Hum and Static in a Vox Wah

YouAre · August 05, 2006, 07:02:41 PM

I have a vox v847 that i'm repairing. Its Mostly stock (the mids and Q resistors were modded within useable ranges). When i have a battery plugged in, i get static when the pot is around the higher end of the range. When i use my godlyke adapter with the 9v clip attachment, i get SERIOUS hum and static, that totally drowns out the guitar signal at all. It gets louder when i turn the wah pot up like when there is a battery. So i'm guessing i have a power/filtering issue.

What do you guys think could be an issue? I think i'll replace the electrolytic cap first to see if that helps. Any other ideas?

thanks,
Murad

jonathan perez · August 05, 2006, 07:25:37 PM

move the wah away from the power strip/wall.

then see/hear what happens, and if it helps.

Paul Marossy · August 05, 2006, 07:33:11 PM

Wah pedals are very sensitive to EMI from wall warts, fan motors and that sort of thing. Make sure your power supply is at least 2 feet away from your wah pedal to minimize the hum.

YouAre · August 06, 2006, 02:29:06 AM

what about the battery issue, when the wah pot is in the higher range of the throw, it gets that static. And is there any type of filtering i can add in for when i add on a power jack?

jonathan perez · August 06, 2006, 02:54:07 AM

same deal, if its too close to either a power strip or wall outlet, itll do the same thing.

also, if youre using like 3 distortion pedals at once, youll get noise hahah

is the pot scratchy? or is it actual emi noise?

YouAre · August 06, 2006, 11:11:20 AM

its the same noise with any pot. is there any way to shield against the noise?

petemoore · August 06, 2006, 11:35:26 AM

New battery.
It may not be shield or Power supply related.
I want to remove scratchy pot from the discussion, when you say 'static' is this static present with the wah mostly toe down and the wah pot is held in 1 position?..
I Ask and tell about scratchy pot which makes 'scritchy' noises when it is ranged past a certain spot...it's wafer may have an 'open' or some strange ramp to it's variable resistance.
If not a bad pot, or power supply wierdness [good new battery will eliminate that as suspect, not a freshly charged Older Rechargable]...
It not a 'wierd problem', it just seems 'wierd'...little statics are 'wierd' to find and discuss...it could be some wire problem [like broken strands that move/conduct/resist], a pot lug loose, basically anything 'wierd' [as in taking beyond taking voltages of actives to determine if they're within operational paramaters, and audio probing to find the area causing the undesirable behaviour...it can be a 'process' which...isn't finished till it's found.
Could be something as simple as a piece of conductive crap went through the rack hole, into a pot and is moving around when the wiper does, or crossing traces under teh board...

YouAre · August 06, 2006, 11:43:21 AM

i'm fairly certain its not a pot problem. I am using a linear 100k pot, not a wah pot, becuase i am debugging the circuit outside of the wah shell.

Could caps be an issue at all?

Paul Marossy · August 06, 2006, 03:51:55 PM

Quotewhen the wah pot is in the higher range of the throw, it gets that static.

Try it with the circuit in the wah shell. You might be picking up noise because it's not in the shell.

QuoteAnd is there any type of filtering i can add in for when i add on a power jack?

A 100uF or 220uF cap across the power supply ought to work. Check out how they do it on the GCB-95 schematic. It's basically a cap and a diode combo.

petemoore · August 06, 2006, 04:00:07 PM

Could caps be an issue at all?
I've not had a staticky non electrolytic, and you said you changed that one? But...I wouldn't look there first [more like near last], or unequivically carve in stone that caps could be ruled out.

jonathan perez · August 06, 2006, 04:49:32 PM

PUT IT IN A SHELL!

that should be your problem...unless of course it SCRATCHING, not HUM...

Paul Marossy · August 06, 2006, 08:14:47 PM

QuoteI've not had a staticky non electrolytic

Yeah. I really doubt that it's a cap.

YouAre · August 08, 2006, 10:23:53 AM

My thanks to you all.

I replaced the pot with a bourns pot.

i took out the original wiring harness, and rewired the entire thing inside the wah shell.

There's no noise anymore! You guys rock

Now my next question......power supply filtering! what do you guys use? I've heard of 470uf cap in parallel with a 220 ohm resistor from the power jack. I
ve also seen the crybaby schem with a 220uf and a .01uf cap in parallel into a resistor whos value i've forgotten (i'm probably off on values for this one). What do you guys use?

Paul Marossy · August 08, 2006, 10:31:56 AM

Great! Glad to hear that it's fixed now.

On the power filtering, the 220uF cap scheme should work fine. Using a 470uF cap wouldn't hurt anything, it's just that it's probably twice what you really need.

uan · August 09, 2006, 11:45:51 AM

i used to have the same problem with my vox v847 which i added adaptor jack into it..
try to use regulated power supply..that work for me..

David · August 09, 2006, 11:56:16 AM

Quote from: Paul Marossy on August 08, 2006, 10:31:56 AM
Great! Glad to hear that it's fixed now.

On the power filtering, the 220uF cap scheme should work fine. Using a 470uF cap wouldn't hurt anything, it's just that it's probably twice what you really need.

I've been using a 100R resistor connected to V+, followed by a 1000uF capacitor to ground. Blissful silence. Overkill be hanged!

Paul Marossy · August 09, 2006, 02:50:49 PM

QuoteOverkill be hanged!

In stompboxes, overkill on power filter caps is OK. Tube amps is another story...

David · August 09, 2006, 03:38:34 PM

Uhhh, yeah. Point taken, sir.

However, I haven't built any tube amps and it seems unlikely that I will. Nevertheless, I've learned to never say never (again)!

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Hum and Static in a Vox Wah