Screening / Shielding a Rear Routed Strat

[Jay Menon]

The control cavity: shielded comprehensively with good quality conductive graphite paint.

The control cavity cover: is also shielded with copper foil and I’ve ensured electrical continuity, by strips of foil that curved over the edge of the cavity and are priced by the cover screws...

Interestingly
Pickup selector switch in bridge position: very mild buzz when I take my hands off the strings/bridge.

However there is significantly more buzz with the pickup selector in the middle position and a lot more buzz with the pickup selector in the neck position.

It’s almost as if the longer cables from the neck pickup are attracting electromagnetic interference. But from Kinman I understand that the pickup cables are shielded...

Is there a simple solution please?

Should I be painting the floor and walls of the pickup cavities? Sure that would leave the 'roof' open - no real Faraday cage created there...

[Wonks]

Yes, the cable from the pickups on Kinmans is the single conductor + braid type, which should minimise noise pickup. First step is to use a multimeter to check that all the ground connections are continuous and with very low resistance readings.

Any area coated with conductive paint and connected to ground will help suck up RF noise that's around, it's not going to make matters worse and can only help. yes, the pickups themselves are sticking out of the cavity shielding but they should be hum-cancelling pickups, so you shouldn't be suffering from 50Hz and 100Hz hum issues.

Mabe you could post a picture of how the pickup wires are connected - especially how their braid screens are grounded.

[Jay Menon]

Thanks Wonks - I'll post a picture - as soon as Imgur is up and running again...

[Jay Menon]

Here are a few pictures:

I did a continuity /resistance test with a multimeter. Jack to jack cable connected to the output socket.

With my multimeter set to 200 ohms, between two of the screws on the rear control cavity plate and the earth of the Jack, I’m actually getting proper continuity (7Ω and 13Ω) with the accompanying audible beep on my multimeter. These are the screws that connect with the folded over pieces of copper foil (At 12 o’clock and 3 o’clock positions in the photograph). On the other two screws I’m getting 28.5Ω and 138.2Ω.

I believe that would indicate that the graphite screening of the control cavity is reasonably comprehensive?

[Wonks]

All looks like it should be to me. The pickup cable screens are all soldered to the back of the volume pot, which looks to be connected to the output jack ground.

The tone pot casings look to be are soldered to the back of the volume pot via wires on the sides of the pots.

Screening paint DC resistances are fine (up to 1meg ohm works for this) and the impedance should be less for AC signals.

Is there a ground wire to the trem claw? I can't see one in the photo but part of the volume pot is hidden and some of the claw is out of the photo so it may well be. Whilst low-impedance pickups like EMGs don't need the strings grounding, Kinmans are still high-impedance pickups and still benefit from this.

[Jay Menon]

Yes Wonks, there is a ground wire from the volume pot to the Tremolo claw - and there is also electrical continuity between those two structures as tested with a multimeter.

So I’m a bit lost as to why I still get buzz when I take my hands off the strings / bridge...

[Studio Support Gnome]

you're the noise source.

when touching grounded parts, it's taking your output to ground..... \
\

[zenguitar]

My gut instinct is that the problem lies with the ground from the trem claw.

20 years ago I would spend hours with a multimeter trying to chase down these grounding problems. Then, in frustration I would rewire the loom anyway and the problem was fixed.

Now, Wonks is far more knowledgable with a multimeter than I am so if my theory is wrong I am more than happy to reconsider, but my thoughts are along these lines; the multimeter uses a 9V battery to test continuity but guitar pick-ups output in the mV range, so a little resitance that wouldn't trouble a 9V signal would have an affect on a much smaller signal.

So for a couple of decades I've rarely wasted time with the meter and just remade the loom with new hook-up wire and desoldered and cleaned the pot/switch wiring tags with a solder sucker, solder mop, and a file. The exceptions are when that would devalue a collectable instrument.

A couple more thoughts...

When checking the string ground don't forget the mechanical connections as well as the soldered ones. Some trem springs and trem blocks can tarnish which can add resistance. So it's worth giving them a brush up with wire wool where they make contact.

Servicing the pots/switches/jack socket with De-Oxit, Super Servisol 10 or similar (and even wet & dry paper in the jack socket) is always worth a try.

Andy

[Jay Menon]

My gut instinct is that the problem lies with the ground from the trem claw.

