WEIRD scratch noise in Wah POT (changed allready 3 pots and parts from Wah)

richon · March 29, 2011, 04:22:19 PM

I got a Vox 847A (SMD) to mod and it got a WEIRD scratch noise when i turn the POT

so i changed allready 3 pots and at the part or turn of the pot (position of the pot) it make a scratching noise

So... i tested even alphas normal pedal pots and cheap ones and every one makes the same

so i put my tester to measure if there is DC voltage on it.. and it apears almoues 1mV DC ond both (one side and middle side) lugs of the pot

so then i changed the 0.22 (both) to some fine panasonic caps... no good

changed the Inductor
changed Trans
changed 220uF and 4,7uF electrolitic caps
tested with battery and 9V PSU...

so, i'm running out of posibilitys..

so to avoid building another PCB , do you got another idea?
and parts from Wah)

jasperoosthoek · March 29, 2011, 05:05:13 PM

What pots did you use? I've used a cheap but dedecated (with gear) wah pot that scratched right off the shelf. Now I've replaced it with a Dunlop Hotpotz (from Small Bear). Works absolutely fine, it also solved some sweep issues. Money well spent!

Skruffyhound · March 29, 2011, 05:44:48 PM

I don't have any answers, but I can confirm that I had a very similar problem with my Vox, and it's not the pot, I changed to a Hotpotz - I think that's what the Castledine site recommended too, but to no avail. It was my first modded pedal and I haven't looked at it since, so I can't help much. Sorry

richon · March 29, 2011, 06:28:30 PM

Quote from: jasperoosthoek on March 29, 2011, 05:05:13 PM
What pots did you use? I've used a cheap but dedecated (with gear) wah pot that scratched right off the shelf. Now I've replaced it with a Dunlop Hotpotz (from Small Bear). Works absolutely fine, it also solved some sweep issues. Money well spent!

the original Vox Wah pot , the BLACK BEAR (from smallbear) and a 100K-L Alpha and 100K-A Alpha pot were tested with scratch sound

what could it bee?

alanlan · March 29, 2011, 07:00:59 PM

pot scratching usually points to DC current passing through wiper (if not a worn out pot)

R.G. · March 29, 2011, 08:12:15 PM

I don't know exactly what's wrong, but it occurred to me that if the ground end of the pot were intermittent, it would let the caps into the pot and to the follower section float up like input caps on a pedal with true bypass but no pulldowns.

Maybe.

It's a theory, anyway. Not sure what that would do to the wah-ing. Probably nothing good.

joegagan · March 29, 2011, 09:54:23 PM

great. i already didn't like this new generation of wahs. now we see a pattern of scratchy sounds, this is the second or third time this issue has come up ( pot didn't fix it on the previous either) on these smd wahs. i hope dunlop figures it out.

John Lyons · March 29, 2011, 10:45:06 PM

I had a similar problem. I found that if I took a cap about 200pf and put it to ground off the output
it went away. Oscillation? I don't know. It drive me mad but in the end it worked. Non SMD traditional wah circuit
with typical values.

All the ins and outs were short and separated as well.

Skruffyhound · March 30, 2011, 08:28:42 AM

200pf and put it to ground off the output

Thanks for the tip, I'll definately give that a try..... if I ever get back to that project

Paul Marossy · March 30, 2011, 09:43:40 AM

Quote from: alanlan on March 29, 2011, 07:00:59 PM
pot scratching usually points to DC current passing through wiper (if not a worn out pot)

Yeah, most of the time that seems to be the case. IIRC, in a wah pedal you can also connect the pot into the circuit wrong and get scratching. That's what I seem to remember being told once anyway.

Quote from: John Lyons on March 29, 2011, 10:45:06 PM
I had a similar problem. I found that if I took a cap about 200pf and put it to ground off the output
it went away. Oscillation? I don't know. It drive me mad but in the end it worked. Non SMD traditional wah circuit
with typical values. All the ins and outs were short and separated as well.

