Shielding Question for Fender Mustang - Redundant Grounding?

[nodoz]

Hey everybody, first time poster / first time shielder here! Looking for a bit of advice on a problem that I'm having, hoping this might be the right place to get some help.

Earlier this week I did a complete rewire of my 2016 Fender Offset Series Mustang using higher quality components. The pickups have had unpleasant hum at times, so I decided to try shielding it before installing the upgraded electronics. I did a few days of reading on the subject before giving it a go and believe I shielded it correctly, but ever since I finished the rewire, the hum seems to have gotten even worse, changing in intensity depending on where I'm standing in the room or what direction I'm facing. I'm wondering if the issue might lie in some redundant grounding, producing one of those *ground loops* I've seen so many people arguing about.

I built the new wiring harness as a wire-for-wire replica of the factory one, and I'm thinking that perhaps my "faraday cage" might have rendered one / a few of those original ground wires unnecessary, causing said hum. My schematic is as follows:

- The shielding: Pickguard shielded with StewMac pickguard shielding foil. Body cavities shielded with StewMac Copper Tape (which runs up onto the face of the guitar body, where it makes contact with the pickguard foil and completes the "cage"

- Two single coil pickups running to a Switchcraft three-way toggle switch. Both pickups have two-conductor leads. I soldered their coaxial center hot wires to the appropriate toggle switch terminals and their outer braided shields to the ground terminal on the toggle switch.

- There are just two pots this guitar, one for volume and one for tone. Here is where I suspect the issue might lie:

1) A ground wire running from the toggle switch ground terminal to the back of my volume pot
2) A ground wire running from the back of my volume pot to the bridge
3) A ground wire running from the back of my volume pot to a washer that is screwed down into the control cavity, where it makes contact with the shielding tape
4) A ground wire running from the back of my volume pot to the back of my tone pot
5) A ground wire from the back of my tone pot to the ground terminal on my output jack

I'll include a photo of my completed wiring down below as well. I'm hoping to become a convert to the world of shielding, if anyone has any advice or tips they could offer I would really really appreciate it. Thanks!

[nodoz]

Here's a photo of the wiring. This was also before I installed the tone cap onto the tone pot

[newey]

nodoz-

Hello and Welcome to G-Nutz2!

Understand, first of all, that these types of issues can be devilishly difficult to troubleshoot over the internet, since we can't have "hands on" access to your guitar.

But I think you are right to suspect a grounding issue. There are two basic ways of grounding, "star grounding" and daisy-chaining the backs of the pots together, and then to the output jack. Either way will work, but you are employing a mix of the two.

The issue is probably not a "ground loop" or redundancy in the grounding, it's probably a less-than-perfect ground connection somewhere in the circuitous path you have set up.

Since this guitar requires removal of the pickguard to access the innards, you will want to do as much testing as possible before reassembly, so as to (hopefully) avoid taking it on-off numerous times. If you don't have a multimeter, best to get one. Use it to check between the sleeve connection of the jack and the back of each pot, as well as the frame ground of the toggle switch. You should show basically 0Ω between all grounded points. Then check that your shielding and bridge ground are similarly contiguous to the jack sleeve.

Also double check that you do not have any stray strands of wire between the + connection at the V pot and anything else, as well as between the braided shield from the pickups and the toggle switch lugs.

If you still have the guitar assembled, one quick check to do is to press down firmly on the edge of the pickguard at various places, see if the noise is affected at all.

(We are also assuming here that the surrounding electrical environment hasn't changed since you did the shielding- same room, same cord, same amp, etc.)

The last-resort "down and dirty" fix is to hit all your solder joints with the iron again, but I would check things with the meter first, since resoldering won't tell you anything until you put it all back together again.

These are my only suggestions at the moment, again, it's difficult over the 'net. You may want to let others weigh in before you start pulling things apart.

With 2-conductor pickups (i.e., without a separate shield wire), the braided shield is performing double duty, carrying the signal "negative" connection and also acting (theoretically) as a shield against noise. If all else fails, you might try taking that connection straight to the star point, and then from the star point to the jack sleeve lug. While what you have done should work OK (provided all the grounds are good), the fact that the pickups' signal neg. is all the way at the end of the chain of grounds concerns me.

[sumgai]

nodoz,



Hi, and to The NutzHouse!


One thing that strikes me is that you said "Stew-Mac" shielding, then you show a picture of an aluminum shielded pickguard (and control plate). If what you said about the caavity being shielded with copper tape is true, then you're headed for future problems.

