Pick up HUMMM at my bench

[Mars Amp Repair]

Hello Folks,
Well, I have an interesting issue here. I don't typically work on guitars or basses, unless there's an electronic issue that the guitar repair dudes can't handle. When I have, I have bot had any issues with inordinate humm/buzz.
I was working on a Japanese copy of the early 70's Gibson Les Paul bass and an actual early 70's Les Paul Triumph (sometimes referred to as 'Recording bass'.
Both of these basses had very inordinate HUMMMM when in single pickup mode. With the pickups combined & in phase selected, the hum was nearly gone. Now I have a 90's Jazz bass that I use all the time, and in single pickup operation, it buzzes a little, but nothing I feel is inordinate.
I turned off everything on my bench to see if that might be causing the hummm/buzz to no avail.
I also noticed that even with the volumes off, there was then a 60Hz hum that was being picked up by the non-shielded wire from the input jack to its destination.
I replaced the wire with a shielded cable & it improved the humm w/volume all the way down, but didn't eliminate it.
I replaced the input jack with one of the square ones and wrapped it with copper sheeting and grounded that, which attenuated the HUMMM to an acceptable level.
The single pick-up operation was still really noisy and fairly independent of the orientation of the bass.
I decided to try the bass out on an amp I was testing in the back of the shop area and it was even worse. I turned the bass around and it was more directional. Then I realized that all the breaker boxes for the entire store & some for other parts of the building are in that corner!!!
So, I finally took the bass out to the showroom & tried it there...Voila-no buzz from the pickups or humm w/volumes all the way down..DUH!!!.
It seems I never get past the point where sometimes I miss the obvious!!
I guess for some reason, this variety of pickup on these early 70's Les Paul basses just are particularly sensitive to erroneous noise pickup. I haven't checked, but perhaps the pickups are of a higher impedance. Well, for sure these Les Paul basses will absolutely be the quietest on the planet!!!
Glen

[The Dude]

Often, on older guitars, I find the volume pot wiring "different" from modern wiring. I've said it was "wrong" in the past and have been corrected, so I refrain from saying "wrong". At any rate, the output of the volume control is not taken from the center tap, but rather one end. This still works as a volume control in that it lowers volume, but it does not ground the guitar output when the control is all the way down. This wiring scheme can cause the symptom you describe. The easiest way to tell if this is the problem is to turn volume controls all the way down and measure continuity across the jack.

[Mars Amp Repair]

Well, Yeah.
I've not found the correct schematic for this partiular bass, but it has a coil in it that works part of the tone circuit that could be coupling some of the hum, too. Also, the pick ups are multiple coil taps, too.
I guess since they seem to operate fine outside of 10ft from what is essentially a massive 60Hz generator, that's good enough for me .
it does prove one thing for sure, when you put either of these basses out of phase for the pickups, the hum does cancel very effectively.
Crazy stuff this guitar world!
Thanx, Glen

[Mars Amp Repair]

Pics & Diagram. I guess the Japanese copy isn't really a copy of the 'Recording' variety.

[Chuck H]

As soon as I read "Gibson" I had a suspect. Gibson used inductors in tone circuits on some guitars. This could be part of the additional hum you're hearing. That little copper foil tape isn't going to be up to the task of shielding. I would try properly shielding the control cavity.

[Helmholtz]

You just can't shield a low frequency magnetic field with copper or aluminum foil.
You would need a closed Mu-metal "cage".

[Mars Amp Repair]

Thanx guys, agreed abt absolute shielding.
As mentioned, once I got both of these basses away from the breaker box wall, they were relatively quiet.

[Chuck H]

Thanx guys, agreed abt absolute shielding.
As mentioned, once I got both of these basses away from the breaker box wall, they were relatively quiet.

As Helmholtz mentioned, standard shielding can't abate the low frequency EMI. But properly shielding the control cavities in these instruments can't hurt. You may be able to mitigate hum further by implementing the inductor sandwich (what Mike Sultzer called an idea I used for an attenuator circuit). You need to determine the value of the inductor in the circuit. Then replace it with two inductors of half value in series out of phase. They need to be arranged or shielded in a way that mitigates mutual inductance so that they don't cancel each other relative to value, but still allows the opposing phase to cancel atmospheric EMI. I know this can work because I've done it. But it's a new experiment each time.

[Mars Amp Repair]

All valuable information. Again, fortunately, I won't have to go down that road.
BTW, the Japanese Les Paul, not actually being a 'Recording' copy, did not have an inductor in it.
As with the Gibson, it's pickups for some reason are just super sensitive.
Thanx again.