FDP Forum / Help me understand ground noise/ 11 messages in thread.

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Paul L



New Jersey, USA

It looks just like a Telefunken U-47!
May 10th, 2017 10:48 PM        

So, I bought a prewired kit for my Tele, including a 4-way switch. At the same time, I swapped out the stock pickups for a set of Vintage Noiseless. Everything appears to be wired correctly and the switch functions properly, but there is a loud hum in every position except for the neck pup by itself. When the bridge pickup is engaged (by itself or with the neck) I hear the loud hum, but it goes away when I touch the control plate. Any ideas or suggestions?



Mick Reid

Contributing Member
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Australia

American-made in Oz!!
May 11th, 2017 12:23 AM        

Do you have a wire connecting the bridge (underneath) going the rest of the grounds in the control cavity? (back of pot)<br /> <br /> Also, is the nickel cover on the neck pickup grounded separately?<br /> For a 4-way switch, there should be 3 wires coming from the neck pickup: 1 hot (coil) 1 ground (coil) and 1 ground (cover).<br /> <br /> [edited to add link]<br />



Paul L



New Jersey, USA

It looks just like a Telefunken U-47!
May 11th, 2017 10:31 AM        

Thanks! Do I need to scrape away the chrome plating under the bridge where the ground wire is to be connected?



wrnchbndr

Contributing Member
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*

New Jersey

I'm back with the otters again
May 11th, 2017 11:07 AM        

No. You just need to have a contact that is going to be dependable. I run a bare wire under the bridge and insert about 1/2" into one of the wood screw holes that keep the bridge attached to the body.<br /> <br /> Be sure that you don't have the output jack wired backwards. We've all done that.



Peegoo

Contributing Member
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******

I've got my own

double-cross to bear
May 11th, 2017 02:30 PM        

Also--you need to lift (disconnect) the ground side of the coil from the cover of the neck pickup.<br /> <br /> This isolates the cover from the coil.



Paul L



New Jersey, USA

It looks just like a Telefunken U-47!
May 12th, 2017 12:00 AM        

So I lifted the ground for the cover and soldered it to the back of the volume pot. I also ran a ground wire from under the bridge to the pot. There is still a slight hum that goes away when I touch either the bridge, control plate, or the strings. But it's 1000X better than it was! <br /> <br /> Thank you guys so much! I really appreciate your help.



Mick Reid

Contributing Member
***

Australia

American-made in Oz!!
May 12th, 2017 05:12 AM        

Great news.<br /> <br /> Are the control & pickup cavities shielded? (with shielding paint or copper foil)<br /> If so, is there a ground wire connected to the shielded cavity? (usually with a solder lug screwed into the body) See linked pic below.<br /> That may or may not fix it, but I always ground to the shielding.



Paul L



New Jersey, USA

It looks just like a Telefunken U-47!
May 12th, 2017 10:48 AM        

No, they're not. Easy enough to fix, though. Thanks again!



Peegoo

Contributing Member
**********
**********
******

I've got my own

double-cross to bear
May 12th, 2017 03:16 PM        

Touching the strings and the guitar getting quiet is how it's supposed to work.<br /> <br /> There will always be some noise. The string ground uses the capacitance of the human body as a noise filter. So things are working as they should.<br /> <br /> Shielding the cavities will help a bit too.<br /> <br /> IMPORTANT: shielding the control cavity will make some positive improvement. But unless you shield the drilled bore that holds the output jack, you're not completely shielded. If you cannot shield the 7/8" diameter bore, a simple solution is to use a length of coaxial wire between the controls and the jack. <br /> <br /> If you don't have coaxial wire, twist the two conductors into a tight helix and apply heat shrink to keep them twisted tightly. A twisted pair is pretty remarkable in that it helps to reject induced noise in a signal line.



Paul L



New Jersey, USA

It looks just like a Telefunken U-47!
May 12th, 2017 07:10 PM        

Thanks, Peegoo. I'm pretty sure that's how Mojotone wired the harness (with the two output leads twisted together and shrink wrapped). I probably will shield the cavities at some point, but I think I can live with it for now. <br /> <br /> Thanks again to all of you for helping me to get this right.



amphead4



Cincinnati, USA

May 16th, 2017 07:10 AM        

"A twisted pair is pretty remarkable in that it helps to reject induced noise in a signal line."<br /> <br /> Peegoo, maybe I need to research this. Even though I twist them too, I seem to recall that with a twisted pair, the rejection works when it's a balanced line and neither conductor is grounded. But it's been a long time since I studied that.



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