Even without any kind of electrical current being passed through the materials, two dissimilar metals never play nice together. In this case, both materials are highly conductive elecrically speaking, so they tend to be "submissive" to currents. What that means is the eventually, even a minute amount of current (coming from a pickup or two) can cause electrolysis, which in turn degrades one of the materials. Whether it takes 10 minutes or 10 years to start giving you grief, that's a crap shoot. But before I went any further, the first thing I'd do is pick one material, and make the both cavity and pickguard are covered with the same stuff.

Oh, and a "simple" screw into the sheilding isn't good enough in my book. Call me antediluvian, but I prefer to think that there's a good reason they invented solder, and soldering irons.

For the rest of it, newey covered you well - ground loops are a description of what happens well outside of the guitar, never within it.

Last thing I'll mention is that your wires themselves are more than large enough in diameter (wire size guage). In the future, you can safely use 20 or even 22 guage wiring, no problem. That makes running and solder wires much easier, trust me on that one.

HTH

sumgai

[nodoz]

Jan 31, 2020 7:21:26 GMT -5 newey said:

nodoz-

Hello and Welcome to G-Nutz2!

Understand, first of all, that these types of issues can be devilishly difficult to troubleshoot over the internet, since we can't have "hands on" access to your guitar.

But I think you are right to suspect a grounding issue. There are two basic ways of grounding, "star grounding" and daisy-chaining the backs of the pots together, and then to the output jack. Either way will work, but you are employing a mix of the two.

The issue is probably not a "ground loop" or redundancy in the grounding, it's probably a less-than-perfect ground connection somewhere in the circuitous path you have set up.

Since this guitar requires removal of the pickguard to access the innards, you will want to do as much testing as possible before reassembly, so as to (hopefully) avoid taking it on-off numerous times. If you don't have a multimeter, best to get one. Use it to check between the sleeve connection of the jack and the back of each pot, as well as the frame ground of the toggle switch. You should show basically 0Ω between all grounded points. Then check that your shielding and bridge ground are similarly contiguous to the jack sleeve.

Also double check that you do not have any stray strands of wire between the + connection at the V pot and anything else, as well as between the braided shield from the pickups and the toggle switch lugs.

If you still have the guitar assembled, one quick check to do is to press down firmly on the edge of the pickguard at various places, see if the noise is affected at all.

(We are also assuming here that the surrounding electrical environment hasn't changed since you did the shielding- same room, same cord, same amp, etc.)

The last-resort "down and dirty" fix is to hit all your solder joints with the iron again, but I would check things with the meter first, since resoldering won't tell you anything until you put it all back together again.

These are my only suggestions at the moment, again, it's difficult over the 'net. You may want to let others weigh in before you start pulling things apart.

With 2-conductor pickups (i.e., without a separate shield wire), the braided shield is performing double duty, carrying the signal "negative" connection and also acting (theoretically) as a shield against noise. If all else fails, you might try taking that connection straight to the star point, and then from the star point to the jack sleeve lug. While what you have done should work OK (provided all the grounds are good), the fact that the pickups' signal neg. is all the way at the end of the chain of grounds concerns me.

Thanks for the thorough response newey, I appreciate your help!

- The guitar is currently assembled + set up so I'll try the trick of pressing down on the pickguard first. And you're correct, same exact electrical environment, no variables. 

- I have a multimeter that I used to test the conductivity across all of my shielding, but I didn't use it to check between the various ground points. I'll start there when I remove the pickguard and see if anything is giving me a suspiciously high reading.

- I'm relatively confident in my soldering ability, have years of experience and good variable temp station, but a re-solder is definitely a good idea if nothing else yields a clear suspect. 

There are two basic ways of grounding, "star grounding" and daisy-chaining the backs of the pots together, and then to the output jack. Either way will work, but you are employing a mix of the two.

Does the metal control cavity cover and the contact it makes with the copper shielded cavity render the ground wire daisy-chain between the two pots unnecessary?

With 2-conductor pickups (i.e., without a separate shield wire), the braided shield is performing double duty, carrying the signal "negative" connection and also acting (theoretically) as a shield against noise. If all else fails, you might try taking that connection straight to the star point, and then from the star point to the jack sleeve lug. While what you have done should work OK (provided all the grounds are good), the fact that the pickups' signal neg. is all the way at the end of the chain of grounds concerns me.

So is the basic concept with star grounding to send all grounds to a single fixed point within the guitar and then wire that fixed point directly to the output jack ground lug? Sorry if the answer is obvious, it's my first season in the big leagues haha. About to read that "quieting the beast" article btw!

[nodoz]

Jan 31, 2020 12:42:01 GMT -5 sumgai said:

nodoz,



Hi, and to The NutzHouse!


One thing that strikes me is that you said "Stew-Mac" shielding, then you show a picture of an aluminum shielded pickguard (and control plate). If what you said about the caavity being shielded with copper tape is true, then you're headed for future problems.

