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Thoughts On Shielding And Grounds


Rockhorst

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I decided to go a little bit apeshit on my MiM strat. Don't know why cuz it already was pretty amazing (it holds itself pretty well against my PRS in sound quality and definitely defeats it in the fearceness department!). I guess I just needed something to do. So I decided to try out the shielding as presented in the guitarnuts article...and failed. I didn't have enough copper tape to shield the whole cavity, but I did have copper paint. So I decided to go for a tape shielded pickguard, painted cavity with some copper strips running across it. I tested it at some point and it was pretty noiseless. I don't know what I did after that, but I can't get it to quiet down anymore. Thing buzzes like mad and is unplayable. I pulled out the tape and decided to start looking for more copper tape.

While I wait for the tape, maybe someone here has some thoughts on shielding? Basic summary of what I did:

-Shielding with a combination of paint and copper tape (conducting adhesive)

-Disconnected the tone controls all together.

-I decided not to put in the DC protection capacitor. No stock strat has it and I don't see players dropping dead by the millions.

-Star grounding, which comes down to lifting the pickup ground wires from the volume pot casing and connecting it to the pot's ground terminal, since the tone pots where taken out.

One thing that may be trivial: because I didn't want to solder/desolder everytime, I tested it with long wires running from volume pot to input jack and connected the wires to the jack using metal clamps. Granted, that might give some noise. Also, the volume pot took quite some heat from the soldering (but still gives good readings on the ohmmeter).

One thing I don't fully understand about the guitarnuts article: one makes a Faraday cage of the cavity and grounds the cage to the bridge/trem. The audio circuit inside stays isolated from this, if I interpret the article corrrectly. Wouldn't that leave the audio circuit floating?

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One thing I don't fully understand about the guitarnuts article: one makes a Faraday cage of the cavity and grounds the cage to the bridge/trem. The audio circuit inside stays isolated from this, if I interpret the article corrrectly. Wouldn't that leave the audio circuit floating?

If you are having a problem with hum and/or buzzing, then you must have something wrong with your grounding. Are you sure that all of your shielding has continuity between itself and ground? Or, another common mistake people often make is reversing the hot & ground wires on the output jack, which has the same symptoms.

The idea behind star grounding is to ground your circuit in only one place, and not have redundant paths to ground, which some people believe causes ground loops. The Farady Cage is what shields the circuit from EMI & RFI, is connected to ground and is effectively part of the ground system. However, it does little or nothing for noisy pickups that like to pick up every interference that comes their way. That is a function of the pickups themselves. That is why I have some Fender noiseless pickups in my Strat, because I have never been able to really truly ever eliminate hum from any single coil pickups.

Edited by Paul Marossy
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By using tape on the underside surface of the pickguard you are doing two things.

1. As Paul M mentioned, shielding the circuit from EMI and RFI.

2. Grounding the individual components that are mounted on the pickguard.

I'll bet the components (switch, pots and possibly pickups) are being grounded twice, through their respective ground wires AND through their metal parts contacting your shielding tape. When I put conductive tape on my pickguards I always make sure to isolate all those components. I cut a large enough space from the tape so the metal parts like switch base, pot cases / shafts, and whatever else that is already grounded otherwise, will not touch the tape.

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I'll bet the components (switch, pots and possibly pickups) are being grounded twice, through their respective ground wires AND through their metal parts contacting your shielding tape. When I put conductive tape on my pickguards I always make sure to isolate all those components. I cut a large enough space from the tape so the metal parts like switch base, pot cases / shafts, and whatever else that is already grounded otherwise, will not touch the tape.

Interesting point you make, since the guitarnuts article that I referred to earlier actually stresses that the pots should touch the copper foil, for instance at point 17:

17. Remove any wires which are soldered from the shell of one pot or switch to the shell of another. These wires are ground loops because the bodies of the controls are also electrically connected through the foil on the back of the pickguard.

