Continuity testing

Continuity testing

Postby shadowboxer » Tue Jun 10, 2014 5:25 pm

Have a question on this. I've got a guitar wired Garcia style. I was just messing around with the multimeter on my guitar and I get continuity when I test the bridge or the strings. But when I test the volume knob, which is metal, I don't get continuity. On ohms it reads something like 16.5. I had some problem in the past with getting buzzing noises while touching the volume knob which was the reason for testing this in the first place. It's not doing it now. Is this lack of continuity with the volume knob normal? How is it normally grounded under Wolf/Tiger wiring?
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Re: Continuity testing

Postby softmachine72 » Tue Jun 10, 2014 5:36 pm

the way the tiger is wired the body of the vol pot body(case) should not be grounded.
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Re: Continuity testing

Postby milobender » Tue Jun 10, 2014 7:52 pm

The body of the pot has been grounded in every guitar I've ever seen... by the connection to the shielding. Are you sure about Garcia's? It would be very strange to actually isolate the body of the pot.
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Re: Continuity testing

Postby softmachine72 » Wed Jun 11, 2014 1:42 am

milobender wrote:The body of the pot has been grounded in every guitar I've ever seen... by the connection to the shielding. Are you sure about Garcia's? It would be very strange to actually isolate the body of the pot.

Garcia's Guitars were not every other guitar . His guitars use a 10 AWG wire ground buss from ground lug on the vol pot to the ground lug on the output Jack. This allows for all components, body, tailpiece and p-up cavity to ground at a a single ground point to avoid ground loop noise.

When two or more devices are connected to a common ground through different paths, a ground loop occurs. To prevent ground loops, all signal grounds need to go to one common point and when two grounding points cannot be avoided, one side must isolate the signal and grounds from the other.
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Re: Continuity testing

Postby hippieguy1954 » Wed Jun 11, 2014 3:18 am

If the control cavity is well shielded, the pot is also grounded to the shielding when the pot is installed. You might just be getting some static. Does this happen when you are on a carpeted floor?
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Re: Continuity testing

Postby shadowboxer » Wed Jun 11, 2014 5:23 am

Yes, I do get some static-like noise out of that pot but only when I turn it and only when there is a note being played. No static/crackle when the strings aren't vibrating and I turn the volume knob. Happens in a carpeted and non-carpeted room. Might be a dirty/bad pot. But the original problem was when the guitar briefly went through a period when it started humming when I touched the volume knob or tone knobs. Was fixed somehow when I poked around at some of the wires around the jack. That problem hasn't come back.

Let me ask this: Should the bare ground wire touch the painted-on shielding inside the cavity or should it not touch it? If the shielding is suppose to be grounded, I don't think mine is. The bare ground wire floats. It probably would make contact near the exit to the jack but I put some electrical tape between the ground wire and the shielding so i don't think it is making contact right now.
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Re: Continuity testing

Postby hippieguy1954 » Wed Jun 11, 2014 7:58 am

shadowboxer wrote:Yes, I do get some static-like noise out of that pot but only when I turn it and only when there is a note being played. No static/crackle when the strings aren't vibrating and I turn the volume knob. Happens in a carpeted and non-carpeted room. Might be a dirty/bad pot. But the original problem was when the guitar briefly went through a period when it started humming when I touched the volume knob or tone knobs. Was fixed somehow when I poked around at some of the wires around the jack. That problem hasn't come back.

Let me ask this: Should the bare ground wire touch the painted-on shielding inside the cavity or should it not touch it? If the shielding is suppose to be grounded, I don't think mine is. The bare ground wire floats. It probably would make contact near the exit to the jack but I put some electrical tape between the ground wire and the shielding so i don't think it is making contact right now.


Ok, so it's not static. We know that now.

Yes, the shielding should be grounded. There should be continuity between shielding and any ground. All the grounds should have continuity with each other. Actually, there should be a separate wire going from the shield to the bare floating ground wire. Just like all the other ground wires go to it.

