Pink Purple Fuzz (Bearfoot, BJFE) oscillation problem

[Solder of Destiny]

I built the three-knob Pink Purple/Fuchsia Fuzz from this stripboard layout:

I used an MPF4393 in Q1 and an MP38A in Q3. I used 1n4001 as D1 and D2. Everything else is as in the layout (or at least that is my belief).

The pedal works but I'm getting oscillation in the higher reaches of the fuzz pot with the Nature in certain spots. The oscillation persists even with guitar volume rolled all the way off.

It's the same issue as another forum user experienced in this post. They never found a solution.

Here are some photos of my build. It's in a test box I've successfully used many times so I know the 3pdt switch, jacks, power and grounds are wired correctly and working.

Other people who've built the effect from the same source as me have reported similar oscillation problems in the comments there, so I'm guessing it might be an issue with the actual layout rather than something I've done wrong, but others in the comments report their builds are working well with no oscillation so I dunno.

Maybe some experienced fuzz builders here have some suggestions for fixing oscillation issues in general that I could try? Or perhaps can see an issue with the layout itself.

 

[Solder of Destiny]

Anybody out there?

 

[jwin615]

Could try a few things. Try touching a 20-220pf cap between lugs 1-2 and lugs 2-3 of the nature pot with the pot positioned to a section where it oscilates.
Could also try adding a miller cap to Q2.

 

[Solder of Destiny]

Could try a few things. Try touching a 20-220pf cap between lugs 1-2 and lugs 2-3 of the nature pot with the pot positioned to a section where it oscilates.
Could also try adding a miller cap to Q2.

Thanks for the suggestions. The only cap I have in that range is a 100pf and it doesn't really help, although it does change the pitch of the whistle a bit!

 

[Solder of Destiny]

So, I rebuilt this pedal as the four-knob version and there are no oscillation problems. A bit of a shame, though, as I much preferred the Nature knob to the separate bass and treble pots and I don't think I'll be able to get it into a 1590B, which would have easily fitted the three-knob one. Ah well, you can't win 'em all.

 

[Chuck D. Bones]

Those long wired could be the problem. With a high-gain circuit, we need to minimize couple between the input and output. The input wire in particular should be short and routed away from everything except the chassis. I prefer solid wire for internal wiring so that it stays put.

What does the toggle switch do and what is that circuit board stuck on the outside?

 

[Solder of Destiny]

I've since disassembled this build to reuse some parts so I can't test that theory. However, I did move wires around in case it was a lead dress issue and it made no difference. I'm not sure that was the cause (for whatever that's worth). The side circuit is this (switched to off):

 

[Chuck D. Bones]

So we're done troubleshooting this circuit then.

 

[Solder of Destiny]

So we're done troubleshooting this circuit then.

Yep. See #5

 

[MichaelW]

So, I rebuilt this pedal as the four-knob version and there are no oscillation problems. A bit of a shame, though, as I much preferred the Nature knob to the separate bass and treble pots and I don't think I'll be able to get it into a 1590B, which would have easily fitted the three-knob one. Ah well, you can't win 'em all.

That's funny, I've been thinking about re-building mine from 3-knob to 4-knob. What about separated Bass and Treble did you not like? I built mine on the PPCB Fuchsia board. One of my absolute favorite fuzzes.

 

[Solder of Destiny]

That's funny, I've been thinking about re-building mine from 3-knob to 4-knob. What about separated Bass and Treble did you not like? I built mine on the PPCB Fuchsia board. One of my absolute favorite fuzzes.

It's a great fuzz. It's not that I dislike the separate controls, but for me the Nature control functioned very well and gave me the range of tones I needed. If it ain't broke and all that. The only reason I changed was in the hope of fixing thr oscillation issue. Mission accomplished.

 

[Chuck D. Bones]

Pots are not ideal devices. We might think that grounding the wiper (pin 2) would provide 100% isolation between pins 1 & 3. It doesn't, which means there is an unintended feedback path from the output stage to the input stage. That by itself could lead to oscillation. We can get every setting that the NATURE pot gets by using the proper combination of BASS & TREBLE.

 

[HamishR]

I built one following Chuck's ideas and it is a fantastic fuzz/OD. Just wonderful. It has a few more knobs but IMO they are worth it. I have built it and it works - I dislike having INPUT anywhere the 9V rail. Seems to me to be asking for problems. So while it may be off topic I can strongly recommend this version. I like the original but like this a lot more. I also recommend soldering transistors in place once you know they are good. While I generally have faith in the circular bakelite Ge transistor sized sockets i find the strip sockets to be unreliable in the long term. The BC550C definitely doesn't need to be socketed, and IME if you have good 2N5952s you should be ok soldering them in too. And for Ge the 2N1308 is generally as reliable as they get. So I would suggest socketing the 2N1308 but solder it in place once you know it's good.

BTW- for the electrolytics I tend to use those intended for SMD construction. They work in this layout. But allowing enough length in the leads of regular electrolytics so that you can bend them over can work too. And the clipping knob really takes this pedal to the next level - outstanding.