Jazzmaster Hum Issue - Help an electronics doofus out

[bierce85]

I'm at my wit's end with this one - The harness "works" as intended however when in the lead circuit the bridge and middle positions hum louder than the actual pickup output. The neck pickup is also very noisy compared to my telecaster, but not nearly as loud as the bridge/middle. Touching either of the pickups also increases the volume of the hum.

The odd thing is if I pull the pickguard off the body and move it around at different angles at times I can get all 3 positions whisper quiet, with the middle hum canceling as it should, and touching the pickups now decreases the noise as it should. The problem is I have no idea why moving it around is temporarily fixing the problem. I've tried wiggling every little connection and nothing exposes itself as the culprit. Here's some pics of the harness. This is a 62-63 style inca silver build I completed recently with Antiquity I + harness I had lying around which I purchased from an eBay seller (1469 music). I have the strings grounded by running a wire into the tailpiece route and clamping it into the vibrato rather than running a wire to the bridge thimbles, but I don't think that's the issue as wiggling that wire doesn't change anything.

Some pics:

[Embenny]

The odd thing is if I pull the pickguard off the body and move it around at different angles at times I can get all 3 positions whisper quiet, with the middle hum canceling as it should, and touching the pickups now decreases the noise as it should. The problem is I have no idea why moving it around is temporarily fixing the problem.

It sounds to me like a component (like a potentiometer or switch lug) is touching your shielding when you close everything up. The fact that it's quiet and working as intended when taken apart, then humming and behaving oddly when closed back up again sounds like it pretty much couldn't be anything else.

You have to figure out what is touching where...and then either rotate some pots/components, or just place an electrical insulator in between the two (like some electrical tape on top of the shielding in the spot that's causing the issue, to keep it from making contact).

A lot of the time, guitars can end up with a component touching against the wall of the cavity somewhere when the pickguard in finagled into place to line up all the screw holes. That only becomes a real issue when the walls of the cavity suddenly become electrical conductors

[timtam]

To be clear, is it consistently quiet off the body, or is the bad noise still sometimes there, no matter what wires you jiggle ? FWIW, I put heatshrink over all exposed wire at pot/switch terminals, and over the cap legs, especially in guitars with cavity/pickguard shielding that have a high potential for shorts. The vendor who wired your harness in "vintage-style" used techniques that were OK in the 1950s, but not how a beginning electronics student would be taught to wire it today.

[bierce85]

It sounds to me like a component (like a potentiometer or switch lug) is touching your shielding when you close everything up. The fact that it's quiet and working as intended when taken apart, then humming and behaving oddly when closed back up again sounds like it pretty much couldn't be anything else.

To clarify the whole thing hums when the pickguard is pulled off as well, but I'll move it around and sometimes its quiet. So the problem does not only occur when installing the pickguard on the body. I don't really understand the "touching the shielding" thing as in the past the only problem that's ever caused for me is the whole thing grounding out/going quiet and that's easy to isolate and fix. Why would it cause a loud hum?

[bierce85]

To be clear, is it consistently quiet off the body, or is the bad noise still sometimes there, no matter what wires you jiggle ? FWIW, I put heatshrink over all exposed wire at pot/switch terminals, and over the cap legs, especially in guitars with cavity/pickguard shielding that have a high potential for shorts. The vendor who wired your harness in "vintage-style" used techniques that were OK in the 1950s, but not how a beginning electronics student would be taught to wire it today.

It hums off the body as well, but I'm able to wiggle it around and get it to shut up periodically until it moves again.

[timtam]

OK, so the problem is a bit more complicated. Trouble-shooting is always more systematic if you understand how the circuit works, which requires a schematic (not a wiring diagram) - see below. That shows clearly how the lead and rhythm circuits are almost entirely separate, save for sharing the neck pickup, ground, lead/rhythm switch, and the output jack. So whether you have issues in just one circuit or both often leads you to just one circuit's components or the shared ones as the problem source.

So re-do your testing with just the lead or rhythm circuit selected. See if you can isolated the hum scenarios to just one circuit, or both. If the problem is isolated to one, work through the wiring of each of the components in just that circuit. If it's in both, work through the shared components. From what you said originally, the bridge pickup/wiring sounds like it might be implicated - if so, the problem would be just in the lead circuit, and with just that pickup selected. If that's the case, un-screw/disconnect it - any change in noise for the remaining circuitry ? Connect it directly to the guitar cable (eg with alligator clip test leads) - does it hum too much ? But if none of that points to that pickup being the real problem source, adjust your strategy according to the above principles.