If you had a problem that was "fixed" by poking around, you can bet that is were your problem (or part of your problem) is and should be looked into further to see what you did to "fix" it and if it is still intermittently happening. Probably is. Then see if the pot is still a problem.
Last edited by hippieguy1954 on Wed Jun 11, 2014 8:02 am, edited 1 time in total.
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Re: Continuity testing

Postby milobender » Wed Jun 11, 2014 8:00 am

It does sound like a dirty pot... I'd try just replacing it and see.
Yes, the painted shielding does need to be connected to circuit ground. I use a main ground bus/wire and connect the body shield to that, but whatever the scheme, it must be connected or it won't be effective.

edit: sorry for the redundancy "o) we must have been typing at the same time, and hippyguy's faster :lol:

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Re: Continuity testing

Postby hippieguy1954 » Wed Jun 11, 2014 8:04 am

I think between the two of us we nailed it. :roll:
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Re: Continuity testing

Postby shadowboxer » Wed Jun 11, 2014 12:08 pm

Thanks for the replies. I'll try to get this sorted out soon and let you know the results.

As for the time when it was humming when I touched the volume knob, I'm guessing that it may have been before I put the electrical tape between the shielding and the ground cable. At that time, if the bare ground cable was touching the ground spade as well as the hot spade on the input jack, would that have caused that hum? I'm thinking maybe that was what was going on at the time but could be wrong. When I poked around in there, I might have moved the ground wire away from the hot/tip connection.
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Re: Continuity testing

Postby hippieguy1954 » Wed Jun 11, 2014 12:26 pm

Yes, if the bare ground cable was touching the ground spade as well as the hot spade on the input jack it would make a hum for sure and pretty much screw up your whole signal.
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Re: Continuity testing

Postby TI4-1009 » Wed Jun 11, 2014 6:12 pm

Are we sure that on Tiger the ground bus isn't soldered to the pot case? I've never seen a photo that far into the control cavity on Tiger to be sure. Has anyone ever seen it, or talked to anyone who's seen it?

I know there's a Tiger trib belonging to someone on the board that was wired by a luthier close to Brawer and the bus on that one is soldered to the volume pot case. That's how I did my Tiger and Wolfish.
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Re: Continuity testing

Postby tatittle » Thu Jun 12, 2014 12:58 am

When two or more devices are connected to a common ground through different paths, a ground loop occurs. To prevent ground loops, all signal grounds need to go to one common point and when two grounding points cannot be avoided, one side must isolate the signal and grounds from the other.

>> I was once considering doing a common ground point on all my guitars, and then I read an article pointing out that there is always a common end at the output jack/cable. :idea: The real problem is when plugging into 2 separate amps with both grounded, or effects etc that get power sources from different outlets...this is where 2 prong adapters help (on all but 1 plug). But the guitar circuit may be a smaller version of the same issue, if the outlets have a common ground point in the building wiring. Certainly wont hurt and its easy to do that way.

" Garcia's Guitars were not every other guitar . His guitars use a 10 AWG wire ground buss from ground lug on the vol pot to the ground lug on the output Jack. This allows for all components, body, tailpiece and p-up cavity to ground at a a single ground point to avoid ground loop noise.

>> This could be confusing to some I think. The wire attached to the lug provides the ground link to vol pot, which is practically the same as attaching it to the back of the pot. Here the back of the pot will have continuity from either the shielding or a direct connection from the blade/lug...e.g. they are often bent back to touch the back of pot on Fender Strats. Im not sure what affect not grounding the pot would have, especially with shielding...never tried it; and I certainly don't know about Garcia's actual guitar.

If the shielding isn't grounded I believe it acts like an antanae for noise! Not good. Shielding often connects to the pots by default as the pot secures against it. If there is removeable pickguard design more care is needed to ensure the cavity side shielding has continuity.

If you're like me its hard to know how important some of these things are because I've never tried anything but the typical wiring models. Only when I have a problem do I go in and see what is wrong.
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Re: Continuity testing

Postby tatittle » Thu Jun 12, 2014 2:16 am

Being sleepless again, I checked the schematics we have all seen by now and found that the Wolf diagram has a wire from the vol pot ground lug to the pot body, but the Tiger does not. It may be that it was just left out of the Tiger schematic since it cannot be seen in the picture, but that is merely a guess assuming this didn't change from Wolf. The shielding paint would provide a connection to the vol. pot so it wouldn't be nec'y on the other hand. But it looks like the copper wire stops after the tone pot to me which is really a surprise...Ill take another look, maybe something is obscuring the view of it past that point.

...it looks like a 2nd bare heavy gauge wire is run from the 10 ga copper at that end to my eyes, I assume it runs to the vol pot just as the schematic shows. Take a look...doesn't the copper stop right at that red wire on bottom right of photo? It looks like the red wire is below the copper to me, rather than above obstructing view of the copper.
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