Help with troubleshooting a DIY Wah pedal

[squeakywhite]

Hi everyone,

I am having trouble getting my diy Wah pedal working. It's my first pedal build and I have been trying to solve the problem on and off for the last few months.

Symptoms: When the box is on with the pedal down, I get a quiet guitar signal coming through to my amp. Lifting the pedal up past about 70% travel gives a harsh squealing noise.

With the box switched off, I get clear, loud guitar signal but still get a quiet squeal with the pedal up.

As far as I can tell, there are no signs of any Wah effect.

Here is a quick video showing the squeal: https://www.youtube.com/watch?v=kW2b3SqmAGU

I have checked all components and wiring, and also simplified the wiring by removing the 3PDT switch and wiring the circuit directly.

The circuit I am building is the Basic Wah Circuit from Geofex:
http://www.geofex.com/article_folders/wahpedl/wah1.gif

Full circuit details here:
http://www.geofex.com/article_folders/wahpedl/wahped.htm

I assembled the circuit on perfboard (probably should have breadboarded first): 
https://www.dropbox.com/s/xulhta1aslpkiwq/Overview.jpg
https://www.dropbox.com/s/459if3c02eo3c2x/Circuit%20Top.PNG
https://www.dropbox.com/s/i6u1ypxoofqqzz9/Circuit%20Bottom.PNG (The soldering is a bit rough but it looks as though I have continuity in the circuit)

I am using 2N3904 transistors for both Q1 and Q2. I have made no modifications to other resistors, capacitors, etc.

I have checked voltages at each terminal of each component:
https://www.dropbox.com/s/dxhzpe7i8q3u8i3/Circuit%20Voltages.PNG

With an audio probe, I get a squeal right at the input of the circuit.

Please let me know if I have missed anything or if you require further information.

Any help is much appreciated!
Sam

[Transmogrifox]

Basically your pedal is unstable.  This happens from too much gain and/or phase in the wrong place.

Your wiring and resistor colors look correct.  Your voltages measure correctly except for the base at Q2.  There are only two things I can call into question:
1)  Make sure Q2 is properly oriented.  From your picture it looks correct, but doesn't hurt to check.  I'm guessing the odd voltage on the base of this tranny is just because it's unstable and your meter picks it up at whatever it averages to.  You should see something that is ~0.6 or 0.7 volts greater than what you measure at the emitter.  If the circuit is unstable, then this measurement won't tell you anything more than the fact that something is wrong (which is obvious), but won't tell you exactly what is wrong.
2)  Your inductor.
3)  Power supply -- Maybe you should add 100uf across the battery terminal on the board.  Small chance there is some cross-coupling through the battery internal resistance.  I have never tried to build this classic Vox/Crybaby circuit without power supply filtering, so I am uncertain whether this might contribute to stability problems.
4) Change the tantalum capacitors to poly film.  They may have some parasitic impedance effects that this circuit doesn't like.  Maybe even accidentally got wrong cap value?  These caps are the main contributors to the circuit's resonance "Q".  If you did, say 0.022uF instead of 0.22uF, that might make it squeal.  Double check these.

One comment about the inductor -- the mounting is aiming EMI like a ray gun right at your transistors.  If you can stand it up on end like a water tank that may help.  Obviously try the easiest stuff first, like wrap it up in foil and take a wire from the foil to ground.  That would be a good way to troubleshoot this hypothesis.

The other thing is maybe you just got a really high-Q inductor, and you may need to add some series resistance to it.  Try adding 47 ohms in series with the inductor to see if that tames things down a little bit.

Output loading can also have an effect on stability.  You might try adding 100k in series with the output (temporarily just as a troubleshooting measure).  That will just give some clues about what is making it go unstable.

The 3904's should be fine.  That's what the Morley BH uses.

[squeakywhite]

Thank you very much Transmogrifox!

You were spot on about the capacitors. It turns out I can't follow simple instructions and had used 0.01uF instead of 0.22uF. I will keep your other advice in mind and be sure to triple check my next stomp box before complaining that it isn't working.

Cheers,
Sam

[Transmogrifox]

It's good to hear it was simple.  I hope it's everything you hoped for   Hard to go wrong with that classic circuit.