Wah Pot - Why would you put a small capacitor across lugs 1 & 3?

[Flying]

I have just got a second hand Joe Bonamassa wah, and noticed a small capacitor across lugs 1 and 3 of the wah pot. I’ve never seen this before, either in images or schematic for this wah or any other wah, and it looks like a modification, not from the factory. The sweep had some crackle so I thought the capacitor was to try and reduce the level of the crackle or tame the high end of the sweep.

I disconnected one leg of the capacitor and in the heal down position there is a noticeable ‘mechanical’ sound, like a switch or a clunk (It sounds mechanical, but isn’t the mechanical sound of the pedal, it’s very loud in the audio signal). The pot hadn’t reached the end of its travel, and if the rack gear is disconnected and the pot rotated by hand I can sweep beyond this area.

So the capacitor is masking this issue, but I don’t see how, could anyone shed any light on this for me?

 

[Robert]

Is the wiper open by any chance when rotated to the bad spot?

 

[Flying]

Is the wiper open by any chance when rotated to the bad spot?

I don’t think my multimeter is refreshing fast enough, I have occasionally got an open circuit when I rotate the pot, but not repeatably. And the area in question I’m guessing is quite narrow as I can hear it as I go through it, but I can’t land on it!


But if there is an open circuit how is a capacitor bridging pins 1 and 3 going to mask that?

 

[Feral Feline]

What is a signal but energy?
What is a capacitor but a receptacle for storing energy?

I suppose the cap releases that bit of signal-energy while the pot is rotated through that narrow little "signal blind-spot".
Just a wild stab at how the cap might be masking the problem. I don't know.

 

[darwin999]

The cap is a high pass filter. at frequencies above fo = 1/(2*pi*R*C), the high frequencies predominately run thru the cap (C) over the pot (R). It not only tries to hide the pot's motional noise, but when the pot is fixed it also alters the sonics of the pedal - as frequency goes above fo, they increasingly don't see the effect of that pot.

 

[andrewsrea]

^ This. From the look of it, it appears to be a small value in the picofarads. From what we know of Bonamassa (he dislikes brittle trebles and prefers thick, dark tones), that is a 100pF ish cap to take out the RF (Radio Frequencies) and shrill highs.

 

[Flying]

The capacitor is 470pF between lugs 1 and 3 of the pot. If it follows the quintessential wah circuit, one lug goes to ground and the other is connected to the active filter and the output.

I’m trying, but failing, to get in contact with the seller, although it's not been 24 hours yet. I have a Budda Wah, but this Joe Bonamassa one is quite different, and in many ways I prefer it tonally and the sweep, but I can’t help feel that things aren’t right. So I’m torn between trying to return it, and taking it on as a project as I'm intrigued and want to work out what’s going on. I know there are SMD on the other side of the board, and wondering if there is a missing capacitor on there and they ‘fixed’ it at the factory. But I don’t want to mess about with it if I’m to return it.