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Screening / Shielding a Rear Routed Strat

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Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Tue Aug 28, 2018 2:24 pm

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...
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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Tue Aug 28, 2018 3:01 pm

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.
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Re: Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Tue Aug 28, 2018 5:19 pm

Thanks Wonks - I'll post a picture - as soon as Imgur is up and running again...
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Re: Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Tue Aug 28, 2018 7:41 pm

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?

Image

Image

Image

Image
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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Tue Aug 28, 2018 9:39 pm

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.
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Re: Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Tue Aug 28, 2018 10:51 pm

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...
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Re: Screening / Shielding a Rear Routed Strat

Postby Studio Support Gnome » Tue Aug 28, 2018 11:21 pm

you're the noise source.


when touching grounded parts, it's taking your output to ground..... \
\
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Re: Screening / Shielding a Rear Routed Strat

Postby zenguitar » Tue Aug 28, 2018 11:44 pm

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 :beamup:
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Re: Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Wed Aug 29, 2018 6:43 am

zenguitar wrote: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 :beamup:

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)?
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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Wed Aug 29, 2018 8:16 am

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?
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Re: Screening / Shielding a Rear Routed Strat

Postby ef37a » Wed Aug 29, 2018 8:21 am

Are you sure the AMP is actually earthed?

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Re: Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Wed Aug 29, 2018 9:16 am

Thanks Wonks and Dave

All my other guitars (including two strats and two Les Pauls) are dead silent.
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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Wed Aug 29, 2018 10:19 am

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?
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Re: Screening / Shielding a Rear Routed Strat

Postby Hewesy » Wed Aug 29, 2018 10:32 am

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.

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Re: Screening / Shielding a Rear Routed Strat

Postby Hewesy » Wed Aug 29, 2018 10:33 am

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?

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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Wed Aug 29, 2018 10:49 am

They are Kinmans, so should be hum-cancelling pickups. Unless the winding machine added the wrong number of turns to the cancelling coils.
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Re: Screening / Shielding a Rear Routed Strat

Postby Jay Menon » Wed Aug 29, 2018 11:01 am

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 ...
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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Wed Aug 29, 2018 11:16 am

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Re: Screening / Shielding a Rear Routed Strat

Postby Wonks » Wed Aug 29, 2018 11:27 am

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!)
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Re: Screening / Shielding a Rear Routed Strat

Postby zenguitar » Wed Aug 29, 2018 11:29 am

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.

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