Backplate induces hum into single coils + dummy coil circuit

[guitarnerdswe]

I have a Music Man guitar with passive HSS pickups, but an active buffered output and a dummy coil for the 2 single coils. It seems to have developed a problem, being quite susceptible to picking up noise. When troubleshooting, I noticed that when the back plate for the control cavity is installed, the single coils are much noisier than without the back plate (the humbucker is fine).

The back plate is made out of metal (so it acts like an antenna as expected), but grounding the backplate doesn't remove the buzz it picks up when using the single coils (it does when using the humbucker though). So basically the backplate just induces hum into the dummy coil, and the whole single coil and dummy coil combo is the most quiet with the backplate completely off the guitar. I can't wrap my head around this, why it acts like this and what might be done to solve it. I'm pretty sure it didn't use to be like this. Any and all suggestions and theories are welcome.

Here is a picture of the setup and a short video demonstrating the issue. The backplate is connected to the ground wire on the guitar and was checked for continuity.

Since the audio is terrible, I've attached a proper recording with timestamps as well at the bottom.

SOUND CLIP:

on.soundcloud.com/mGwsc

0-7 s = no backplate
7-11 s = backplate installed
11-14 s = no backplate
14-19 s = backplate installed

[mrmcruff]

Hi,

Where is the dummy coil located and what does it look like? I see something that appears to be a coil on the board in the cavity, is that it?

Have you done any recent work to the wiring?

[JohnH]

My guess would be that the metal backplate messes with the properties of the dummy coil for both its inductance and tendency to pickup up hum. Because of geometry of the coil or its placement or different winding arrangement, the dummy coil is affected differently to the coils which it combines with, plus the pickup coils are not under the plate, Hence the plate is throwing off the balance of them. The Humbucker is OK because they are two equal coils in about the same position and orientation and they are not directly below the plate.

[mikecg]

Hello guitarnerdswe,

Music Man claim that some of their guitar backplates are aluminium, or aluminium lined, so I'm guessing yours may be aluminium?
From your photo, I can see that some overspray (black paint) extends some way onto the rear face of the plate.
There appears to be a small area of clean metal under your crock clip connection, and I assume that that is an original feature that makes contact with a ground clip connected to the cavity ground. I ask because, if that single ground point fails for some reason, the overspray on the back of the plate will prevent it from making contact with the conductive paint lining the cavity and extending up to the plate mounting rebate. 

If, at some former time, the guitar was quiet, and now it is noisy, obviously, something has changed - perhaps the original location was electrically 'quiet', and now it is noisy? Perhaps you have moved house recently? If not, then the guitar is definitely the problem - perhaps a dry joint on the circuit board, or an open circuited dummy coil could produce an effective 'hanging hot' that is picking up noise? Also, switch contacts can oxidize over time, so the application of some switch cleaning fluid may be worth a try.

 

[guitarnerdswe]

mrmcruff, JohnH, mikecg, thanks for replying guys! I've summarized my responses below:

The dummy coil is indeed the circular cooper-coloured object close to the output jack. All the wiring is from the factory.

The backplate is made out a non magnetic metal, so aluminium is probably correct. It's free from paint around the screw hole where the clip is, and when mounted properly, it makes contact with the grounding wire that the other clip is connected to (I used the alligator clips during the recording to make 100 % sure there was a proper ground connection when moving the plate around, and I did check that there was continuity from the plate all the way to ground at the output jack).

I haven't moved, the rest of the setup is unchanged and I've checked all the cables (power and audio). I'm fairly certain that the noise level it has now with the backplate off, is what it used to be with it on. When adjusting the trim pots for the dummy coil it still "does its thing", IE removing 50/60 cycle hum. But whenever the backplate is on, it just adds all this high frequency noise.

If the dummy coil was open circuited ("hanging hot"), would it still be able to cancel some 50/60 cycle hum like it does?