Even without any kind of electrical current being passed through the materials, two dissimilar metals never play nice together. In this case, both materials are highly conductive elecrically speaking, so they tend to be "submissive" to currents. What that means is the eventually, even a minute amount of current (coming from a pickup or two) can cause electrolysis, which in turn degrades one of the materials. Whether it takes 10 minutes or 10 years to start giving you grief, that's a crap shoot. But before I went any further, the first thing I'd do is pick one material, and make the both cavity and pickguard are covered with the same stuff.

Oh, and a "simple" screw into the sheilding isn't good enough in my book. Call me antediluvian, but I prefer to think that there's a good reason they invented solder, and soldering irons.

For the rest of it, newey covered you well - ground loops are a description of what happens well outside of the guitar, never within it.

Last thing I'll mention is that your wires themselves are more than large enough in diameter (wire size guage). In the future, you can safely use 20 or even 22 guage wiring, no problem. That makes running and solder wires much easier, trust me on that one.

HTH

sumgai

Thanks for taking the time to respond Sumgai, means a lot!

- Very interesting point in regards to the different shielding metals! I used this aluminum shielding foil on the pickguard just b/c it was more cost effective than using the conductive copper tape for the entire project, I hadn't considered that potential issues could arise due to the dissimilar metals. If I were to re-shield, would copper tape in place of the aluminum on the back of the pickguard suffice, or would it potentially cause an issue due to the metal control cavity cover? 

Here's the foil I was using by the way: www.stewmac.com/Pickups_and_Electronics/Shielding/Pickguard_Shielding_Foil.html

- Duly noted on the screw point. Would you suggest adding some solder over that washer to create a stronger bond between it and the cavity shielding?

- The wires I used are actually 22 AWG wires! They're just inside that fat vintage-style cloth pushback sleeve, hence the larger look. It's just what I had lying around the house. They solder like an absolute dream, but you're correct, I have had trouble running them once or twice in guitars that are a little less generous in regards to routed space. 

[newey]

Does the metal control cavity cover and the contact it makes with the copper shielded cavity render the ground wire daisy-chain between the two pots unnecessary?

Not owning a Mustang (I do have a 2016 Duo-Sonic reissue), I had forgotten about the metal control plate on those guitars.

The basic rule is that one should not use shielding to conduct signal. Assuming the metal control plate makes good contact (check it!) with the shielding, and assuming that the shielding is properly connected to the jack sleeve, then you could in principle remove the daisy chain between the pots. But in your specific case, the pickup braided wire (which carries signal, mind you) would no longer be directly connected to the jack neg.- you would be relying on contact between the switch frame and the pickguard shielding for your negative signal connection.

So, before you cut out the daisy chaining, you would need to run a separate signal wire from the braid to a/the grounding point.

Which would, then, basically leave you with a star-grounding scheme; as I said before, probably best to pick one style or the other. But while that would be "best practices", no guarantee it solves your noise problem, which could lie elsewhere.

So is the basic concept with star grounding to send all grounds to a single fixed point within the guitar and then wire that fixed point directly to the output jack ground lug?

You are learning quickly, grasshopper . . .

BTW, with respect to the "shielding the beast" article, don't get all hung up on the "safety cap" included therein until after you have perused our detailed further discussion of that concept (upshot is that it's probably unneeded unless you use ancient tube amps without a three-pronged power cord).

[ashcatlt]

Star grounding is not significantly different from bus grounding, which is basically what you have here. But you have to be able to trust the bus. I personally never trust a wire soldered to the back of a pot. If it was me, I’d collect all the signal grounds at the ground terminal of the V pot and let the pot bodies find ground through the shielding, since that’s really their function.

[nodoz]

Feb 1, 2020 0:26:13 GMT -5 newey said:

Does the metal control cavity cover and the contact it makes with the copper shielded cavity render the ground wire daisy-chain between the two pots unnecessary?

Not owning a Mustang (I do have a 2016 Duo-Sonic reissue), I had forgotten about the metal control plate on those guitars.

I actually own one of those reissue Duo-Sonics also! The H-S model in Surf Pearl, it's been my #1 for live shows + touring over the last few years. That guitar is actually next on my list for a rewire, the toggle switch + push / pull tone pot are a little unreliable currently. I'd like to shield it too, but I'm going to hold off until I get this Mustang all sorted out first. I'm starting a Jazzmaster build a few months from now too, so I'm slowly working my way up through the ranks of schematic complication and difficulty until then. The JM is the final boss haha

The basic rule is that one should not use shielding to conduct signal. Assuming the metal control plate makes good contact (check it!) with the shielding, and assuming that the shielding is properly connected to the jack sleeve, then you could in principle remove the daisy chain between the pots. But in your specific case, the pickup braided wire (which carries signal, mind you) would no longer be directly connected to the jack neg.- you would be relying on contact between the switch frame and the pickguard shielding for your negative signal connection.

