Wiring Help!

February 16th, 2012, 01:12 AM
I'm not sure if this is the place for this. If it isn't I'm sorry mods!
On to the question. I have an Epi Lp Jr that I recently got a p90 for. I routed and got it all soldered up but there is an incredible hum. I understand it is a p90 and they do that, but this hum is louder than the pickup. I tried re-shielding, checking my grounds. And then finally touching various parts. Touching the bridge does nothing. Touching the strings makes the hum louder. But, touching the input jack reduces the hum considerably. I am using a two pronged switchcraft styled jack. Do I ground the + or the -? Also, should I ground it to the casing on the volume pot? Is there anything else I should ground while I'm in there? Thanks!

February 16th, 2012, 08:21 AM
The - side of things needs to be to ground. The sleeve contact part of the jack should be to ground, and the tip of the jack needs to be the positive (hot).

There should be a wire that runs from one of the bridge post screws to the back of a pot to ground the strings.

Either volume or tone pot can have grounds to them and they should all have a wire on their backs tiring them all together electrically. Unless the pots are mounted in a metal plate that is, as that would connect them all together as well.


February 16th, 2012, 01:10 PM
Everything metal on that guitar needs to be electrically connected to ground. You should be able to take one lead of an ohmmeter and touch the ground side of the jack (or the barrel of a guitar cord that's plugged into the guitar) and any metal part of the guitar and get less than 10Ω of resistance. If you get more from anything, you have a grounding problem. Fix that problem and you'll likely fix your hum.

February 16th, 2012, 04:30 PM
Thanks y'all! I already connected the ground to bridge. I guess tonight after work I'll just grounding anything metal haha. I'll let you guys know how she turns out.

February 18th, 2012, 02:09 AM
So I grounded the - of the input jack, and I grounded the two pots and the only thing that changed was now the hum increases whenever I touch anything metal: strings, bridge, even the tuning pegs. So... any idea whats wrong?

February 18th, 2012, 10:23 AM
Sounds like you have a positive wire touching ground somewhere.

So to the OUTPUT jack, you should only have two wires, one from the middle toggle switch terminals goes to the tip, and the one from the back of a pot goes to sleeve.

Verify continuity with all that. And when you test the + side of the jack (the tab that contacts your cable jack tip), you should only get your pickup resistance readings of 7-10 KΩ depending if you are in selector switch position T or R.

Not to confuse but check this out and look at how the pots, jack and switch are wired. You most likely have one pickup + wire to each vol. pot, one ground wire from each pup to the back of a pot.

Disregard how the caps on the tone pot are wired from cap to pot in this diagram as there are different ways that can be done. They are also wired, commonly on Gibson/Epiphone, from tone pot to vol. pot.


Are you sure the new P90 is good ?? The problem happened when you installed that right ?

February 18th, 2012, 10:34 AM
So I grounded the - of the input jack, and I grounded the two pots and the only thing that changed was now the hum increases whenever I touch anything metal: strings, bridge, even the tuning pegs. So... any idea whats wrong?

Let me explain "grounding". If you already know this, I apologize for sounding condescending.

"Ground" in electronics is the return path for electricity to flow. Electricity won't flow if it cannot complete a circuit. A switch, for instance, opens the circuit and electricity won't flow through that circuit until the switch is closed again. "Ground" gets its name from the earth, to which all electricity eventually flows.

You should be able to plug your guitar into your amp and read < 10 ohms of resistance from any metal part of the guitar to the ground prong on the amp's power plug. If you can get this reading, your guitar should be properly grounded. If you do get this reading and you still have the noise problem, try plugging your amp into a different socket in the house to see if the problem goes away. If it does, then you have a problem with the ground wiring on the first plug. If the noise doesn't go away, take the whole setup to another house or building and plug it in there and try it. If the noise goes away your house has a ground problem.

Be very careful if you find your house has a grounding problem. Improper grounding of electronic things that require a good ground can kill you if you touch the wrong thing at the wrong time. An example: touching your guitar strings while plugged into an amp and touching a microphone that's plugged into a PA system with your lips. Your body is the link between those two ungrounded circuits and if the voltage potential is high enough, you get zapped. Or killed.

If your house has a grounding problem, you need to get a qualified electrician to fix it ASAP. Don't plug your guitar amp into an ungrounded system. If you were to touch anything electric while touching the guitar, you might be electrocuted.

You can get an inexpensive tester to check home wiring at any Home Depot or Lowe's. Cheap insurance.

Now, on to your guitar's ground circuit: The "-" side of the jack is where all grounds inside your guitar run to, electrically speaking. All those black wires that congregate on the back of the pots are the grounds from all pups, the bridge, and anything else that needs a ground. Then one wire runs from the pots to the negative side of the jack. That side of the jack makes electrical contact with the barrel of the input cable; the other end's barrel makes electrical contact with the amplifier's ground circuit and out it goes to the power plug and eventually to earth through the house wiring. If you have all of these conditions, you have a properly grounded guitar. At this point touching the strings should have no effect on any humming or static noise.

Some hum comes from the pups. If you have humbuckers that hum should be minimized. Single coils like most found on Fenders are more susceptible to hum and there's not a lot you can do about that. This hum will not change by touching or letting go of the strings.

March 10th, 2012, 06:12 AM
It sounds like you have the signal and ground to the output jack swapped.