[mikecg]

Hello again guitarnerdswe,

Good to hear that you are carefully checking your ground continuity connections. I suggest you check all the pickup ground and shield connections in the same way.
'Dry' solder joints can cause all sorts of problems on 'aged' circuit boards. The usual suspects are those joints associated with the larger (heavier) components that rely, only on their connecting wires, for support, or those connections that use very fine gauge wires. Vibration, over a long period of time, acts on the mass of the larger components, and the solder joint interface finally ruptures, in a way that is often very difficult to spot without a strong magnifying glass. Once the joint is fractured, oxidization will finish the job, and a joint that looks ok from a distance will actually be completely non-conducting! So the strategy, when looking for dry joints, is to 'wiggle' the suspect larger components, one by one, with the circuit powered up, and in the case of audio equipment, using your ears, as a fault detector. The solder joints on blade switch lugs, also get a fair amount of vibration with use, and so these should also be checked for dry joints. Switches, can wear, and break internally, and so these are also candidates for continuity checks. 

It sounds like there is more than one trimpot for adjusting the dummy coil function, and it would be helpful to know how many there are, and how they effect the coil's function. Even the humble trimpot, being a mechanical component, can fail with repeated use.

One final warning regarding PP3 9 volt batteries. These can sometimes appear to be in good condition when measured unloaded, with a high impedance digital voltmeter, but will not function correctly when loaded 'in-circuit'. So always check your battery voltage in-circuit with the guitar lead plugged in to the guitar. Circuit function can be unpredictable when powered by 'weak' or dying batteries.

[guitarnerdswe]

Mar 11, 2024 17:52:42 GMT -5 mikecg said:

Hello again guitarnerdswe,

Good to hear that you are carefully checking your ground continuity connections. I suggest you check all the pickup ground and shield connections in the same way.
'Dry' solder joints can cause all sorts of problems on 'aged' circuit boards. The usual suspects are those joints associated with the larger (heavier) components that rely, only on their connecting wires, for support, or those connections that use very fine gauge wires. Vibration, over a long period of time, acts on the mass of the larger components, and the solder joint interface finally ruptures, in a way that is often very difficult to spot without a strong magnifying glass. Once the joint is fractured, oxidization will finish the job, and a joint that looks ok from a distance will actually be completely non-conducting! So the strategy, when looking for dry joints, is to 'wiggle' the suspect larger components, one by one, with the circuit powered up, and in the case of audio equipment, using your ears, as a fault detector. The solder joints on blade switch lugs, also get a fair amount of vibration with use, and so these should also be checked for dry joints. Switches, can wear, and break internally, and so these are also candidates for continuity checks. 

It sounds like there is more than one trimpot for adjusting the dummy coil function, and it would be helpful to know how many there are, and how they effect the coil's function. Even the humble trimpot, being a mechanical component, can fail with repeated use.

One final warning regarding PP3 9 volt batteries. These can sometimes appear to be in good condition when measured unloaded, with a high impedance digital voltmeter, but will not function correctly when loaded 'in-circuit'. So always check your battery voltage in-circuit with the guitar lead plugged in to the guitar. Circuit function can be unpredictable when powered by 'weak' or dying batteries.

The battery was actually one of the first things I swapped, but I'll try and swap it again. I'll also give a wiggle to whatever I can wiggle (SMD components are kinda impossible to wiggle).

The wiring for the single coils is basically like this:

• Each pickup has 3 wires exiting
  
  • Hot (to 5-way switch)
    
  • Shield (straight to ground)
    
  • Ground wire that goes:
    1. Into circuit board (via solder pads)
       
    2. Through trimpot
       
    3. Into circuit board again
    4. Through dummy coil
    5. Don't know, but not straight to circuit ground
       

One pickup uses the CW direction around the dummy coil, the other CCW (since they're RWRP like a regular neck/middle strat). After measuring with a multimeter, I'm pretty sure there is some circuitry in the signal path whenever the signal from the pickup ground goes "into" the circuit board. 

There is about 35k of resistance between pickup ground (measured on the pickup) and the actual circuit ground. Twiddling with the trimpots does not change that. The dummy coil itself has 0.47k resistance.