I think I remember when you were having this problem. Or maybe it was someone else with the same problem. It's a curious thing, don't know why or how that can happen. Maybe it is an oscillation thing happening somewhere in the circuit. Power supply maybe?

richon · March 30, 2011, 10:23:08 AM

well the 220pF cap didn't help but nice to know how to tame some highs in a wah

sure will build another PCB to fix it completly

Electron Tornado · March 31, 2011, 07:18:52 PM

Does the pot scratch throughout its range of travel, or only in a certain spot?

If it's only in one spot, move the pot slowly to find where it gets noisey. I have a similar problem in a Dimebag wah. It came with the problem stated as "scratchy pot", but it's not the pot. When one of the controls on the side (don't recall which one off hand) is in a certain position, the pot will sound scratchy as it moves over a certain spot. What's going on is that at those settings, a usually quiet circuit starts creating a lot of white noise. Sweep the pot by that spot quickly and it sounds like scratching.

I narrowed the source of the noise down some, but still scratching my head (so to speak)

zombiwoof · March 31, 2011, 11:02:59 PM

Quote from: joegagan on March 29, 2011, 09:54:23 PM
great. i already didn't like this new generation of wahs. now we see a pattern of scratchy sounds, this is the second or third time this issue has come up ( pot didn't fix it on the previous either) on these smd wahs. i hope dunlop figures it out.

The new Vox V847A's are made in China, not by Dunlop. That's what the OP has, although some others are reporting the problem with Dunlop SMD wahs. I stick with the vintage-type wahs, I just bought a Sepulveda CA Thomas Organ Crybaby, this thing is like 40 years old, all original and the pot isn't scratchy and it sounds great. I also have an earlier Dunlop-made V847(modded) and a BBE Wah, and no such problems with any of them. They can keep their modern SMD construction wahs as far as I'm concerned.

Al

richon · April 01, 2011, 08:40:32 AM

well,

just finished a completed PCB of a VOX WAH (NOT PCB STYLE) parts...

tested, and finally no scratch at all..... I EVEN USED THE SAME PARTS , ALL THE SAME PARTS as the scratching PCB

how come, does a PCB generate the scratch.... weird magic!!!

see ya.

R.G. · April 01, 2011, 10:36:37 AM

Quote from: richon on April 01, 2011, 08:40:32 AM
just finished a completed PCB of a VOX WAH (NOT PCB STYLE) parts...
tested, and finally no scratch at all..... I EVEN USED THE SAME PARTS , ALL THE SAME PARTS as the scratching PCB
how come, does a PCB generate the scratch.... weird magic!!!

Magic does exist - but only inside the human mind. Outside the human mind, in the real world, there is only the laws of physics, as nearly as anyone has been able to determine. So there is a reason. We just don't know what this reason is yet. Enough work with measuring devices would eventually turn up the reason.

richon · April 01, 2011, 02:43:41 PM

i liked the MAGIC world version better....

Paul Marossy · April 01, 2011, 04:01:29 PM

Quote from: richon on April 01, 2011, 02:43:41 PM
i liked the MAGIC world version better....

twabelljr · April 01, 2011, 06:45:35 PM

Quotei liked the MAGIC world version better....

I wish there really was one. Then I would actually be able to play my guitar and sound good! Good job on fixing your pedal!!

richon · April 02, 2011, 09:32:52 PM

3 years ago I found THAT sound...

guess what, just a Telecaster a cable and a Fender Deluxe Tweed Clone...

sometimes a Overdrive (based on a lot of ideas from this forum) and thats it...

althoug I keep looking at sounding more and more complex!!!! but the TELE keep sounding awesome

Phorhas · April 03, 2011, 04:49:03 AM

I have encountered a similar problem once and the source for the weird noise were actually microphonic from a squeaky rocker

News:

WEIRD scratch noise in Wah POT (changed allready 3 pots and parts from Wah)