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Hum problem, fixed but not understood

lark_z

lark_z

Supporting Member
Feb 26, 2020
1,757
3,098
Overland Park, KS
#1
My first parts bass (let's call it "#1") has had some hum when I'm not grounded to it. I know this is not unusual. Noticeably more than the other parts bass (#2). Both are shielded with aluminum tape in the control cavity and pickup cavity. Both have foil on the rear of the pickguard.

Also, the hum really increases on #1 if I rest my thumb on the upper pickup screw, if I'm not grounded. I mean the buzz jumps up in volume if I leave my thumb on that screwhead and take my left hand off the strings.

Now, #1 gets noticeably noisier this week, all the time.

So, I took the pickguard off and it looks mostly fine. I expected to see some physical interaction one of the pickguard screws and the wiring, but there is none. There is some bare wire showing on the bridge grounding wire and it may have been touching the shielding, which shouldn't matter, right?

Checked all grounds with VOM and it's all good.

I did two things. Wrapped the bit of bare wire on the bridge ground with electrical tape. And added one more bit of foil to the control cavity extending it onto the front of the body, to ensure that the pickguard foil is grounded also. There was already one of these. Not sure how to verify that the pickguard foil is/was really grounded. It's only grounded when it's installed and then you have no access.

Reassembled and it's quieter than it's ever been and no hum increase when I touch the pickguard screw.

Very pleased, but a bit puzzled.
 
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#2
Likely the extra shielding up to the pickguard helped.

Your body is conductive. Anything conductive placed in an EM noise will pick it up. If that conductive thing has a path to a circuit, it will transmit the noise to the circuit. If you touch grounded metal on your bass, you are transferring the noise your body picks up to ground. If you touch the pickup screw (or any other ungrounded metal part on the bass), you are basically turning the pickup screw into a short antenna, which radiates out all the noise from your body - and of course the screw happens to be right next to your pickup, so you're essentially transmitting the noise to your pickup. This is why it's good practice to ground any metal on the bass that you are likely to touch, since doing so channels the noise to ground instead of allowing it to cause a problem.
 
#3
lark_z said:
My first parts bass (let's call it "#1") has had some hum when I'm not grounded to it. I know this is not unusual. Noticeably more than the other parts bass (#2). Both are shielded with aluminum tape in the control cavity and pickup cavity. Both have foil on the rear of the pickguard.

Also, the hum really increases on #1 if I rest my thumb on the upper pickup screw, if I'm not grounded. I mean the buzz jumps up in volume if I leave my thumb on that screwhead and take my left hand off the strings.

Now, #1 gets noticeably noisier this week, all the time.

So, I took the pickguard off and it looks mostly fine. I expected to see some physical interaction one of the pickguard screws and the wiring, but there is none. There is some bare wire showing on the bridge grounding wire and it may have been touching the shielding, which shouldn't matter, right?

Checked all grounds with VOM and it's all good.

I did two things. Wrapped the bit of bare wire on the bridge ground with electrical tape. And added one more bit of foil to the control cavity extending it onto the front of the body, to ensure that the pickguard foil is grounded also. There was already one of these. Not sure how to verify that the pickguard foil is/was really grounded. It's only grounded when it's installed and then you have no access.

Reassembled and it's quieter than it's ever been and no hum increase when I touch the pickguard screw.

Very pleased, but a bit puzzled.
Click to expand...
My guess is the pickguard was not actually reliably grounded before. Shielding that isn’t grounded can actually increase noise by catching interference/noise and introducing it to the pickup coil nearby (same with the ungrounded pickup screw as @dwizum pointed out). Does the pickup screw go through some part of the shielding foil at some point?
 

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