratch when turned. I think I read this is sometimes inherent (?) to the design and component selection so won't worry too much about it.
Build docs with schematics (better resolution source than me trying to snapshot it).
 
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I was just playing on the word squeal to promote consent being important. I have no troubleshooting tips for you besides that having 2 high gain circuits both on will pretty much always squeal. Consider a good noise gate if you want quiet without stomping both of em off and on at once.

The crackling pot is a "feature" on zvex pedals, it's inherent.
 
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Nostradoomus said:
I was just playing on the word squeal to promote consent being important. I have no troubleshooting tips for you besides that having 2 high gain circuits both on will pretty much always squeal. Consider a good noise gate if you want quiet without stomping both of em off and on at once.

The crackling pot is a "feature" on zvex pedals, it's inherent.
Click to expand...
Thanks,
Good to know the inherent Zvex.
There was only this pedal on. On the other hand it gas two separate parts, drive/dist side and boost side. So in a way that is probably same as two drives in a row.
I will have to check if any other in the chain cause the noise despite being off, since only git and amp does not make the noise in the pedal.

Again thanks, it helps.
 
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