So, before you cut out the daisy chaining, you would need to run a separate signal wire from the braid to a/the grounding point.

Ok I think I'm starting to wrap my head around all this now. So in the scenario where the shielding + control plate are properly conductive, would I still need to run a wire from the common grounding point (let's say the volume pot) to the output jack neg, or is the ground signal already reaching the output jack neg by the nature of both the pots and the output jack being attached to the same control plate?

Also, if all of the shielding and the control plate are both properly conductive, does that make my ground wire that is currently running from the volume pot down into the cavity shielding superfluous? Trying to sort out the flow of electricity in my head:

Pickup neg signal is run to toggle switch ground --> toggle switch ground is wired down to volume pot --> volume pot comes into contact with the cavity cover which grounds the cavity cover + tone pot + output jack --> cavity cover is in contact with the cavity shielding which grounds all the cavity shielding --> cavity shielding is in contact with the pickguard shielding which grounds the pickguard screws and completes my cage to shield against and absorb extraneous hum. Bridge is also wired to the volume pot to ground the bridge, strings, etc.

If that line of logic is correct, wouldn't the wire from my volume pot down into the cavity shielding only serve to create an extra "loop" in path? 

Thanks again for letting me pick your brain, it's been frustratingly difficult to find trustworthy intel about this stuff and this is the first place I feel like I've received any nascent advice. Thankfully I'm much better at playing the fkn thing than I am at understanding how it all works lol

[nodoz]

Feb 1, 2020 12:47:36 GMT -5 ashcatlt said:

Star grounding is not significantly different from bus grounding, which is basically what you have here. But you have to be able to trust the bus. I personally never trust a wire soldered to the back of a pot. If it was me, I’d collect all the signal grounds at the ground terminal of the V pot and let the pot bodies find ground through the shielding, since that’s really their function.

I've often wondered why the ground terminal of the V pot is usually just soldered to the pot body while all those ground wires get soldered to the back of the pot.

Is there any prevailing logic or tangible advantage that comes with not utilizing the V pot ground terminal? I'd 100% prefer to solder all my grounds to the ground terminal as opposed to the pot. Much simpler to execute and almost always cleaner. 

Thank you ashcatlt!

[sumgai]

Feb 2, 2020 4:11:21 GMT -5 nodoz said:

Feb 1, 2020 12:47:36 GMT -5 ashcatlt said:

Star grounding is not significantly different from bus grounding, which is basically what you have here. But you have to be able to trust the bus. I personally never trust a wire soldered to the back of a pot. If it was me, I’d collect all the signal grounds at the ground terminal of the V pot and let the pot bodies find ground through the shielding, since that’s really their function.

I've often wondered why the ground terminal of the V pot is usually just soldered to the pot body while all those ground wires get soldered to the back of the pot.

Is there any prevailing logic or tangible advantage that comes with not utilizing the V pot ground terminal? I'd 100% prefer to solder all my grounds to the ground terminal as opposed to the pot. Much simpler to execute and almost always cleaner. 

I think you've picked up on most of what's been said, so I'll just recap it for you, and that should answer your question.

The big thing about signals running through conductors is that they meet resistance. It may be small, even mighty small, but nonetheless.... It turns out that mixing signals along that resistance path creates problems, usually of the noisy kind. Shielding should conduct "noise", and nothing else. The back of a pot is, indeed, a shield for the pot's innards. So why does everybody and his brother solder to pot backs? Because it's convenient, that's all.

I'm with ash (and others), I prefer not to use a pot back for just that reason. It's not so much that it's "against best practices" to use a pot's back, there's also the (sad) fact that most pots today are flimsy, and heat can damage them much more easily than pots made in the 50s and 60s. For me, it's best to avoid that possibility, and just use the washer that is espoused in the "Ring method" of gathering grounds.

As to using the metal plate that holds the controls and output jack, that is indeed a shield as well. Being made of a very thick metal (instead of a piece of .015 or .020 inch thick foil), it's probably OK to use it for signal as well, it should be of a very low resistance (and impedance, since we're speaking to frequencies). However, just because the parts are mounted to that plate, that doesn't mean that a good signal path has been established. The metals used in pot construction are not "zero resistance" by any means. And the plate itself has a finish coating on it that must be scratched away for better electrical contact. None of these pitfalls inspire me to use the plate as a signal conductor. Hence, I'd run a separate wire from the ring (washer) to the jack's ground terminal, and be done with it.

HTH



sumgai

[nodoz]

very clear advice, followed that completely. thank you homie!!