How can a broken pickup make tone pot acting like volume?

[colin0154]

I got a P bass from my friend that the tone pot is acting like a volume pot.

My first guess is that the capacitor is broken, or the wrong wiring somehow short it. After checking the control cavity, I can tell the wiring is correct and will not cause a short, unless it is due to a soldering issue.

I test the desoldered old capacitor using a multimeter, and it's fine and nothing wrong. I have tried putting a new capacitor on the bass, but the problem still presist.

This P bass can still make sound when tone pot wide open. And if I disconnect the capacitor, meaning only hot wire into tone pot lug 2 but nothing else, the P bass still makes a sound (the tone pot doesn't affect it anymore).

After some searchings on the forum, some of them fixed it by changing the pickup. But I'm wondering, how does a broken pickup (open coil) turn tone pot into volume pot? Wouldn't an open coil just directly kill the sound?

P Bass Tone pot acting like volume...
Tone pot causes volume drop on Mustang PJ - need help please

 

[Matt Liebenau]

Is the control cavity shielded? If so make sure none of the lugs on either pot are touching the shielding. With a Pbass you’ll have to examine everything with the pickguard and controls out.

 

[slogo]

I went through this a couple of years ago. One of the winding wires on one of the pickups is broken. Your electronics are fine.

 

[Tim Skaggs]

The two halves of a PBass pickup are wired in series, making one continuous path through one lead, through the half/coil to which that lead is connected, through the link wire between the two halves/coils, through the other half/coil, and out the other lead wire.

If either lead to the pickup halves, either coil, or the link wire between the two coils is broken, there will be no continuity and you won’t get any sound out of the pickup.

The part of your troubleshooting quoted below indicates the capacitor HAS continuity (it should not have) or the lug to which the capacitor was attached was grounded in some way before you disconnected the cap.

This P bass can still make sound when tone pot wide open. And if I disconnect the capacitor, meaning only hot wire into tone pot lug 2 but nothing else, the P bass still makes a sound (the tone pot doesn't affect it anymore).

The diagram below is how the bass should be wired.

In other words, the tone pot acted like a volume control, you took the cap off the tone pot, and now it doesn’t act like a volume control. So, basically, you have proven the problem is the cap or the tone pot.

 

[mcnach]

I got a P bass from my friend that the tone pot is acting like a volume pot.

My first guess is that the capacitor is broken, or the wrong wiring somehow short it. After checking the control cavity, I can tell the wiring is correct and will not cause a short, unless it is due to a soldering issue.

I test the desoldered old capacitor using a multimeter, and it's fine and nothing wrong. I have tried putting a new capacitor on the bass, but the problem still presist.

This P bass can still make sound when tone pot wide open. And if I disconnect the capacitor, meaning only hot wire into tone pot lug 2 but nothing else, the P bass still makes a sound (the tone pot doesn't affect it anymore).

After some searchings on the forum, some of them fixed it by changing the pickup. But I'm wondering, how does a broken pickup (open coil) turn tone pot into volume pot? Wouldn't an open coil just directly kill the sound?

P Bass Tone pot acting like volume...
Tone pot causes volume drop on Mustang PJ - need help please

Not the pickup.

Is the cavity shielded? Most times I've had strange issues it came to having a terminal/cable/wire strand touching bits it should not.

 

[mcnach]

The two halves of a PBass pickup are wired in series, making one continuous path through one lead, through the half/coil to which that lead is connected, through the link wire between the two halves/coils, through the other half/coil, and out the other lead wire.

If either lead to the pickup halves, either coil, or the link wire between the two coils is broken, there will be no continuity and you won’t get any sound out of the pickup.

The part of your troubleshooting quoted below indicates the capacitor HAS continuity (it should not have) or the lug to which the capacitor was attached was grounded in some way before you disconnected the cap.



The diagram below is how the bass should be wired.

In other words, the tone pot acted like a volume control, you took the cap off the tone pot, and now it doesn’t act like a volume control. So, basically, you have proven the problem is the cap or the tone pot.

View attachment 7468357

all of this, good post

 

[colin0154]

Well I got the answer. The pickup has a resistance of infinity (open circuit) with some capacitance readings. It's very likely that somewhere internally the wire breaks.

I found a youtube video that's helpful and explaining the reason why it could still make sound with broken internal wire, something to do with capacative coupling or parasitic capacitance. Hope this might be helpful to others.

Broken Guitar Pickup DIAGNOSIS + REPAIR | Fender Style Single Coil Diagnosis + Repair Demonstration (Jump to 3:23)