Hum not canceling

[ThingsAbove]

A friend brought over his 90's Fender MIM J-bass for me to shield because it's super noisy. I shielded all the cavities and checked shield continuity and all that. I've done this a few times before and it's always made a big difference. However when we were done I plugged it in and held it up to a known noisy source and while the higher frequency noise was gone it stilled had the 60hz hum.

It was then that I noticed that having his pickup volumes match made no difference in the hum. He bought this new and has never messed with the wiring. Could the pots be bad or is it likely that it was wired incorrectly from the factory with regard to phase? I'd really like to help him out. He doesn't have the money for a new bass or to take it to a real tech.

 

[gforce9]

I've done several shielding jobs as well and, my knee-jerk reaction is that something that shouldn't be, is touching ground. I've had to take apart and reassemble before. I started covering my grounding jobs (usually aluminum foil) with electrical tape to reduce chances of ground issues. It could also be something else...........

 

[Dirk Diggler]

Yep 60hz sound is the typical ground issue, or should I say lack of ground.
It could also be one of the pots or posts might be touching your shielding?
Good luck,
Dirk

 

[khutch]

Checking the grounding of the various bits is always a good idea. You should be able to verify that the pickups are wired correctly with a visual inspection. I wonder if the factory installed two neck or two bridge pickups? They won't humbuck if they are identical....

Ken

 

[SGD Lutherie]

The early MiM Jazz basses used two neck pickups, and did not hum cancel when they were both on. You can spot these because the pickups are the same length, and the magnetic polarity will be the same on both.

Theresa two kinds of noise you will hear. The low pitched 60/120Hz hum is magnetic field interference. That's removed with hum canceling pickups. The hire pitched buzz is electrical field noise, and that's taken care of with shielding.

Shielding will not get rid of the 60Hz hum however.

If your hum is from both pickups being the same, you either need to replace the pickups, or you can have the magnet polity reversed on one of them, and then wire it in reverse. Many pickup makers that do rewinds can do this for you. I did one not too long ago and it worked out fine.

 

[ThingsAbove]

The early MiM Jazz basses used two neck pickups, and did not hum cancel when they were both on. You can spot these because the pickups are the same length, and the magnetic polarity will be the same on both.

Theresa two kinds of noise you will hear. The low pitched 60/120Hz hum is magnetic field interference. That's removed with hum canceling pickups. The hire pitched buzz is electrical field noise, and that's taken care of with shielding.

Shielding will not get rid of the 60Hz hum however.

If your hum is from both pickups being the same, you either need to replace the pickups, or you can have the magnet polity reversed on one of them, and then wire it in reverse. Many pickup makers that do rewinds can do this for you. I did one not too long ago and it worked out fine.

Yes that describes the situation. After shielding the higher frequency buzz is gone but the low frequency hum is there no matter how I set his volume knobs. He has an early MIM Jazz. The internal routing was different than my Jazz. There was a big open channel between his neck PUP and the control cavity and both his PUPs appeared to be encased in epoxy. The dust covers were integral. I'll help him replace his pickups.

Thank you for the suggestions.