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FDP Forum / Guitar Mods, Repairs, and Projects / Help me understand ground noise

Paul L

New Jersey, USA

It looks just like a Telefunken U-47!
May 10th, 2017 10:48 PM   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

Do you have a wire connecting the bridge (underneath) going the rest of the grounds in the control cavity? (back of pot)

Also, is the nickel cover on the neck pickup grounded separately?
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).

[edited to add link]

see first article

(This message was last edited by Mick Reid at 02:47 AM, May 11th, 2017)

Paul L

New Jersey, USA

It looks just like a Telefunken U-47!
May 11th, 2017 10:31 AM   Edit   Profile   Print Topic   Search Topic

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

(This message was last edited by Paul L at 12:36 PM, May 11th, 2017)

wrnchbndr
Contributing Member
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New Jersey

I'm back with the otters again
May 11th, 2017 11:07 AM   Edit   Profile   Print Topic   Search Topic

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.

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   Edit   Profile   Print Topic   Search Topic

Also--you need to lift (disconnect) the ground side of the coil from the cover of the neck pickup.

This isolates the cover from the coil.

Looky here

Paul L

New Jersey, USA

It looks just like a Telefunken U-47!
May 12th, 2017 12:00 AM   Edit   Profile   Print Topic   Search Topic

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!

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   Edit   Profile   Print Topic   Search Topic

Great news.

Are the control & pickup cavities shielded? (with shielding paint or copper foil)
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.
That may or may not fix it, but I always ground to the shielding.

not my pic, best one I could find online

Paul L

New Jersey, USA

It looks just like a Telefunken U-47!
May 12th, 2017 10:48 AM   Edit   Profile   Print Topic   Search Topic

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   Edit   Profile   Print Topic   Search Topic

Touching the strings and the guitar getting quiet is how it's supposed to work.

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.

Shielding the cavities will help a bit too.

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.

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   Edit   Profile   Print Topic   Search Topic

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.

Thanks again to all of you for helping me to get this right.

amphead4

Cincinnati, USA

May 16th, 2017 07:10 AM   Edit   Profile   Print Topic   Search Topic

"A twisted pair is pretty remarkable in that it helps to reject induced noise in a signal line."

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.

FDP Forum / Guitar Mods, Repairs, and Projects / Help me understand ground noise




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