Guitar Wah Wah Pedal Help!

My VOX Wah Wah pedal (V847) is not working.

The bypass is working. However, when I turn ON the wah-wah effect - there is only a clean sound of the guitar and a very THIN sound of the wah-wah, it sounds more like a VOLUME pedal than it does a wah-wah pedal.

I believe this is the schematic:    http://www.voxamps.com/downloads/circuits/wahwah93.jpg

From what i've gathered, the problem lies in the capacitors (because apparently capacitors are what create the wah wah? i'm not entirely sure myself)

I've replaced the C2 (4.7MF) and C3 (.22MF)

What could be the problem? Please help!

are MF, mF, and uF all the same?


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Grizzly_Bear (author) 3 years ago

I replaced the Inductor. :)
gmoon3 years ago
You've got a Vox/Thomas/Dunlop wah circuit in that pedal--they're all basically the same. Only small differences differentiate each one. It was invented by a guy at the Thomas Organ company, and it's very common ('cause it's good).

In that schematic, MF and uF are indeed the same.

A great article my R.G. Keen explains the technology of a Thomas wah. It's a variable LC filter; one of the few modern audio circuits that actually uses a real inductor in the signal path. An LC circuit has musically cool attributes such as resonance and hysteresis, which aren't so easy to simulate without an inductor. And the article explains the variable part, although it's a little difficult to grasp--basically the circuit fools the inductor into "seeing" the second transistor, the POT and it's supporting components as a variable capacitor.

Anyways... The beginning steps of debugging should always be the same:

-- Check all wired connections. Have you already resoldered some? Look for web photos to be sure you've done that correctly.

-- Capacitors shouldn't need replacing unless:
They are faulty (leaking).
They are old.
You've applied too much voltage OR the power supply polarity was incorrect.

Of course if the wah sounds like a volume pedal then it's probably a good idea to replace C2, but you've done that already.

You might also suspect the inductor, if there's no Wah-ing. Unfortunately, that'll be the most expensive part to replace.

Is the PCB through-part or surface mount? Look for loose components, and be especially carefully you don't (or didn't) fry other parts while you're fitting replacements...

If all else fails, then join diystompboxes.com. There's some very savvy folks there that can help debug the problem.
Grizzly_Bear (author)  gmoon3 years ago
I just bought an 500mh inductor off eBay.

Just out of curiosity, what are some probable causes for the inductor to not work now?
At this point, it's not 100% sure that the inductor is bad. But it's certainly a suspect. I'd still replace any polarized electrolytic cap, if that hasn't been done yet.

AFA an inductor failing, it could be poorly soldered to the PCB, or the very fine wires internal to the inductor might have a manufacturing defect--something that could fail when the pedal got knocked around a bit. They have been know to fail, so it's possible.

If replacing the inductor doesn't solve the problem, just buy a new wah PCB and populate it. You've got all the expensive parts covered--pedal, POT, new inductor. The rest--caps and resistors--should be very inexpensive. Here's one by example; I'm sure you can find many more for sale...
caitlinsdad3 years ago
Capacitor C2 is noted as being an electrolytic type which means there are polarity markings and has to be wired up correctly for the + and - leads. The MF, mf, etc farad units have the metric prefixes and are different multipliers of the farad unit.
Grizzly_Bear (author)  caitlinsdad3 years ago
People have told me that MF mF are the same, and that uF and mF are the same...

Which is it? Where can I find some solid proof?
In your schematic, MF is uF.

Don't trust us? Go to the Schematic Heaven Effects page, check the Wah section. There are 5 versions of the V847 circuit (probably all with small component differences); most or all will use the microFarad (uF) designation. Compare the values with your schematic (or use one on SH).

Old capacitors are often (mis)labeled micro as M or m, rather than u. It was rather indiscriminate, but you see if often.