In the article, one pot is connected to a terminal of a capacitor. On the other end of the capacitor is a connection to all audio signal grounds. I left out this entire part, so maybe that's where I went wrong. On the other hand, the capacitor would act as a short, so the audio signal still would be isolated from the Faraday cage.

More thoughts?

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One thing that may be trivial: because I didn't want to solder/desolder everytime, I tested it with long wires running from volume pot to input jack and connected the wires to the jack using metal clamps.

Did you by chance mix up the ring and tip connections? Reversed wiring on an output jack is Very noisy.

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Interesting point you make, since the guitarnuts article that I referred to earlier actually stresses that the pots should touch the copper foil, for instance at point 17:

QUOTE

17. Remove any wires which are soldered from the shell of one pot or switch to the shell of another. These wires are ground loops because the bodies of the controls are also electrically connected through the foil on the back of the pickguard.

Thats exactly my point. In my case I elected to leave all the ground wires attached and isolate the electrical connectivity to the foil on the back of the pickguard. Just make sure those components are only grounded once.

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My guess is you did like Southpa is stating. You have to either leave enough foil around the holes for the components to touch and create the ground, but have to wires attached to the casings at all or to trim back so the components are not touching the foil and run a ground wire from your casings. But you can't do both. If it is touching the foil, your ground lugs must go to the star and not to the casings.

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I read once somewhere that you also want the pot bodies and switch bodies grounded, so they don't act like antennas for EMI/RFI. But I guess that is a moot point if you don't use the back of the pot(s) as a place to solder wires to ground...

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  • 2 weeks later...

First thing to understand is that you will never remove all the hum with the stock pup`s. If you want total silance then fit Lace or noiseless pups. Star grounding works well when done correctly however the change is not huge. Check your star grounding as people have stated above and check for earth loop paths with a multimeter. Double check the bridge grounding. Some guitars are made with the bridge un grounded and simply grounding the bridge can make a big differance.

The strings act like a big antenna and this noise is some how transfered to the pup`s. Good luck :D

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I didn't work on the shielding for a while because I had some other obligations, but I gave it a try again yesterday. It's driving me raving mad. Please plz assist some more.

Situation is the following: I covered the pickguard and the cavity with copper tape with conductive adhesive. Continuity all round. The foil from the cavity reaches over the edges a little bit so it can make contact with the pickguard. I've checked (and double-checked) that the leads on the input jack are in the right order: hot to tip and ground to sleeve. All the ground wires from the pickups are soldered to the back of the volume pot. The tone pots have been disconnected entirely. I did not include a capacitor to prevent DC shock, as was suggested in the guitarnuts article. I've ensured (by temporarily taping it all of) that no part of the audio circuit is touching the shielding in the cavity. The ground wire from the tremolo spring claw is connected to the foil on the bottom of the cavity.

I tried 2 things (all tests with an unstringed instrument!):

- Leave a hole in the shielding around the volume pot so that it's not connected to the shield. Result: lotsa noise when untouched. No reaction when touching bridge assembly (as expected). Hum goes away when I touch the output jack or the volume pot, i.e. when I ground the audio circuit with my body.

- With pickup grounds still wired to the pot casing I had the pot casing connected the shielding on the pickguard. Result: lotsa noise when untouched. When touching volume or output jack noise goes away. When touching bridge/trem assembly noise increases.

So the way I see it, I've tried all suggestions in this topic. The thing is essentially star grounded at the volume pot, since the tone controls are disconnected. It seems that somehow, in both cases described above, the audio circuit is left floating. I'm lost here people. Please drop some bread crums.

Edited by Rockhorst
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Result: lotsa noise when untouched. When touching volume or output jack noise goes away. When touching bridge/trem assembly noise increases.

Hmmm...sounds like the shielding is hot!

I would suspect that string grounding thing....you should hard wire the ground to the trem/bridge/string to the "star" volume pot.

Of course...this is where you need a multimeter/continuity tester. for this I use my old "analog" needle meter...touching the ground to the jack and foil for instance.

