Grounding Issue

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tommytinker

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Hey guys, bit of a frustrating problem here that I'm sure some of you have encountered. I have been rewiring an Epiphone Dot semi hb. I first replaced the stock pups with Seymour Duncan Phat Cat P-90's. Since they sounded a bit bright I decided it was worth switching out the pots and caps too. The overall tone of the guitar has been improved but I am now left with a buzzing sound that disappears when I touch the strings or any other metal surface on the guitar.

I bought a multimeter and tested the connections which all seem to be sound. The only thing that I can think of is that the wire that I used, which was just 22g Radio Shack wire, could maybe be the cause of the problem. I checked all the usual suspects including the bridge ground which seems fine and my wiring specs were copied directly from the Seymour Duncan site. Any advice would be greatly appreciated. I would prefer to figure the problem out without taking it to a tech if at all possible. Mainly because I'd like to learn. Thank you in advance.
 
Is it a super-loud buzzing, or just the normal hum like you'd get on a strat? The first thing I'm thinking is that P-90s are single coils, and therefore more susceptible to hum issues that you wouldn't have encountered with humbuckers. Odds are that thing's not shielded on the inside, either. You might try using a shielded wire for your connections instead of the cheapo, single-conductor stuff (bottom picture here). Just make sure you ground each shield exactly once.
 
It isn't a really loud sound no. But because it goes away when I touch the guitar proves that it is a grounding issue, no? I have read about shielding but I have no clue how to do it. Read something in another thread about copper tape? Thanks for the link by the way.
 
Normally the bridge is connected to ground -- that's why the hum goes away when you touch the strings. If it's as loud as the strings, then yeah, you have a grounding issue, but so far this sounds normal.

If it were a solid-body, then shielding would be an easier issue. You just line the control cavity with copper tape, foil, conductive paint. etc. It's the same idea as putting all the controls into a metal box. I just don't see opening up a hollowbody and covering the whole inside in foil -- that's why I suggested the shielded wire instead. It may not work quite as well, but a lot of times it's more practical.
 
Okay, I see what you're saying. Only question is if I were to use the shielded wire on every connection what would I do with the actual shielding part? Would I solder the outer shielding to the same place I connect the actual wire? Or would I just ignore the shielding?
 
You would solder the shielding part to the ground (back of pot). The same goes for grounding solid-body guitars - it's not just lining the cavity with foil, but also grounding the foil.

I don't know what's effective on hollowbodies, but I'd imagine the most important thing would be pickup covers. Hollowbodies with P90's and other single-coils are nothing new (Chuck Berry, Eddie Cochran, etc.) but those metal pup covers (which should be grounded) can help.

But if the noise isn't loud enough to affect your playing, just leave it be, it's not worth bothering with. Strat people live with it.

It's not a grounding "issue", it's a 60-cycle hum issue. When you touch a grounded part of the circuit, your body (the hum "antenna") is grounded by the circuitry and the hum diminishes. That's the way I read it. You're not grounding the guitar, the guitar grounds you.
 
did you use star-grounding?

That is all the grounds go to one place together (all the wires make a nice start shape) and then goes to ground from that single location, that can really help avoid ground loops. That and shjeilding can help, but the hum you speak if is not unusual for single coils as a rule.

Daav.
 
The link that I used in wiring my guitar is on the Seymour Duncan site. They are chrome covered P-90s for humbucker sized cavity. Phat Cats

I forgot to mention that when I taper off the volume pots on either pup the noise gets much worse. This doesn't seem normal.
 
tommytinker said:
... when I taper off the volume pots on either pup the noise gets much worse. This doesn't seem normal.
Click to expand...

That part doesn't seem normal to me either. When you turn the volume down all the way, is the hum really loud, like when you put your thumb on the end of a cable? I ran into a situation like that once when I was missing a connection to ground somewhere.

My first suggestion would have been to check for cold solder joints and the like, but from the original post it sounds like you've done that already. From the diagram on SD, it looks like the wires to the pickups also have the shield right there on the outside -- another thing I could see causing a problem is that shield rubbing up against a hot wire someplace in there.

Is there a backplate on this guitar? Can you take a photo of what's in there? Maybe somebody here could see a problem if you post it here.
 
It may be difficult. All of the guts are hanging out of the f hole. No back plate. I might try to get one up though.
 
danw said:
That part doesn't seem normal to me either. When you turn the volume down all the way, is the hum really loud, like when you put your thumb on the end of a cable? I ran into a situation like that once when I was missing a connection to ground somewhere.

It's not quite that loud but it sounds similar.
Click to expand...
 
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