20 years ago I would spend hours with a multimeter trying to chase down these grounding problems. Then, in frustration I would rewire the loom anyway and the problem was fixed.

Now, Wonks is far more knowledgable with a multimeter than I am so if my theory is wrong I am more than happy to reconsider, but my thoughts are along these lines; the multimeter uses a 9V battery to test continuity but guitar pick-ups output in the mV range, so a little resitance that wouldn't trouble a 9V signal would have an affect on a much smaller signal.

So for a couple of decades I've rarely wasted time with the meter and just remade the loom with new hook-up wire and desoldered and cleaned the pot/switch wiring tags with a solder sucker, solder mop, and a file. The exceptions are when that would devalue a collectable instrument.

A couple more thoughts...

When checking the string ground don't forget the mechanical connections as well as the soldered ones. Some trem springs and trem blocks can tarnish which can add resistance. So it's worth giving them a brush up with wire wool where they make contact.

Servicing the pots/switches/jack socket with De-Oxit, Super Servisol 10 or similar (and even wet & dry paper in the jack socket) is always worth a try.

Andy

Thanks Andy - does all this apply even though the guitar otherwise 'works' (apart from the buzz when I take my hands off the strings / bridge)?
Jay

[Wonks]

One question to ask is whether a standard humbucker equipped guitar is any noisier in the
same location?

Whilst the extra hum-cancelling coil on each pickup can do a lot to reduce the hum level, close to a noise source where the noise field strength can vary considerably within an inch or so, they will always let a little hum through. Further away from the source, apart from the field strength being a lot less to start with, the difference in the field strength between the two coil locations is far less, so hum levels are almost equal and almost all of it cancels out.

If you rotate your normal playing position by 180°, does the hum level per coil change at all?

[ef37a]

Are you sure the AMP is actually earthed?

Dave.

[Jay Menon]

Thanks Wonks and Dave

All my other guitars (including two strats and two Les Pauls) are dead silent.

[Wonks]

It's a hard one to diagnose further without having the guitar in front of you. It may be that re-doing all the solder connections will reduce the hum levels. Or it may be an issue with the pickups themselves and the hum cancelling coils aren't working that well on the middle and neck pickups. The pickups do have screened cables, which should work as well as shielding paint would.

What actual model of Kinman pickups do you have installed? Have you checked online to see if anyone else has reported them as relatively noisy?

[Hewesy]

Just a random thought, does the hum lower/diminish if you roll the tone right off?

I added some rather lovely handmade P90's to an electric 12 string earlier this year and had horrible issues with noise. Rolling the tone right off saw the hum vanish - opening the pot up added it back in.

I went nuts trying to chase it out of the wiring - eventually after the help of the legends that are Wonks and Zen realised I was on a hiding to nothing and just kept the tone pots dialled back. One thing I did do though was shorten the wiring between the pickups and selector switch, being a Jazzmaster body style there was a long cable span which I'm sure was adding some noise in. Not applicable to a Strat but just to mention for any random Google searches that might read this...

Now it's usable, and for my style (and probably the fact it is a 12 string so inherently bright) rolling the pots back was livable.

Just a thought.

Hewesy

[Hewesy]

As an aside as Wonks says above, if they are hum cancelling this should see those positions reduce/remove the noise.

Unless they are the vintage style singles with no hum cancelling?? Which is the issue I had with my P90's as above.

What model are they? New or used?

Hewesy

[Wonks]

They are Kinmans, so should be hum-cancelling pickups. Unless the winding machine added the wrong number of turns to the cancelling coils.

[Jay Menon]

I’m not experiencing 50Hz hum at all. The pickups are working fine - brand new set of Impersonator 54s.

What I am experiencing is inductive buzz when I take my hands off the bridge/strings ...

[Wonks]

So these - for anyone interested.

https://kinman.com/model-products.php?p ... %20Pickups

[Wonks]

So I'd paint the pickup cavities with the conductive paint - not forgetting that this will need to be grounded somehow to work best.

(Just realised that it was solder and not missing tape on the control cover - d'oh!)

[zenguitar]

As Wonks said, it's hard to be definative when you don't have the guitar infront of you.