[unreg]

Hi guitarnerdswe.

I used to have an odd noise problem too until I removed the ground loop I had created.

I used to have my guitar’s amp plugged into one outlet in my room; while also having my pedals plugged into another outlet on a separate wall of my room. Using two outlets to power my guitar’s sound caused a sometimes randomly noisy ground loop. This noise survived despite all logic shared with me on this forum.

I ended up purchasing The Truetone 1 Spot adapter from Sweetwater. That allows me to hook up 5 pedals (max) with using only 1 outlet on my power strip. Now, my amp and pedals are plugged into only one power strip and thus they use only one outlet on my rooms walls - and all noise has vanished!


www.sweetwater.com/store/detail/1Spot9v--truetone-1-spot-slim-9v-dc-adapter/


I hope you solve your problem. Thought this may help.

[guitarnerdswe]

Hi unreg! I wish my problem was that simple. 

In the video clip, I'm using a battery powered amp, with the mains power cut off in the entire apartment except for that room, plus everything in that room disconnected from the wall sockets.

Yep, I really went that far just to rule out power issues  Glad you figured yours out though!

[stevewf]

Helmut Lemme's "Inside Electric Guitars" shows a Music Man Silhouette, which, while it doesn't look the same as your guitar, it does have a dummy coil in the cavity. Lemme offers a schematic of the basic active hum-cancelling circuit, calling it "truly an ingenious (even cunning) idea!"

The Silhouette has a dummy coil and impedance converter together in a single unit; perhaps the two are separate in your guitar.

I don't know copyright law, so erring on the side of caution, I stop short of copying any diagram from Helmuth's book and pasting it here.

[guitarnerdswe]

Mar 15, 2024 12:23:05 GMT -5 stevewf said:

Helmut Lemme's "Inside Electric Guitars" shows a Music Man Silhouette, which, while it doesn't look the same as your guitar, it does have a dummy coil in the cavity. Lemme offers a schematic of the basic active hum-cancelling circuit, calling it "truly an ingenious (even cunning) idea!"

The Silhouette has a dummy coil and impedance converter together in a single unit; perhaps the two are separate in your guitar.

I don't know copyright law, so erring on the side of caution, I stop short of copying any diagram from Helmuth's book and pasting it here.

Everything is on the same circuit board (and it's also active, while the Silhouette is passive I think?). The pickups and tone pot on my guitar are passive, but the rest (buffered output, switchable boost, noise reduction circuit) are active.

Do you remember what the purpose of the impedance converter was?

[stevewf]

Mar 19, 2024 15:12:50 GMT -5 guitarnerdswe said:

Do you remember what the purpose of the impedance converter was?

Generally, Helmuth promoted it for removing the effect of the the capacitance of the instrument cable (guitar<->amp), which typically flattens the resonance peak and shifts it lower in frequency. Mr. Lemme sells them.
I wonder if that's the same as a buffer?

In the case of the Silhouette, I have to admit that I don't understand everything, but I think it's part of the overall hum-canceling system, in that with active electronics, the level of the "anti-hum" output from the dummy coil can be mixed appropriately with the "pro-hum" of the normal (magnetic) coil without affecting the sound profile of said normal coil.

[mikecg]

Mar 15, 2024 12:22:11 GMT -5 guitarnerdswe said:

In the video clip, I'm using a battery powered amp, with the mains power cut off in the entire apartment except for that room, plus everything in that room disconnected from the wall sockets.

Hello again guitarnerdswe,

A few additional queries:

1. You say that you are using a battery powered amp, and so there will be no connection to a domestic ground or earth. What effect on noise, if any, does a connection between the guitar ground and the domestic ground, produce?

2. Have you checked for continuity between the guitar ground - i.e. the circuit board ground, and the conductive paint lining the control cavity? 

3. Check for continuity between the ground connection of the jack at the amp end, and the guitar circuit board ground. 

[guitarnerdswe]

Mar 21, 2024 7:23:49 GMT -5 mikecg said:

Mar 15, 2024 12:22:11 GMT -5 guitarnerdswe said:

In the video clip, I'm using a battery powered amp, with the mains power cut off in the entire apartment except for that room, plus everything in that room disconnected from the wall sockets.