One problem that may result from fixing the ground wire thing is that you may end up shorting the guitar (hot to ground) and getting no sound at all. One way to test if the shield is hot perhaps is to touch a wire from the ground (output jack or volume knob that decreases noise) to the strings (or shielding).

It's very hard to know exactly what you have done but it sounds like you rewired it, not just shielded the cavity and as a result something may have gone wrong in this process. Again, it is easy for things to go wrong in a strat when it is reassembled and can be frustrating...everything can test ok till you put it all together...so all the more important to test that things are ok while it is opened up...in case when closing it a hot wire presses against the shielding somehow.

Anyway...not a lot I can offer at this point....keep at it...likely it is something simple

pete

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What I rewired is the following: I disconnected the tone controls, took them out of the circuit (which really brings your neck and middle pickup to live IMO). I also installed a different bridge pickup.

What would be a great first help is if somebody could tell me dead certain which of the 2 cases describe above I should go for: with the grounds at the pot casing, should the pot casing touch the shielding on the pickguard or not? (from previous answers I would suspect not).

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Additional note: the instrument is NOT stringed yet (but I don't see how that would make any difference).

When tapping the PUs with a screw driver you hear pops as expected.

When I lift the pickguard off the problem remains the same.

If I would attach the ground wire from the bridge to the pot, instead of to the bottom of the cavity the audio circuit would be grounded again and the guitar would be playable I guess. But that would leave the shield ungrounded.

EDIT: I connected the pot casing to the shield again and got something different: it is less noisy (but far from silent) then when not connecting pot to shield. Noise still goes away when touching metallic parts. When not touching you have a noise level with a constant 'tick' sound: about every 0.5 second your hear a 'tick' through the amp.

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Additional note: the instrument is NOT stringed yet (but I don't see how that would make any difference).

When tapping the PUs with a screw driver you hear pops as expected.

When I lift the pickguard off the problem remains the same.

If I would attach the ground wire from the bridge to the pot, instead of to the bottom of the cavity the audio circuit would be grounded again and the guitar would be playable I guess. But that would leave the shield ungrounded.

EDIT: I connected the pot casing to the shield again and got something different: it is less noisy (but far from silent) then when not connecting pot to shield. Noise still goes away when touching metallic parts. When not touching you have a noise level with a constant 'tick' sound: about every 0.5 second your hear a 'tick' through the amp.

That would seem to confirm what I read once somewhere about the pot bodies acting like antennas if they are not grounded. On the ticking sound, it sounds like it is picking up some kind of digital pulse from something nearby. I bet if you go to another room that ticking noise will go away. I've even had my wrist watch coming thru pickups before when working on a guitar and it makes kind of a pulsing noise, too.

By the way, when you are conducting your tests, what else is in the room with you? Computer monitors (CRT), flourescent lights, wall warts, transformers, fan motors and stuff like that nearby your guitar will make just about any guitar hum no matter how well it is shielded, and especially if your pickups are pointed at those sources of EMI.

And what kind of pickups are you using? I built a guitar once with a cheap pickguard/pickup combo and it hummed A LOT, and I shielded the heck out of the ENTIRE control cavity and the pickup cavities, too. The problem did not go away until I got better pickups to put in the guitar. By its very nature, any single coil pickup is going to be very susceptible to EMI, but even more so the really cheap ones.

Just throwing out some more ideas...

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Thanks Paul! My house isn't that big so my amp is about a meter from my computer. Never gives me much trouble though, even with single coils provided I don't stand directly next to it. They're also at different angles.

About the antenna thing: I posted a link to an article on that on the previous page of this thread.

PUs I'm using are 2 stock MiMs and a texas special in the bridge. The most important thing for me right now is to KNOW which is the correct way to go:

Option 1: audio circuit completely seperate from shield, shield being connected to ground

Option 2: audio circuit makes contact with shield through the casing of the volume pot and the shield is at one point connected to the bridge

Option 3: something different (brainfart: what would happen if I left the shield ungrounded? My guess is that should work

If I know which one of these is correct, then I can start troubleshouting convergently, instead of having to run every test twice or thrice.

