I've been having trouble with my wah pedal for a while now. It sounds good when I shift it up and down but when it passes through the mid-line I hear feedback come out. Any clue as to how to fix this? (Its a Vox V847)
I would google it. I am only familiar with two repairs on a wah....the potentiometer and the inductor. Neither are very hard to replace, I have never done the latter.

I have an old Thomas Organ crybaby and it does not always react well with other pedals. You might want to try plugging it into the amplifier without any other pedals in your chain and see what it does.

Other than that, you could google that pedal and see if anyone else has had that problem. You could email Vox tech support and ask what the deal is.
Sounds like the potentiometer is getting worn in the mid range area, this is common with well used older wah pedals, and is in fact how Dunlop recreated the EVH wah pedal, but of course not everyone likes that. If you are DIY inclined and have a soldering iron you can pickup a new part and it is a relatively cheap and easy fix, could take it in to get fixed but I personally have no idea if that would be cheaper opposed to buying a new one.

Edit: Could also be some dirt build up, I would first get a can of air and spray the crap out of it and see if it is still buggy.
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