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Wiring problem on Jazz Bass?

P

pitchpocket

Feb 17, 2012
14
0
#1
I've got an early 90's Fernandes JB copy. Sounds great but if I turn the volume down completely on both pots there is still sound coming through the amp. Something has to be wrong.

I looked at the wiring diagram on StewMac for typical jazz basses and it looks completely different to what I have here.

Any ideas?

wiring.jpg
 
#2
Looks like someone changed the neck volume pot. Perhaps a 500K (I can't read it)?

Either way that shouldn't prevent the volume from getting completely turned off. Unless that pot doesn't ever really go to zero ohm allowing no resistance from pickup to ground.... time to test with a multimeter
 
#4
the pot says 250k matching the others. I can see now that it's probably been changed.

Not properly grounded?
I took the pickups out and found under each pup a copper/brass plate wired to a brass plate under the pots. The brass plate under the pots is wired to the back of the bridge pup pot.

The brass plate under the bridge pup has a ground strap that is poked into the body under the bridge. The ground strap was not connected to the plate under the pup (broken solder) and was nearly broken through where it slides under the bridge. Unscrewing the bridge completely broke it off.

I don't doubt it is the source of the grounding problem.

I'll replace it and see if it solves the problem. Where do you get the copper tape?
 
#5
pitchpocket said:
the pot says 250k matching the others. I can see now that it's probably been changed.

Not properly grounded?
I took the pickups out and found under each pup a copper/brass plate wired to a brass plate under the pots. The brass plate under the pots is wired to the back of the bridge pup pot.

The brass plate under the bridge pup has a ground strap that is poked into the body under the bridge. The ground strap was not connected to the plate under the pup (broken solder) and was nearly broken through where it slides under the bridge. Unscrewing the bridge completely broke it off.

I don't doubt it is the source of the grounding problem.

I'll replace it and see if it solves the problem. Where do you get the copper tape?
Click to expand...

It's really quite simple. When a volume pot is properly wired, rolling it all the way down reduces input impedance against the pickup coil to 0 Ohms. At that point, electrons can only flow between the coil itself and ground. Since you're hearing a signal, it is a clear indication that the pots are not grounding the wiper terminal at the full counterclockwise rotation. Check the pots themselves, and the resistance to the ground potential from all points on the bass.
 
#6
I replaced the ground strap and the problem with the pots continue. Turning both the bridge and the neck pup full cancels out the single coil buzz, but turning them both to zero is not effective. It's quieter, but still has output.

I'm no electrician, but I can see the ground is missing from the neck pot and tone mix. All ground wires are posted to the bridge pot and then to the plate. I'll fiddle a bit and post the results.

Thank you!!
 
#7
Squier uses the mounting of the pots to the control plate as it's 'ground' too, and that seems to work.

You may want to solder in ground leads from pot to pot to pot to jack anyways. Not saying that will solve the problem, but it's a good idea.

You still need to test the pots with a multimeter to make sure they are going down to zero resistance with volumes rolled off.
 
#9
pitchpocket said:
Turning both the bridge and the neck pup full cancels out the single coil buzz, but turning them both to zero is not effective. It's quieter, but still has output.
Click to expand...

I think that people are trying to help you figure out why you still hear the pickup sound with the pots turned down. But this statement could be construed to indicate that what you hear is 60 Hz hum with the pots turned down. Which is it: string sound or 60 Hz hum? If the string sound goes away with both pots turned down then the pots are wired and working correctly. Any 60 Hz hum you hear in that condition is due to grounding or shielding issues.

Ken
 
#10
The issue is (was) that if you turned the volume pots to zero, it would be like they were at 4. No hum, but full sound and tone, just turned down.

Anyway, I decided to blow it up and rewire it to spec. Works fine now.

Thanks for your ideas and help. Appreciation!
 
#12
After re-wiring, I found the CTS pot was bad. The volume reduced fine till the stop and then the volume jumped back on. If you backed off the stop it would go to almost zero, but never off.

I replaced them all with new and changed to linear for the volume pots. One of the two I bought was defective (!?) so I swapped one of the mini audios in until the replacement arrives. Already sounds 100% better.
 

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