[ChrisDesign]

Hmmm. You have a great problem.

Do you have another guitar to confirm the hum is from the guitar and not some problem in your rig? A ground loop maybe? You said the problem is with the least circuit, so I guess that throws that option out.

Do you have a multimeter? Does all of your shielding tape form a conductive link to the output jack? Check all cavities and metal hardware (e.g. Bridge and tailpiece) connect to the output jacks ground. Cold solder joints can wreak havoc where there is no connection but the joint looks ok.

If it was me, I would throw the pots, switches, wiring, and jack in the bin. I would buy new CTS pots and carefully wire everything up from scratch. That is probably the wrong approach, but it's all I can think of.

[Embenny]

It sounds to me like a component (like a potentiometer or switch lug) is touching your shielding when you close everything up. The fact that it's quiet and working as intended when taken apart, then humming and behaving oddly when closed back up again sounds like it pretty much couldn't be anything else.

To clarify the whole thing hums when the pickguard is pulled off as well, but I'll move it around and sometimes its quiet. So the problem does not only occur when installing the pickguard on the body. I don't really understand the "touching the shielding" thing as in the past the only problem that's ever caused for me is the whole thing grounding out/going quiet and that's easy to isolate and fix. Why would it cause a loud hum?

Oh, I misunderstood, then.

Having a loud hum that gets louder when you touch the pickups sounds like two potentially separate issues: 1) That components in the guitar that have been wired together with the intention of being grounded are not, in fact, connecting to ground (via the output jack, which is connected to your amp's ground, which is actually grounded). 2) Some of those components may be getting inserted accidentally into the "hot" signal path, which is why I thought they might be touching the shielding. Touching the shielding only results in "grounding out" (short circuiting to ground) if that shielding is actually grounded. If the shielding isn't connecting properly to ground, then touching something in the signal path places it into the hot signal, which adds tremendous amounts of noise (I did this one, with Jaguar claws).

This is one of those cases where you're just going to have to break out the multimeter and start checking EVERYTHING for continuity. Something either isn't connecting where it should, is connecting where it shouldn't, or you've got a malfunctioning component.

It sounds like your rhythm circuit is working properly though? That at least narrows down the path you need to be scrutinizing. It means your output jack is properly connecting to ground, and there's nothing wrong with the neck pickup itself.

Re-reading your description, I've still got a gut feeling that the shielding is getting connected to the hot signal somehow. The fact that moving the pickguard around allows you to find a "quiet" orientation (even when in the middle, hum cancelling position of the pickup selector) makes it sound like your shielding is behaving as an antenna, sort of like how single coil pickups hum more or less depending on their orientation.

[andy_tchp]

Having a loud hum that gets louder when you touch the pickups sounds like two potentially separate issues: 1) That components in the guitar that have been wired together with the intention of being grounded are not, in fact, connecting to ground (via the output jack, which is connected to your amp's ground, which is actually grounded).

Reviewing the pictures I would believe this to be the case, and there's a few issues here.

• You have the cavities lined with copper foil, which the ground leads of the pickups are soldered to(*).

• However, there is no visible 'overlap' of the cavity shielding onto the front/face of the body to connect this to the shielding on the back of the pickguard (and through this the switch/pot/jack bodies). It all needs to be connected, otherwise it's an antenna as Mike suggested. In this case the cavity shielding is the antenna, and it's in the circuit via it's connection to the ground lead on the pickups.
  
  EDIT: I just had another look and zoomed in - is that a wire running from the rhythm circuit mounting bracket into the cavity shielding? If so theoretically that would/should ground everything, but I dislike the concept of soldering to foil and would ditch this wire in favour of overlapping the foil onto the front of the body.

Also worth checking:

• Depending on how thick the foam is under the pickups (and whether it follows the footprint of the whole pickup) when they (the pickups) are installed and tightened down, there's potential for either/both of the pickup leads to intermittently/partially contact the cavity shielding due to the way the leads exit downwards from under the bottom bobbin (which often ends up being a very short section of uninsulated wire if they're cloth covered type). Check this too 'just in case' and ensure that at the very least the hot pickup lead isn't at risk of grounding out.

