Jazzmaster Hum Issue - Help an electronics doofus out

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bjornsynneby
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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by bjornsynneby » Tue Aug 25, 2020 12:59 am

You seem to have the pickups grounded to the cavity and the rest of the circuit is at the pickguard which makes it essential that the both shields (cavity and pickguard) are well connected in order to get full signal. If you have bad contact between the shields it will affect the signal. I would suggest another way of grounding your guitar in order to make your guitar work as it should without hum.

Grounding pickups to the shield creates a ground loop which acts like an antenna. This antenna taps in to electromagnetic waves that comes from dimmers etc. Make all ground connections go to one point - maybe the volume pot? This is called star grounding and done correctly it eliminates all ground loops which are causing hum.

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timtam
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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by timtam » Tue Aug 25, 2020 1:19 am

bjornsynneby wrote:
Tue Aug 25, 2020 12:59 am
Grounding pickups to the shield creates a ground loop which acts like an antenna. This antenna taps in to electromagnetic waves that comes from dimmers etc. Make all ground connections go to one point - maybe the volume pot? This is called star grounding and done correctly it eliminates all ground loops.
Star grounding in amps, and a single mains power source for amps/pedals/etc, are required because there can be a potential difference (ie a voltage difference) between different ground points in those circuits - that is, all grounds may not be at exactly 0.0 volts. If those ground points are linked in a loop/circuit, current can flow in the ground circuit because of that potential difference, producing hum. In contrast, all grounds in a guitar share the same ground potential - that at the output jack. Ergo you can't have a hum-producing ground loop in a guitar, even if you have a physical loop.

JMs / jags have had pickup grounds wired via the shields since the beginning. While copper foil may be a somewhat less secure basis for the same approach, there is nothing fundamentally unsound about it if you can implement it securely/continuously - ground is ground.
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Having said all, I quite like star grounding. But not for the reason that it has any influence on ground loops in guitars. I just think it provides a much neater solution to grounding than having a pile of ground wires all linked to the back of pots (one of my pet hates ... such a 'quick and dirty' 1950s technique, that you don't see anywhere else in electronics). eg Am Pro jag ...
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Last edited by timtam on Tue Aug 25, 2020 1:43 am, edited 1 time in total.
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bjornsynneby
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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by bjornsynneby » Tue Aug 25, 2020 1:38 am

I tried both. Eliminated hum with star grounding in my guitar. It might be that it is just bad contact between the shields that is causing the hum though. Anyone interested in hum cancelling in guitars can read this. https://www.fralinpickups.com/2018/11/1 ... grounding/

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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by timtam » Tue Aug 25, 2020 1:50 am

bjornsynneby wrote:
Tue Aug 25, 2020 1:38 am
Anyone interested in hum cancelling in guitars can read this. https://www.fralinpickups.com/2018/11/1 ... grounding/
A lot of hearsay and half-understood electronics there (like some of the other material written by the Fralin web guy on that site). He tells someone in the comments that noise that lessens when they touch the metal pickup cover means that there is a ground wire missing - 100% wrong. Someone in the comments challenged them to prove the ground loop theory and they declined, even though it would be dead easy. Just take one of the many Telecasters and Jaguars that have a physical ground loop - a wire link between pots and a metal control plate linking the pot bodies. Record it as is, then cut the wire link between the pots. If the ground loop was hum-producing there would be a reduction in hum.
"I just knew I wanted to make a sound that was the complete opposite of a Les Paul, and that’s pretty much a Jaguar." Rowland S. Howard.

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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by bjornsynneby » Tue Aug 25, 2020 2:56 am

Hehe. Yes I have tried that too on all the telecasters I’ve built. My suggestion is to move the two ground cables of the pickups to the back of the volume pot or at least to the pick guard shield. If hum goes down - fine.

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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by bjornsynneby » Tue Aug 25, 2020 4:17 am

If you want to keep the wiring as it is and not enter the mad scientist world of trail and error I suggest you put some strips of conductive tape that are going above the edge of the cavity shield that can make a mechanical connection with the pickguard shield. I think they are called glacier points in the diagram posted by timtam.