Hello again guitarnerdswe,

A few additional queries:

1. You say that you are using a battery powered amp, and so there will be no connection to a domestic ground or earth. What effect on noise, if any, does a connection between the guitar ground and the domestic ground, produce?

2. Have you checked for continuity between the guitar ground - i.e. the circuit board ground, and the conductive paint lining the control cavity? 

3. Check for continuity between the ground connection of the jack at the amp end, and the guitar circuit board ground. 

Hi!

Sorry for the late reply.

1. I A/B:d recorded clips with the whole signal chain connected to domestic ground, vs the domestic ground disconnected. The grounded clips have less overall noise, but the backplate still adds about the same amount of noise when it gets close to the guitar.

2. I tried to, but I couldn't get a reading on it since it's a underneath a clear coat layer. I have another Music Man that has a patch where bare black paint is accessible, and there is no continuity even within the paint itself, so I'm not sure if it's even conductive paint to begin with it? It doesn't even cover the entire control cavity, and on the newer guitars they've skipped it entirely.

3. Yes, there is continuity all the way from the circuit board (and backplate) to domestic ground at the power outlets.

I also tried swapping cables, batteries, lifting the domestic ground on just the guitar rig, and lifting audio ground, no change in the noise.

[unreg]

Apr 13, 2024 11:22:13 GMT -5 guitarnerdswe said:

Mar 21, 2024 7:23:49 GMT -5 mikecg said:

Hello again guitarnerdswe,

A few additional queries:

2. Have you checked for continuity between the guitar ground - i.e. the circuit board ground, and the conductive paint lining the control cavity? 

Hi!

Sorry for the late reply.

2. I tried to, but I couldn't get a reading on it since it's a underneath a clear coat layer. I have another Music Man that has a patch where bare black paint is accessible, and there is no continuity even within the paint itself, so I'm not sure if it's even conductive paint to begin with it? It doesn't even cover the entire control cavity, and on the newer guitars they've skipped it entirely.

Hi again,

This somewhat happened to me too. While my guitar is an ESP LTD M-155, it met life on a Korean assembly line and so isn’t the same ?quality as yours, mine had similar paint issues. Similarly, its electronic cavity had only a few spots painted AND it also did not return any signs of continuity. A member here, frets, guided me to make my own conductive paint. Maybe she’ll visit and help here too; I don’t quite remember the ingredients specifically.

EDIT: Ok, I just looked it up. She used:

Jan 19, 2021 14:57:35 GMT -5 frets said:

Hi Unreg ,
First, I’m sorry your having trouble with it.  You’ve followed the recipe perfectly.  3 teaspoons of graphite powder to one ounce of Acrylic paint. The only thing I can offer is that I use a particular brand of graphite powder, AGS.  Here’s a pic.

But I would not think other brands would cause a problem.  I don’t k ow about the quartz.  I haven’t been too helpful, all I can say is using the AGS graphite, my cavities are quite conductive.  

That’s from this frets thread:
guitarnuts2.proboards.com/thread/9306/make-own-shielding-paint?page=1

Sorry, I hid my terrible remembrance in this spoiler:

SPOILER: Click to show

(There was black paint, probably black nail polish, and some small canister of iron flakes/powder from Ace Hardware; I mixed those two together, removed most of the hardware, painted, and now have an electronics cavity where the paint completely lining the walls IS conductive.)

Though, remember… my problem had a major relation to the external ground loop I had created in my room; but, after the painting and reassembly, my guitar was at a somewhat quieter state, you know bc my external ground loop still existed!

[col]

guitarnerdswe A thought: are you certain that the back plate is not making contact with any of the components? I just wonder if there is an intermittent dry joint or some other faulty component being activated by pressure from the plate.