• There's very little clearance for the 3-way switch in the cavity when everything is assembled, particularly if the solder terminals on the switch were splayed outwards to give yourself a bit more space while wiring everything up. When the cavity is fully shielded with foil (like in the pics) I've encountered the solder terminal for the neck pickup's hot lead making contact with this shielding, which leads to a 'no output' condition when the neck pickup or middle position is engaged (and much frustration/confusion, especially when all of the wiring was tested and working perfectly before screwing the pickguard down and stringing up the guitar )
  
  
  *(Don't do it this way BTW, solder to an actual ground terminal on the 3-way switch or some other convenient spot that's actually designed to be resilient mechanically when soldered to).

[andy_tchp]

If it was me, I would throw the pots, switches, wiring, and jack in the bin. I would buy new CTS pots and carefully wire everything up from scratch. That is probably the wrong approach

Yeah.

It is.

Over a grounding/circuit completion issue you're suggesting that the OP throw away the brand new CTS pots he/she already has on hand, and purchase...new CTS pots. While also throwing away the (usually) failure-proof for life Switchcraft switches and output jack.

[Embenny]

Having a loud hum that gets louder when you touch the pickups sounds like two potentially separate issues: 1) That components in the guitar that have been wired together with the intention of being grounded are not, in fact, connecting to ground (via the output jack, which is connected to your amp's ground, which is actually grounded).

Reviewing the pictures I would believe this to be the case, and there's a few issues here.

• You have the cavities lined with copper foil, which the ground leads of the pickups are soldered to(*).

• However, there is no 'overlap' of the cavity shielding onto the front/face of the body to connect this to the shielding on the back of the pickguard. It all needs to be connected, otherwise it's an antenna as Mike suggested. In this case the cavity shielding is the antenna, and it's in the circuit via it's connection to the ground lead on the pickups.

...
*(Don't do it this way BTW, solder to an actual ground terminal on the 3-way switch or some other convenient spot that's actually designed to be resilient mechanically when soldered to).

I read this thread on my phone, so I admit I didn't inspect the photos but wow, good catch. Grounding pickup leads to cavity shielding is not something you should ever do. Like you said, using a ground terminal or the case of a switch or volume pot is the way to go, since they're mechanically reliable points to use.

You also identified that the cavity shielding doesn't go up onto the guitar top to connect to the pickguard shielding (which is how I like to do it), but there's a chance the pickguard shield is grounded via the case of the pots and switch if it goes right up to the edge of those components.

As always, a multimeter is your best friend. If there's continuity from pickguard shield to the case of the volume pot, and from that case to the output jack ground, it's fine.

[andy_tchp]

You also identified that the cavity shielding doesn't go up onto the guitar top to connect to the pickguard shielding (which is how I like to do it), but there's a chance the pickguard shield is grounded via the case of the pots and switch if it goes right up to the edge of those components.

As always, a multimeter is your best friend. If there's continuity from pickguard shield to the case of the volume pot, and from that case to the output jack ground, it's fine.

The pickguard shielding will 100% be grounded via the pot casings/output jack body, but the cavity shielding is still suspected as 'floating' if it's not contacting the pickguard cleanly.

I've since made a quick edit to my post as it looks like there might be a wire from the rhythm circuit bracket soldered to the foil in the rhythm circuit cavity that would join the two shields, awaiting clarification (and yep, a thorough check for continuity with a DMM )

[bierce85]

I've since made a quick edit to my post as it looks like there might be a wire from the rhythm circuit bracket soldered to the foil in the rhythm circuit cavity that would join the two shields, awaiting clarification (and yep, a thorough check for continuity with a DMM )

You are correct, there is a ground wire running from the bracket. Here's another angle:


As for soldering to the foil - I only did that after reviewing a bunch of photos of old Jazzmasters, but perhaps that's different since they used a more robust shielding plate?

[bierce85]

Thanks guys for all the input - I'm going to run some more foil over the top of the body tomorrow and see if that changes anything!

[andy_tchp]

Yeah, those are actual brass tubs/plates on vintage guitars so a bit thicker and more able to 'take' (/retain) solder. And timtam raised an excellent point about stuff deemed acceptable in the 1950s that isn't today.

First I would thoroughly investigate any potential accidental shorts to ground through the bottom of the pickups where the wires exit/3 way switch (or anything else at all 'hot' that goes near to the shielding), followed by ditching the wire from the rhythm circuit bracket to the tape and getting a decent overlap happening onto the top of the guitar body, then get those pickup ground leads and vibrato ground wire soldered to something substantial (doesn't matter what, ground is ground, provided they are all properly connected.)


(And the copper tape you're using definitely has conductive adhesive, right?)