Humming guitar query

Kalimna

Greetings folks,
I have a twin humbucker LesPaul copy that is giving out a lot of background hum when strings are not being touched. I'm assuming that there's a grounding problem somewhere, so any advice in diagnosing would be great.

The issues are : 
Background hum both pickups, nearly to zero when strings touched.
Hum goes to zero if tone rolled back to 0 when either pickup selected.
Hum goes to zero when both pickups selected if either tone control set to zero.
There is also a very noisy static like sound if the 3-way switch cavity cover is rubbed, but not if the main control cavity cover.

Im happy to go delving in with a soldering iron, but would take any advice.

Cheers,
Adam

TINMAN82

Should’ve bought authentic 

ICBM

That’s a shielding issue, not grounding. The grounding is fine if touching the strings stops it.

Kalimna

Ok, so out with the conductive paint? Or copper foil?
If you dont mind me asking, how does the tone control at zero cut the hum? Specificaly when both pickups selected, same volume but only one tone at zero.
Cheers,
Adam

ICBM

The tone controls filter the buzz to ground. (It is buzz, not hum, if it’s the kind of noise that’s stopped by touching the strings.) When both pickups are selected, the tone controls are in parallel if either both volumes are up full or you have ‘50s’ wiring, so both work at the same time.

Check what the wiring is like, in particular the long runs to and from the switch, they’re the biggest cause of noise. Ideally it should be either metal braided shield cable (like a vintage Gibson) or a foil-shielded multi-core cable (like a modern Gibson) - if it’s multicore, check that the shield is connected, and that there’s the minimum of exposed cores at each end.

Foil shielding might be necessary in the cavities, but if the wiring is all thoroughly shielded it probably isn’t. I don’t find paint very effective, and messy.

Also, does turning the volumes down to zero stop it? If not, the volume controls are wired backwards - a lot of copies are like this. It’s noisier in every situation except when both controls are up full. If so, swap the input and output wires - the pickups should go to the end terminals, and the switch to the middles.

Kalimna

Thank you @ICBM for the thorough explanation. Yes, the buzz does stop completely if the volume turned to zero.
I think my terminology was inaccurate - i'd thought that maybe the shield part of the wire wasnt connected at the switch end (and therefore the grounding), and that would explain the interference from static generated at the switch cover.

Lastly, because i like to at least try and understand things, why does touching the strings reduce the buzz if you are insulated from the floor?

Thanks again,
Adam

Kalimna

TINMAN82 said:

Should’ve bought authentic 

Helpful, as always

ICBM

Kalimna said:

Thank you @ICBM for the thorough explanation. Yes, the buzz does stop completely if the volume turned to zero.
I think my terminology was inaccurate - i'd thought that maybe the shield part of the wire wasnt connected at the switch end (and therefore the grounding), and that would explain the interference from static generated at the switch cover.

Lastly, because i like to at least try and understand things, why does touching the strings reduce the buzz if you are insulated from the floor?

The capacitance of your body acts as an aerial if it’s not grounded - try bringing your finger very close to (but not quite touch) the wiring in the guitar without holding the strings and you should hear the noise increase. But when you also touch the strings, the capacitance is then part of the ground and filters off the buzz in the same way as the tone caps.

Definitely check the switch frame is grounded - if it’s not it will be very noisy. Even if it is, an open-frame switch is not very well shielded, so it may be useful to do the cavity - but make sure it doesn’t short the switch contacts. It might be worth just doing the inside of the cover, if you run a wire to the switch frame so it’s trapped under the edge of it.

Kalimna

Thank you. The information you willingly pasd on is greatly appreciated. The Uncle Doug of Glasgow