[mikecg]

Apr 13, 2024 11:22:13 GMT -5 guitarnerdswe said:

Mar 21, 2024 7:23:49 GMT -5 mikecg said:

Hello again guitarnerdswe,

A few additional queries:

1. You say that you are using a battery powered amp, and so there will be no connection to a domestic ground or earth. What effect on noise, if any, does a connection between the guitar ground and the domestic ground, produce?

2. Have you checked for continuity between the guitar ground - i.e. the circuit board ground, and the conductive paint lining the control cavity? 

3. Check for continuity between the ground connection of the jack at the amp end, and the guitar circuit board ground. 

Hi!

Sorry for the late reply.

1. I A/B:d recorded clips with the whole signal chain connected to domestic ground, vs the domestic ground disconnected. The grounded clips have less overall noise, but the backplate still adds about the same amount of noise when it gets close to the guitar.

2. I tried to, but I couldn't get a reading on it since it's a underneath a clear coat layer. I have another Music Man that has a patch where bare black paint is accessible, and there is no continuity even within the paint itself, so I'm not sure if it's even conductive paint to begin with it? It doesn't even cover the entire control cavity, and on the newer guitars they've skipped it entirely.

3. Yes, there is continuity all the way from the circuit board (and backplate) to domestic ground at the power outlets.

I also tried swapping cables, batteries, lifting the domestic ground on just the guitar rig, and lifting audio ground, no change in the noise.

Hello again, guitarnerdswe,

"Curiouser, and curiouser", said Alice!

A few more questions, if I may:

Did you buy this guitar from new, and if not, was it sold to you with clear-coat inside the control cavity, or did you add this clear-coat yourself, during a renovation?

The reason I ask, is that the standard conductive graphite loaded paint (it used to be called 'Aquadag', and was developed for coating the external surfaces of cathode ray tubes - remember those?), was (is) a mixture of colloidal graphite in distilled water with an acrylic binder. This paint film is not particular stable, and in its original application was not designed to be over-coated. It is quite possible that the clear coat has, over time, created micro-cracking of the graphite paint, and effectively destroyed its conductive properties. Normally, the conductive paint would make contact with the the pot bushings as they pass through the guitar body, and the pot bushings would be connected to the circuit board and guitar ground. Closure of the control cavity with a grounded conductive cover, produces what is termed a 'Faraday cage' that effectively isolates everything inside the control cavity from external capacitively coupled electrical interference. However, in your case, it would appear that the lining of the control cavity is now non-conducting, and therefore, the 'cage' cannot be 'closed'. Under these conditions your backplate is acting, not as a shield, but as an aerial!

If this is indeed the case, you have two 'ways to go', you can re-coat the cavity with 'Aquadag', and here's a link:

www.amazon.co.uk/Conductive-Graphite-Paint-Electroplating-Electroforming/dp/B07X66HVBL/ref=asc_df_B07X66HVBL?tag=bingshoppinga-21&linkCode=df0&hvadid=80195721622954&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583795267134962&psc=1

Or you can line the cavity with sticky back copper foil, making sure that all pieces are soldered together, and here's another link:

www.amazon.co.uk/Qrity-Conductive-Shielding-Induction-Electrical/dp/B0BT9M5FGV/ref=sr_1_5?crid=1N7JWFITZ9A6M&dib=eyJ2IjoiMSJ9.PLaKlG40JjABoHUESbJpQjNj4rGrUky4xgG8R0u8BlRKSf0CSDgwhYoWMUG-0TE9hRM8s4gQPCpsZe-s-3ZveQ_WTQv0gbDhyAl1aENJcAARaiylSB7iIbCNc00jCpyJadBKRh-Kzw0ZJZzz08bQwszAqPM96QgpVJaqvRpt6MjowyapjsXM9lfbeIcfs7_kKWMA-9pjyK1zZYRKti7XZPXTlDb0sTjU3kqQ_pRcaIc.Q1LZYrF3SgS0o-aUT0h46ww0cfs43fpQocr6GwZPOks&dib_tag=se&keywords=copper+foil&qid=1713134753&s=diy&sprefix=copper+foil%2Cdiy%2C119&sr=1-5

[bigez]

did you try asking in the music man forums? those guys really know these guitars well and can probably troubleshoot quickly.

