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wah problem

BrianGWN

Great 'Double White' North ~ Electronics Specialis
Joined
Jul 15, 2001
Messages
1,401
I have a friend with a VOX wah that is giving him problems, model V847. I know there are quite a few around here who are technically inclined, maybe someone might be up on this...
This wah has a tendancy to want to pick up AM radio broadcasts. Apparently the problem is most pronounced if the wah pedal is in the low-frequency emphasis position, fully down at the heel end. I suppose I would interpret this to maybe indicate some kind of ground integrity or shielding problem, but I haven't really ever dabbled with wahs so I don't know for sure.

I was going to try to do a little research, gave one site a quick look, but did not see any reference to this in their common problems list.
This model does not have a DC power connector (currently only internal 9V battery) so the owner was already interested in me doing some kind of surgery to add in a DC connector, such as the 2.1mm same as the Roland Boss stuff. At a quick glance inside things look to be in order and the unit does function, does do the wah thing. Any thoughts on what to look for regarding the cause of AM radio interference?? Thanks...
 

Toecutter

Member
Joined
Feb 21, 2003
Messages
520
BrianGWN,
Check to make sure you have a good solid ground connections on the input and output connectors. An easy thing to try, is tighten the nuts on both connectors. I have an old Cry Baby that had this same problem, tightening the nuts solved it.

Good luck.
 

BrianGWN

Great 'Double White' North ~ Electronics Specialis
Joined
Jul 15, 2001
Messages
1,401
Hmmm... He did mention something about one of the input or output jacks coming loose at one point and having to tighten them.
Now that you mention it I noticed there did not appear to be any explicit soldered connection from some portion of the pedal metal "shell" to some circuit ground point. If the physical contact between the 1/4 jacks and the "shell" is being relied on to establish the electrical connection between the shell and the circuit signal ground, then yeah I could see how a problem could develop.
When/if I get to modding it to have a DC power connector, I'll try some stuff to improve the whole ground integrity, scrape/clean around the 1/4 jacks. As well I think there was a bolt with nut through the "shell" up towards the toe-end of the pedal - If all else fails I could try making a connection from that bolt to some signal ground point.
 

YTLP

New member
Joined
Mar 25, 2003
Messages
652
I found this while looking for info on modding my Crybay Wah. Maybe it will help here...

"If you've converted to true bypass, replace the 68K series input resistor with a lower value, say 47K. This gives a slight increase in volume. Too low a value is likely to result in the pedal picking up radio interference. This can be addressed by adding small value (10 - 20pF) capacitors between the collector and base of each transistor."
 

John Catto

Active member
Joined
Jul 15, 2001
Messages
3,609
Check that all the input- output leads are shielded and grounded at ONE end. Ferrite beads on the leads would be a good idea as well.

Hi Brian !:)

Also what YTLP said seems sound to me and while you're at it check the cable he has going to the pedal. Just the cable would be enough to introduce the radio stuff.

There may be something buried in here as well, this is a GREAT Wah resource.

http://www.geofex.com/Article_Folders/wahpedl/wahped.htm
 
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BrianGWN

Great 'Double White' North ~ Electronics Specialis
Joined
Jul 15, 2001
Messages
1,401
Hi there Mr. The Diodes. (By the way, Texas did eventually get your tape, hand delivered back in March ;))

>>while you're at it check the cable he has going to the pedal<<
Ah yes, must not forget to check the simple stuff, must not forget the old school of thought about "trust no one" or "trust no other cable".

Hmmm, all good stuff to consider. I should try to track down a schematic for that V847 beast.
I'm heading out in a minute and don't know if I'll be back online this evening, so, if John or anyone else happens to know where one could go sniffing for Vox wah schematics, please do drop another note. Thanks all.
 

BrianGWN

Great 'Double White' North ~ Electronics Specialis
Joined
Jul 15, 2001
Messages
1,401
John Catto said:
Brian, this is the V847 schematic
http://www.voxamps.com/support/circuits/wahwah93.jpg.
Here's some mod stuff specific to the V847 http://www.green-fuz.freeserve.co.uk/wah.html
Thank you John, you are a scholar and a gentleman! :yay
By the way, the first URL you gave did not work. Eventually I noticed the problem, you accidentally included a trailing period character. I think you meant:
http://www.voxamps.com/support/circuits/wahwah93.jpg
...Okay so maybe the jury is still out on the scholar part, hee hee. ;)
 

ashbass

Active member
Joined
Jan 13, 2003
Messages
1,914
Here is a bit from Geoffrey Teese's troubleshooting page:

"Q: My wah picks up a local radio station.
A: Try adding a 10pF or 20pF capacitor between the Base and Collector of the transistor(s). (One cap for each transistor.) This could knock out enough of the ultra-high frequencies to solve the problem. If not, your wah might need specific EMI/RFI filtering."

http://www.realmccoycustom.com is his site.

Teese was (is?) an authorized VOX repairperson and has lots of great stuff to help you out on his site.

I did what he suggested (also on the troubleshooting page) to cancel hum when I added an ac adapter jack and the hum was gone. Great stuff.

ash
 
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