Username: Password:

Author Topic: Cavity screening issue - not sure what the problem is, static?  (Read 2536 times)

Twinfan

  • Light Heavyweight
  • ******
  • Posts: 10528
OK, here's the scoop.

I have a guitar with a screened control cavity, where screening foil has been used throughout.  The floor and walls are covered, as is the 'ledge' at the top which the cavity cover sits on.  The rear of the plastic cover is also completely covered in the screening foil, which touches the foil on the 'ledge'.  So I have a completely screened cavity - floor, walls and roof.

With the guitar plugged into an amp, if I touch the cavity cover or screw heads with anything I get a 'crackle' which sounds a bit like static.  Not a problem when I'm playing, but if I'm between songs and the guitar brushes my leg it's really annoying!

Any idea what's the cause and fix?  I've never had this problem before, but I've never had a guitar that's been screened on the back of the cavity cover before.  Is this the problem???

hermetico

  • Guest
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #1 on: April 21, 2010, 03:38:01 PM »
Ok. Your cavity is well shielded but... did you ground it?
I mean, you have to put some screw in one wall, and throw a jumper from there to your common grounding spot.

Twinfan

  • Light Heavyweight
  • ******
  • Posts: 10528
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #2 on: April 21, 2010, 03:52:45 PM »
Good point - I'm not sure.  I guess I need to check!

Philly Q

  • Light Heavyweight
  • ******
  • Posts: 18109
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #3 on: April 21, 2010, 04:26:27 PM »
But if the metal parts of the pots/switch are in contact with the shielding foil, the foil should already be grounded (I agree a wire would make 100% sure, belt and braces).

It's crackling when you touch the plastic backplate, which suggests static rather than a shielding issue as such.  You'd think the backplate would be in contact with the foil somewhere, but maybe it's insulated by the glue sticking the foil to it.

This may sound silly, but I've heard that when you have static crackle from a plastic scratchplate, wiping it with an anti-static laundry sheet (e.g. Bounce) will fix it!
BKPs I've Got:  RR, BKP-91, ITs, VHII, CS set, Emeralds
BKPs I Had:  RY+Abraxas, Crawlers, BD+SM

Twinfan

  • Light Heavyweight
  • ******
  • Posts: 10528
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #4 on: April 21, 2010, 05:44:11 PM »
Just bought some Bounce, now known as Lenor, so I'll find out!  :lol:

FELINEGUITARS

  • Middleweight
  • *****
  • Posts: 6609
  • London & Southeast's Number 1 BKP stockist
    • http://www.felineguitars.com
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #5 on: April 21, 2010, 05:55:29 PM »
BAdly done screening can cause problems of its own - layers of tinfoil with questionable electrical continuity can be a problem

If it is static caused by brushing across a pickguard I would suggest a layer of adhesive copper foil across the underside of the plate with a wire soldered directly to the copper and going to the back of a pot
With foil shielding I used to be a bit OTT and soldered each piece to the next (when using copper foil) to ensure continuity.
These days I find it easier to use Nickel paint which is expensive but works very nicely
www.felineguitars.com - repairs & custom built
Great fretwork!
Buy your BKPs & Earvana from ME!

Twinfan

  • Light Heavyweight
  • ******
  • Posts: 10528
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #6 on: April 21, 2010, 08:43:36 PM »
Thanks everyone - looks like I've sorted it  :)

I tried the Bounce first - no difference whatsoever.  And now my guitar smells of "Lemon Fresh" or something equally pansy-fied  :roll:  :lol:

I then managed to trace the problem to the four screws holding the plate on.  So I've added a strip of adhesive copper tape into the screw holes and out into the cavity, with a nice long overlap to ensure continuity of signal.

Voila - no crackling!  Not quite sure why they were the problem, but clearly they were.....

Philly Q

  • Light Heavyweight
  • ******
  • Posts: 18109
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #7 on: April 21, 2010, 08:51:14 PM »
Weird!



(And what's wrong with lemon fresh, I'd like to know?  :wink: )
BKPs I've Got:  RR, BKP-91, ITs, VHII, CS set, Emeralds
BKPs I Had:  RY+Abraxas, Crawlers, BD+SM

hermetico

  • Guest
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #8 on: April 22, 2010, 01:10:35 AM »
Thanks everyone - looks like I've sorted it  :)

I tried the Bounce first - no difference whatsoever.  And now my guitar smells of "Lemon Fresh" or something equally pansy-fied  :roll:  :lol:

I then managed to trace the problem to the four screws holding the plate on.  So I've added a strip of adhesive copper tape into the screw holes and out into the cavity, with a nice long overlap to ensure continuity of signal.

Voila - no crackling!  Not quite sure why they were the problem, but clearly they were.....

One possibility is that the foil didn't cover the whole walls so, the "tap of the cage" was little bit open, allowing RF monsters to come inside.
BTW, I also use conductive paint for shielding the walls (copper paint) and copper foils just for the pickguard.

Twinfan

  • Light Heavyweight
  • ******
  • Posts: 10528
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #9 on: April 22, 2010, 08:40:41 AM »
Shielding is wierd.  I've had guitars with no shielding at all that were silent, and then this one that was quite noisy until I fixed it.

Bizarre.


Copperhead

  • Lightweight
  • ***
  • Posts: 530
Re: Cavity screening issue - not sure what the problem is, static?
« Reply #10 on: April 22, 2010, 03:22:54 PM »
I use adhesive backed thick copper foil, and solder a ground directly to it.
BKPs: Nail Bombsss, Mississippi Queen, BKP-91, Cold Sweat, mystery humbucker, Supermassive