[unreg]

Apr 14, 2024 17:59:51 GMT -5 mikecg said:

Closure of the control cavity with a grounded conductive cover, produces what is termed a 'Faraday cage' that effectively isolates everything inside the control cavity from external capacitively coupled electrical interference. However, in your case, it would appear that the lining of the control cavity is now non-conducting, and therefore, the 'cage' cannot be 'closed'. Under these conditions your backplate is acting, not as a shield, but as an aerial!

If this is indeed the case, you have two 'ways to go', you can re-coat the cavity with 'Aquadag', and here's a link:

www.amazon.co.uk/Conductive-Graphite-Paint-Electroplating-Electroforming/dp/B07X66HVBL/ref=asc_df_B07X66HVBL?tag=bingshoppinga-21&linkCode=df0&hvadid=80195721622954&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583795267134962&psc=1

Or you can line the cavity with sticky back copper foil, making sure that all pieces are soldered together, and here's another link:

www.amazon.co.uk/Qrity-Conductive-Shielding-Induction-Electrical/dp/B0BT9M5FGV/ref=sr_1_5?crid=1N7JWFITZ9A6M&dib=eyJ2IjoiMSJ9.PLaKlG40JjABoHUESbJpQjNj4rGrUky4xgG8R0u8BlRKSf0CSDgwhYoWMUG-0TE9hRM8s4gQPCpsZe-s-3ZveQ_WTQv0gbDhyAl1aENJcAARaiylSB7iIbCNc00jCpyJadBKRh-Kzw0ZJZzz08bQwszAqPM96QgpVJaqvRpt6MjowyapjsXM9lfbeIcfs7_kKWMA-9pjyK1zZYRKti7XZPXTlDb0sTjU3kqQ_pRcaIc.Q1LZYrF3SgS0o-aUT0h46ww0cfs43fpQocr6GwZPOks&dib_tag=se&keywords=copper+foil&qid=1713134753&s=diy&sprefix=copper+foil%2Cdiy%2C119&sr=1-5

This seems to be a great explanation of the problem. Now I remember “Faraday cage”; thank you mikecg!

OP, you may also create your own shielding paint, with frets’ recipe, in my post, above, as a third option; just create that Faraday cage (which will eliminate your backplate from being an aerial); it truly helps tons!

[frets]

I can’t emphasize enough the needed quality of graphite powder. The brand I recommended is by far the best.

[guitarnerdswe]

Thanks for all the replies guys!

unreg, frets & mikecg, doing a proper application of graphite powder is definitely on my list of things to try if turns it the circuit board isn't at fault!

I bought the guitar new, and the whole "patchy conductive paint with a clear coat"-business in the control cavity is factory stock.

I might be missing something from the physics point of view, but could someone explain why the backplate would act purely as an antenna, just because the conductive paint is no longer working? The backplate is still connected with a wire to the ground of the output jack.

Could the existing (now presumed non-conducting) conductive paint mess with a new coating of conductive paint, if it turns out that parts of the existing paint is still conductive?

col, absolutely sure. I've held the plate a distance from the guitar just to sure, and at weird angles just be certain it's doesn't touch anything it's not supposed too. That also how I figured out it was proximity to the dummy coil that induces the hum.

bigez, I haven't asked on the MM forums, but I did ask Music Man CS about the problem directly. They refuse to give any other answer than "If something happens when you turn the trim pots, it's working" and won't respond any further. They also completely ignored all the details of the issue.

[unreg]

Apr 25, 2024 13:37:15 GMT -5 guitarnerdswe said:

I bought the guitar new, and the whole "patchy conductive paint with a clear coat"-business in the control cavity is factory stock.

My guitar was also bought new. It did arrive with patchy conductive paint… there were big sections of the electronics cavity that wasn’t even painted. But, mine didn’t, I think, have a clear coat.