It might just be a poor string ground, but standard guitar wiring is seriously wishy-washy. Standard circuits from the original 50's designs have been improved with screening tape and paint, so you end up with multiple ground paths together with a bunch of inductors, resistors and capacitors that interact in all sorts of ways that can vary depending on how switches and pots are set.

That's why it's so frustrating trying to chase these problems down.

Andy

[Jay Menon]

Thanks everyone, for being so helpful.

Before I unsolder all the connections and put it together again - and just to make things clear ...

The guitar works fine, the pickups sound beautiful. My only problem is that when I take my hands off the strings/bridge the guitar starts buzzing.

Is this symptomatology consistent with a faulty earth connection to the spring claw? If so, then I shall take everything apart and put it together again (painful job).

[zenguitar]

That's the problem Jay, there's just no way to offer you any certainty. Such are the whims of passive guitar wiring.

The only possible additional test I could suggest would be a lead with a crocodile clip on one end connected to the sleeve of a 1/4" jack plug at the other. You could clip it to a string at the headstock and plug the jack plug into an amp or mixer. That would give you a direct string ground. If that resolves the problem when you take your hands off the strings you can be confident you have a string ground problem. But It won't tell you exactly what the details of the problem are. However, if you have a flux pen it would be worth reflowing the string ground connection to the back of the pot first.

Now I hope you see the frustration that lead me to my practice of pulling out the loom, cleaning and servicing the pots, sockets, and switches and rewiring from scratch. In 20 plus years that has only failed me once (and I am convinced that the niggling low level buzz on that strat is more to do with the roller nut, on paper it can take 11's but I have a feeling that a mechanical buzz too low to pick up by ear is affecting the string vibration just enough to come through on the pick up when you up the gain).

Every other time, it has always been quicker to rewire than spend hours with a meter trying to track down the specific cause.

Good luck.

Andy

[Jay Menon]

Thanks Andy and Wonks - I can totally see your points...

A scratch plate based pickup set is so much easier to re-wire - but I'll get down this weekend and re-do this guitar, hopefully things will work better. I'll also use the opportunity to re-paint the pickup cavities.

Is shielding the pickup cavities standard practice on a Les Paul for instance? (I ask since an LP is course conceptually similar to a rear routed strat)

[Wonks]

As the buzz comes from taking your hands off the strings, that shows that the strings are grounded properly. Your body is being given a good connection to ground, and either a) any electrical noise your body is picking up/giving off is taken to ground so it doesn't affect the pickups or b) your body now acts as an effective soak for a lot of the EMI floating around (or a mixture of both).

Shielding the pickup cavities isn't standard for a Les Paul (though it's a good idea for those fitted with P90s). Gibson keep changing their control cavity arrangements, but on my very new 2018 LP Classic Goldtop (with P90s), they've got the pots connected to a metal shield plate (like they have done in the past), which helps. they've also added a small tag board, which minimises the soldering they have to do to the back of the pots.



With covered pickups with shielded cables, there's not a lot of point in adding more shielding to the pickup cavities. Even with uncovered humbuckers, you normally don't get enough noise problems to make shielding the cavities necessary. Also on most guitars with non scratchplate mounted pickups, it's near impossible to shield the cable passageways to them, so you'll never manage complete shielding coverage. But every bit of extra shielding will bring the noise levels down slightly, so if you are getting noise issues, then it's certainly worth adding as much shielding as you can.

The noise has to come from somewhere though, so unless you are near to a powerful radio mast, it's worth checking around your house for any sources of electrical noise which can be turned off, especially when recording, e.g. dimmer switches and fluorescent lights.

[Hugh Robjohns]

My only problem is that when I take my hands off the strings/bridge the guitar starts buzzing.

Picking up on a point Max (SSG) made in his characteristically brief and enigmatic way earlier, the problem could be that the issue here is actually the grounding (or lack of it) of you as the player.

For example, when you touch the (already effectively grounded) strings your body will become grounded, and thus act as an additional shield around all the guitar's sensitive high-impedance bits. But when you become ungrounded you lose that shielding ability and/or you may even act as a aerial to inject noise into the back of the guitar.

You could do some informative experiments with a separate grounding lead onto your wrist, for example, and/or with a grounded sheet of tin foil between you and the back of the guitar to see what effect that has on your unwanted noise problem. That might then give some clues as to the direction to move in to fix this...

H