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All I can think of is to tear it apart into individual components, check them thoroughly with a meter as well as continuity of each wire (ground and hot). When you are sure everything is functioning properly AND you know which wire is which :D, rebuild the system. If its still not functioning like its supposed to then post a picture of your pickguard and wiring. We'll fix this thing yet! :D

Checking pots? I like to use an old style non-digital meter (set to Ω), touch between various contact combinations, including pot casing, while turning the shaft. Look for the resistance "sweep" on the meter. Same method with switches, btw, wiring a strat switch can get confusing too! It may sound obvious, but still hasn't been mentioned in this thread, what wiring diagram are you using? Make sure you are following it the right way round. I've done it before myself!

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Pot is 100% functional, all relevant parts have been checked for continuity and the switch works fine. Really peeps, I doubt that's where the problem is (but I'll double check later). Pictures coming up within the next 12 hours B)

By the way, this 'mod' was just something I wanted to try out. The bit of noise from a strat never bothered me that much, the article just got me curious. So if at some point I really need my strat again (it sorta is my main axe) I'm just gonna rip all the shielding out, string it up and play that thing :D . But it would be nice if we got it fixed before I get to that point :D If only to satisfy curiosity...

Edited by Rockhorst
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Thanks Paul! My house isn't that big so my amp is about a meter from my computer. Never gives me much trouble though, even with single coils provided I don't stand directly next to it. They're also at different angles.

About the antenna thing: I posted a link to an article on that on the previous page of this thread.

PUs I'm using are 2 stock MiMs and a texas special in the bridge. The most important thing for me right now is to KNOW which is the correct way to go:

Option 1: audio circuit completely seperate from shield, shield being connected to ground

Option 2: audio circuit makes contact with shield through the casing of the volume pot and the shield is at one point connected to the bridge

Option 3: something different (brainfart: what would happen if I left the shield ungrounded? My guess is that should work

If I know which one of these is correct, then I can start troubleshouting convergently, instead of having to run every test twice or thrice.

What you want to end up with when you are all done is to have all grounds connected together in one place, otherwise known as "star grounding". The pot bodies and switch bodies should be connected to the shielding, and the shielding should be connected to ground. Having the shielding not connected to ground will not do much for you. Ideally, all of the pickup cavities and the control cavity should have continuity between them and ground. That way, you get a Farady Cage.

After all of that is accomplished, your pickups will still be the weak link. But you will have done everything humanly possible to quiet everything from extraneous noise.

Edited by Paul Marossy
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Pictures, as promised. Not to clear but I hope you can make stuff out. Two notes: the soldering on the pot isn't the prettiest ever, but it holds up on continuity check, second you can see the copper foil removed underneath the volume pot. After the above discussion I guess I should put it back.

shield02.jpg

shield01.jpg

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Simple method,well, this is MY method anyway and it hasn't failed me yet :D.

Cut away the tape from ALL metal contact areas on your pickguard that would otherwise require grounding. That includes all pots, switch, pickups etc. Then run ALL ground wires (switch, pickups, bridge or tremblock AND shielding) to volume pot. You can treat the tone pots as one ground, ie. run a wire from farthest tone pot to the case of the next one then to the vol pot. Lastly, run a wire from the vol pot casing to output jack. To ground your shielding you can crimp an eye lug to a wire and then use a small wood screw to hold the lug in contact with the copper tape in the bottom of your control cavity, the other end is soldered to the vol pot casing.

Tone pots should also be grounded to themselves. Third lug is pushed up in contact with the pot casing and then soldered. Thats the surest way I know to ground a guitar.

Edited by Southpa
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Two notes: the soldering on the pot isn't the prettiest ever, but it holds up on continuity check, second you can see the copper foil removed underneath the volume pot. After the above discussion I guess I should put it back.

If all of your ground wires are soldered to the back of the volume pot and the volume pot doesn't have continuity with the the shielding, then that could be a large part of your problem with noise right there.

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