Apr 25, 2024 13:37:15 GMT -5 guitarnerdswe said:

I might be missing something from the physics point of view, but could someone explain why the backplate would act purely as an antenna, just because the conductive paint is no longer working? The backplate is still connected with a wire to the ground of the output jack.

That is interesting; perhaps your company never planned on grounding the backplate via connection with the grounded paint on the cavity walls? Did you check continuity from your backplate to your jack’s ground? If not, then perhaps that grounding wire isn’t working well… maybe it’s broken or maybe one of its solder spots has chilled.

Apr 25, 2024 13:37:15 GMT -5 guitarnerdswe said:

Could the existing (now presumed non-conducting) conductive paint mess with a new coating of conductive paint, if it turns out that parts of the existing paint is still conductive?

I don’t believe so. Since my cavity was painted by myself 4 times… and also because ground loops don’t/can’t exist inside guitars; that’s what I’ve learned here; as long as your cavity coating is grounded, that’s sufficient.

[mikecg]

Hello again, guitarnerdswe,

Very sorry to hear that Music Man were so unhelpful - not a good sign IMHO. By the way, I have done a search on the Music Man forum, and there seems to be quite a few queries regarding 'noise' problems, so maybe your problem is not unique, and if the good people (?) at Music Man are aware - maybe they are keeping stumm?

You ask how the back-plate can act as an aerial, when it is connected to ground. Well, take a look inside a mains powered radio with an aerial. The wire from the aerial will be routed through a tuning coil - an inductor consisting of a short length of wire and very often terminated at the chassis ground. At high enough radio frequencies, even a short length of wire has significant impedance, and so your guitar lead outer screen will provide sufficient impedance at radio frequencies, to allow the back-plate to act as an aerial.

With the cavity non-conducting, any rf signal picked up by the back-plate will be capacitively coupled to the circuit board. But if the cavity is at the same potential as the backplate, there is zero electric field inside the cavity, and if the field is zero then there is no charge transfer between the 'Faraday cage' and the circuit board, so capacitively coupled rf noise is minimized. I hope this explanation helps.

Personally, I would avoid using conducting paint, and go for the copper foil solution - just my preference.

[newey]

Apr 25, 2024 15:27:38 GMT -5 unreg said:

Could the existing (now presumed non-conducting) conductive paint mess with a new coating of conductive paint, if it turns out that parts of the existing paint is still conductive?

Either something conducts well (only a few Ohms resistance at most) or it does not. Adding more conductive paint shouldn't increase the resistance.

[unreg]

newey, I didn’t say that.

[newey]

Yeah, that was guitarnerdswe. Not sure how I screwed that up . .

[stevewf]

Apr 25, 2024 15:45:58 GMT -5 mikecg said:

Personally, I would avoid using conducting paint, and go for the copper foil solution - just my preference.

Hey mikecg, can you say why? I'm considering using paint on the next guitar, after having always used copper foil. Thanks.

[mikecg]

Hello stevewf,

Well, lets take a look at the 'sheet resistance', for a start:

1 oz copper foil is around 34 micrometres thick and has a sheet resistance of around 0.0005 Ω/square.

The 'Cat Music' conductive paint claims to provide a sheet resistance of around 8 Ω/square.

Lower is better!

Next, making a reliable connection:

No problem with copper foil - just spot solder all the pieces together, and solder to a ground lead. If the control pots are supported by the cavity they will also be in direct contact with the foil.

Connecting a ground lead to conductive paint needs to be done with care. A small wood screw and a solder tag will work, but don't over-tighten the screw, or you may end up with the solder tag slightly below the paint film, having made a slight depression in the wood surface, and having 'punched through' the thin conductive paint layer, rendering the connection open, or at best, high resistance. Probably best to always overcoat the screw/solder tag area with a decent 'blob' of conductive paint, in an attempt to mitigate this problem.

Next product reliability:

Copper is copper - you can see what you are buying, and there is a good chance that it will work first time.

Paint can be anything - you won't know what you have until you apply it - and if it fails - things could get very messy.

And Finally:

I have a roll of copper tape that will probably see me out, and I don't have any conductive paint!