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?...

 

[Mcknib]

High-pitched squealing as you know can be caused by a few things, quite often its the wiring layout so that's the first thing I check especially with highish gain pedals

You dont want signal carrying wires running paralell or too close to power wires you want them to cross like this + and not run close together like this =

All the usual rules would apply short wires etc

I try to find out if it's radiated ie from external devices or conducted from a circuit component not as easy as hiss where if you turn your guitar vol down and it still hisses it's in your circuit /s

I usually use the not so sophisticated method of touching solder pads and wiggling wires to see if I can find it

I'll dab my finger on and off pads to try and narrow it down

There's some info here although it does get techie

Parasitic oscillation - Wikipedia

en.wikipedia.org

 

[fig]

Swine waves ?

 

[Coda]

Ok. So I insulated (put electrical tape around) the jacks, and it helped a little. Now the oscillation only happens when not playing. Playing the guitar through the pedal is just fuzz. As soon as I stop playing, however, the screeching starts. I poked around inside the pedal a bit, and found that when I touch the legs of Q2 (2N5458) the screech disappears. There is still a decent amount of hum, but I’m not worried about that, as the input is not plugged into anything, and the back cover is off the pedal.

Short of hiring someone to hold their finger on Q2 wheneverI play this pedal, what can I do to remedy the issue?...

 

[Barry]

Ok. So I insulated (put electrical tape around) the jacks, and it helped a little. Now the oscillation only happens when not playing. Playing the guitar through the pedal is just fuzz. As soon as I stop playing, however, the screeching starts. I poked around inside the pedal a bit, and found that when I touch the legs of Q2 (2N5458) the screech disappears. There is still a decent amount of hum, but I’m not worried about that, as the input is not plugged into anything, and the back cover is off the pedal.

Short of hiring someone to hold their finger on Q2 wheneverI play this pedal, what can I do to remedy the issue?...

Is it soldered in or in a socket?

 

[Coda]

Is it soldered in or in a socket?

Socket.

 

[Barry]

Socket.

You might try tinning the legs to see if it will seat better, or solder it in

 

[Coda]

You might try tinning the legs to see if it will seat better, or solder it in

Now that you mention it, it does sit pretty loose in the socket. I’ll see what I come up with. Thanks for the tip.

 

[Coda]

You might try tinning the legs to see if it will seat better, or solder it in

The transistors sit a bit more firmly, but the squeal remains. Maybe I can put heat-shrink around the legs?

 

[Chuck D. Bones]

Heat-Stink tubing is not gonna help.
Can we see pix of the inside?
Grounding is critical in pedals, particularly the high-gain ones. All pots, jacks & switches need to make electrical contact with the case. If the inside of the case is painted, then you need to sand or scrape off the paint near the holes. Do the sanding on the inside, unless you're going for the "relic'ed" look. Makes sure both jacks are grounded to the board.

For other things that can cause squealing, check out the movie "Deliverance."

 

[Mcknib]

That's all folks

 

[temol]

From my limited experience squealing ma by caused by:
- circuit itself (unstable),
- pcb layout,
- wiring (in/out wires proximity, etc)
- wrong value of the component
- misbiasing


Put a buffer (source follower) at the input and see if it helps.

 

[Chuck D. Bones]

You can do the same thing by putting a buffered-bypass pedal in front of your Pulse Wave.

How about some photos?

 

[Coda]

I took the tape off the jacks. Here’s the pic. Funny how such a simple looking build has given me such issue. Maybe it’s because I haven’t put the knobs on yet?...

 

[temol]

You can also try using sielded wire for the IN/OUT connections.

 

[Chuck D. Bones]

If you do that, make sure you terminate the shields to ground at the jacks.

I don't see anything wrong in the pic. R11 is 33K instead of 27K, but that won't matter. Those long transistor leads can act like antennae. You can shorten them up.

How about measuring some DC voltages? Let's start with Q2 D & S. D should be around 3.5V and S should be around 2.1V.

 

[Coda]

If you do that, make sure you terminate the shields to ground at the jacks.

I don't see anything wrong in the pic. R11 is 33K instead of 27K, but that won't matter. Those long transistor leads can act like antennae. You can shorten them up.

How about measuring some DC voltages? Let's start with Q2 D & S. D should be around 3.5V and S should be around 2.1V.

I am reading D: 3.7 and S: 1.3.

 

[Coda]

I also shortened the legs on the transistors, and shortened the input/output wires, and it still screeches. Maybe I’ll turn it into a Saved by the Bell-themed pedal...

 

[Chuck D. Bones]

I am reading D: 3.7 and S: 1.3.

Those are pretty far off. The board is squealing during your measurement, right? That's probably why the measurement is off. Make sure the board is very clean on both sides, particularly around Q1, under the socket. Looks like those transistors are good, so I would recommend removing the sockets and soldering the transistors in. There is a trace that connects to R11 (close to the output), which runs between B & C on Q1. Any stray coupling between that trace and Q1 could do this. Does the squeal change pitch when you turn the DISTORTION control?

Those "1N34A" diodes look like schottky to me. I have yet to see a Ge diode in an orange glass package. Where did you get them? It's possible they are the culprit. Not real likely, but possible.

 

[Coda]

Those are pretty far off. The board is squealing during your measurement, right? That's probably why the measurement is off. Make sure the board is very clean on both sides, particularly around Q1, under the socket. Looks like those transistors are good, so I would recommend removing the sockets and soldering the transistors in. There is a trace that connects to R11 (close to the output), which runs between B & C on Q1. Any stray coupling between that trace and Q1 could do this. Does the squeal change pitch when you turn the DISTORTION control?

Those "1N34A" diodes look like schottky to me. I have yet to see a Ge diode in an orange glass package. Where did you get them? It's possible they are the culprit. Not real likely, but possible.

The diodes came from Tayda. I’m trying to find the rest of them.

The squeal changes pitch slightly with the control. As for Q2, if the numbers are that far off, should I solder it in place?...

Also, for giggles, I just put the other two 2N5458’s that I have in the pedal and measured the voltage one was D/S of 3.96/1.63, and the other was 3.96/1.26.