Fuzz Wah oscillation help needed

[SonicVI]

Maybe somebody can help me out. This is a bit complicated.  I recently bought a 70's Thomas Cry-Baby Stereo Fuzz-Wah very cheap as it needed new pots (one for wah and one for volume). The way these pots are mounted and connected to the gears you cant just stick in two off the shelf wah pots without modification.  So, I decided I'd elimnate the volume function and use a wah pot only, and while I was at it mod the heck out of the thing.

Here is the stock pedal schematic:


As you can see the input goes to the fuzz on/off first (Vox distortion booster circuit, similar to Si fuzz face) which goes on to either the wah or to an output buffer.  So the fuzz>wah>buffer basically.
What I've done is:
-I totally rewired the pedal so both the fuzz and the wah are true bypass.
-The buffer is wired to the output of the wah and is modded to a JFET buffer.
-I put in a rotary switch to select fuzz before wah or wah before fuzz (Juggler circuit from geofex)
-Modded the fuzz to NPN (OC140) Ge Tone Bender "MK1.5" with fixed fuzz amount (1K resistor instead of pot)

-The wah by itself works fine
-The fuzz by itself works fine
-The wah after fuzz setting works fine
-The wah before an external Ge Tone Bender pedal works
-An external wah before the fuzz doesn't really work, as expected, but doesn't cause any problems.

The problem is, when the wah is before the fuzz I get an oscillation that basically sounds like a chainsaw motor.
I suspected maybe I had a problem with the effect order switch so I used alligator clip wires to connect directly from the in/out jacks to the circuit in/out component leads and the problem still occurs, so I think my wiring is all ok.  I also tried bypassing the wah buffer with no luck.    So even just the wah straight into the fuzz causes the oscillation.  Could there be some kind of ground loop bewteen teh circuits as since they are on the same circuit board?  Could the output of the wah be too great for the input of the fuzz? Any other ideas for things to check?

[Paul Marossy]

Here's some of my thoughts:

1. Are you using a battery or a wall wart to power this thing? If wall wart, maybe it needs some power supply filtering.
2. Is there a track(s) on the PCB that you need to cut as a result of making everything true bypass and eliminating the volume pedal part?3. Are you sure that the pot(s) is wired correctly?
4. Does it behave this way if you are in a different physical location?

It seems that it has something to do with the wah being before the fuzz, but it may not necessarily have to do with the circuit order. Case in point: I recently had something drive me nuts with a new pedal I built (English Channel) that suddenly had a lot of hum and I couldn't even use the wah pedal (which was before it) because it would just squeal like crazy. At first I thought maybe it was my wiring because I did a few little tweaks to this new pedal. I even added shielded wire to the input/output and that didn't help. I added another filter cap to my pedal board's regulated power supply, and that only helped very slightly. I finally found that the problem was that my wah pedal was just picking up EMI from a bunch of wall warts in my studio/work area/computer room and it was somehow causing a lot of hum to get into the system as well as causing that wild oscillation. When I moved everything about 8 feet away from where I was having these problems, everything was very quiet and there were no more problems whatsoever! I thought the problem was that I screwed something up, but it wasn't me at all!

[SonicVI]

Here's some of my thoughts:

1. Are you using a battery or a wall wart to power this thing? If wall wart, maybe it needs some power supply filtering.
2. Is there a track(s) on the PCB that you need to cut as a result of making everything true bypass and eliminating the volume pedal part?
3. Are you sure that the pot(s) is wired correctly?
4. Does it behave this way if you are in a different physical location?

Only been using battery so far. 
I had to cut one track in the fuzz and one in the buffer. Removing the volume part just required removing the pot and R22 (and J2, the stereo output).
Pretty sure the pot is wired correctly. I don't think the wah would work at all if it weren't  The points marked N1, N2, etc are labeled solder posts on the pcb, it makes all the connections very easy to follow. N5 to wiper of wah pot, etc... All components are labeled on the pcb.
Haven't tried any other locations. I've never had any oscillation problems with any other pedals and the fuzz and wah alone are not excessively noisy.

If it's a ground/power problem between the fuzz and wah circuits then it's perplexing that it only happens when the wah is before the fuzz.

[Paul Marossy]

Hmm... yeah, this is a weird one. It definitely sounds like some sort of low frequency oscillation happening, though.

[SonicVI]

I tried this with an adapter and the motorboating went away, but then I got howling instead. So I changed the buffer back to the stock BJT buffer and the howling is gone.  But, oddly, when I use a battery I still get the motorboating.  I added a 100uF filter cap from V+ to ground and it didn't really help.   Any ideas why a battery would cause this oscillation but a DC adapter wouldn't?   The battery is good.

[Paul Marossy]

I tried this with an adapter and the motorboating went away, but then I got howling instead. So I changed the buffer back to the stock BJT buffer and the howling is gone.  But, oddly, when I use a battery I still get the motorboating.  I added a 100uF filter cap from V+ to ground and it didn't really help.   Any ideas why a battery would cause this oscillation but a DC adapter wouldn't?   The battery is good.

Huh, that is even more weird. 

I have a hunch that it has something to do with your power supply. I'm thinking that maybe it has something to do with the power supply (impedance I think). You seem to have inadvertantly created something that's acting kind of like the "Stab" control on the Fuzz Factory. In the case of the Fuzz Factory, I believe that the "Stab" control changes the impedance of the power supply by adding more resistance to it, and that causes it to go into an oscillation mode. At least that's how I understand it...

[SonicVI]

I dunno, I gotta keep working on it, but at this point I'm pretty much stumped. 

[John Lyons]

Try a large cap across the power rails, maybe 330uf and see if that helps.
You can just touch it to the points to see.

John

[Paul Marossy]

Try a large cap across the power rails, maybe 330uf and see if that helps.
You can just touch it to the points to see.

John

Worth a try.

[SonicVI]

I tried a 470uF which was the closeset I had to 330uF and nodice. So I tried the next biggest that I had in my stock which was 1500uF/25V and it did the trick. The problem is the thing is physically too big!  Gonna have to find a smaller one.

[John Lyons]

Interesting.
I guess the circuit for whatever reason just needs a lot more power filtering.
In most power supplies the filter is 1000uf so why not 2000+uf ?!
At any rate..it works...
You could try (2) 470ufs in parallel, maybe 940uf is enough?

john

[Paul Marossy]

That's cool that adding a big filter cap worked, but it doesn't make much sense that it would be required now when it wasn't required before... 

[SonicVI]

It makes no sense to me whatsoever.

[Paul Marossy]

It makes no sense to me whatsoever.

Yeah, very strange.