Also sometimes when you tape the shielding like this there can be lacking continuity between patches of tape. My experience is that one have to press the tape hard when taping. When this is done you can check for continuity with a multimeter. You can check between the output jack and different points of the shields. And ofcourse between the bridge pickup ground and the output jack ground but that will be hard since you have to connect the both shields to try this.

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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by jvin248 » Tue Aug 25, 2020 5:00 am

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Best practices when shielding a guitar:

-Run cavity tape to the top edge of the cavities so when the pickguard is screwed down you have an enclosed cage. See pictures below. Best to have those tabs where there is a screw hole so the pickguard screw can clamp down for better contact.

-Run one wire from the back of the volume pot to connect all other ground wires, solder them together and wire nut+tape/shrink tube. Or solder all the wires to the back of the pot. Sand a spot on the back of the pot, put some solder there, put solder on the ends of the wires, only then solder the wire(s) to the pot. That way no overheating and messy soldering problems.

-Replace the wires from the volume pot to the jack with shielded cable, especially if the jack is outside the faraday cage of tape shielding (Strats for example), some guitars this can be 50% of the noise floor. No, twisting those two wires does nothing. Scavenge old RCA "VCR" hookup wires if you need to. Would you run a pair of wires from the guitar to the amp, or do you buy that fancy shielded cable to get there?

-When you touch the strings/etc of the guitar and it sounds like you are holding the bare end of the amp cord in your hand -- you have the hot circuit touching the ground circuit. How close is the tip of the jack to the shielding when installed in a JM? When touching the guitar and the hum drops, that is normal. Just the way it is.

-If you 'move things around' and the ground problem gets better or worse, whatever you moved around has the problem. Look there! As noted in another post, electrical tape can insulate against some component touching, but if the problem is a pot or switch lug then over time the lug will wear through the tape -- best to bend the lug or rotate the pot some so no contact can happen.

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Once you get your basic system sorted out so it works.... You might want to investigate lining the inside of the pickup covers with foil and running a jumper from those to the pickguard shielding. You need to be extra careful not to nick the pickup bobbin wires while doing this or you'll kill a pickup. You also need to be careful not to short out any solder points. This can protect the more vulnerable JM "antennas" (they cover a large area compared to a narrow Strat pickup) from receiving unwanted signal. This is an advanced level of shielding with higher risks so proceed with caution.

You can also search around the internet and find research articles showing aluminum Faraday cages actually work better than copper. Copper is traditional for guitars and looks luxurious in glamour shots but an inexpensive roll of Nashua Aluminum Flashing tape is about $7 with enough content to shield two dozen guitars works too, or per the research, works even better. Wrinkle the edges and check continuity as you put it down, same as you might do with copper.

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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by bierce85 » Wed Sep 09, 2020 11:13 am

UPDATE - Ran a ground wire today connecting the pickups and tailpiece ground to the volume pot and the guitar now seems to operate normally. The pickups are still really noisy but totally silent in the middle position. I still get a buzz through the amp if I touch the pickup pole pieces but that's only if my hands arent grounding the guitar by touching the strings or any of the metal components. Pretty sure thats normal?

Anyone else feel like their Jazzmaster pickups are significantly noisier than tele/strat etc? Maybe antiquity I's are particularly noisy?

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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by Bradley-Jazz » Wed Sep 09, 2020 11:25 am

Yes, it’s my experience that JMs are typically nosier than strats and teles. Big wide pickup coils seem to pick up electromagnetic noise more.

Sounds like you’ve got it fixed - nice one!
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Embenny
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Re: Jazzmaster Hum Issue - Help an electronics doofus out

Post by Embenny » Wed Sep 09, 2020 11:33 am

Yep, the fat/wide/shallow coils are ideally designed to pick up noise. Maximal surface area for minimal signal to noise ratio compared to other single coil designs.
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