Troubleshooting bridge pickup that stopped working

[DownRight]

Hello all,

Suddenly tonight while playing my 2003 Mojo, the bridge pickup stopped working. I'm not quite sure what's going on, and am hoping you might be able to help me troubleshoot the problem.

I've opened up the back and have confirmed all of the wires from the pickup are still connected. I carefully wiggled each one (looking for a broken wire inside of the insulation) with the guitar plugged in, but no audible change.

I also considered that the 3way pickup selector its self could be the issue, and to be honest, I'm still not sure.

Interestingly, if the tone knob is at 0, I can faintly hear the guitar with the bridge pickup selected. With the tone at 10, it's completely silent.

I went ahead and changed the battery as well, just in case, but again, no change.

Thank you, I appreciate any help you can provide!

#PickupIssues

[DownRight]

Oh interesting discovery... the problem is only present with the push-pull knob in the in position.

The neck pickup works as expected in either position.

[mmmguitar]

the problem is only present with the push-pull knob in the in position.

So everything works with the push-pull “up” to split the pickups but, with it down, the only issue is the absence of bridge humbucker signal? Does the selector switch middle position ground the entire signal - or does it sound the same as selecting the neck humbucker by itself?

The issue you’re describing indicates that a wire for one of your bridge humbucker coils is somehow shorting to ground (no sound = signal wire shorting/grounding out against something; whereas buzzy noise = grounding wire has come loose); with the other coil working as expected once the push/pull shuts the faulty one “off” at their common connection point - But without it in front of me, I can’t be certain whether the fault lies in a switch connection, or the pickup coil itself. If you have a soldering iron, I’d suggest desoldering the bridge humbucker series connection from the middle lug of the push/pull, to see if the full humbucker signal comes back though. An iron and a multimeter could diagnose this in seconds - However, I don’t know what tools or repair resources are available to you.

[DownRight]

So everything works with the push-pull “up” to split the pickups but, with it down, the only issue is the absence of bridge humbucker signal?

Yes that's exactly correct.

Does the selector switch middle position ground the entire signal - or does it sound the same as selecting the neck humbucker by itself?

The selector switch middle position sounds the same as the neck humbucker by its self.

If you have a soldering iron, I’d suggest desoldering the bridge humbucker series connection from the middle lug of the push/pull, to see if the full humbucker signal comes back though. An iron and a multimeter could diagnose this in seconds - However, I don’t know what tools or repair resources are available to you.

Ok no problem. I have a soldering station and multimeter here, but admittedly I've never really taken the time to learn how pickup wiring works so I'm not quite sure what to test.

Thank you for the help, I really appreciate it!

[mmmguitar]

I have a soldering station and multimeter here, but admittedly I've never really taken the time to learn how pickup wiring works so I'm not quite sure what to test.

No problem: Just set your multimeter for the 20kΩ range and measure the resistance between the pickup selector switch connection for the bridge pickup hot wire (According to the attached image, it will probably be the black one) and where the green/bare shield wire coming from the bridge pickup four conductor wire bundle is soldered to the common grounding lug USM used.

The Mojo bridge pickup is a Seymour Duncan JB which should give a reading between 16-17k. Now keep one probe on the selector switch connection and move the ground probe over to the red and white wires soldered to the DPDT portion of the tone pot: This is the humbucker's series connection; and taking a measurement at these two points should isolate a coil and produce a reading of around 8k. Also test the resistance between the series connection and the end of the wire soldered to the common ground point: It should produce a similar reading.

If all three readings measure as expected, then the pickup is good; and the fault may be to do with the push/pull pot. If one or more measurements are off, then the issue likely lies with the pickup and/or its wires; and we can proceed from there.

If, by chance, your bridge pickup has a red "hot" wire and a green/black series connection, I apologize for the undue confusion - The way OEM pickup contracts worked with Parker Guitars is that they would order F-space/Trembucker-space pickups of the same magnet orientation and wind direction; then just reverse the wiring of one during installation so they'll play nice with each other. This creates a situation in which the wire color codes between the neck and bridge pickups can seem random, at first glance.

(Click to enlarge and show where I circled what I hope are connections resembling the ones in your Mojo)

[DownRight]

Good morning @mmmguitar

Thanks for the info... your picture matches my Mojo exactly from what I can tell, but here's a picture of mine as well in case anything stands out:

Here are the results of my testing with my multimeter set to 20kΩ:

Measuring between the pickup selector switch connection for the bridge pickup hot wire and the common ground:

Code: Select all

push-pull "down" = no reading
push-pull "up" (split) = 8.92k

Measuring between the selector switch connection and the red and white wires soldered to the DPDT portion of the tone pot:

Code: Select all

push-pull "down" = 9.1k
push-pull "up" (split) = 8.92k

Measuring between the red and white wires soldered to the DPDT portion of the tone pot and the common ground:

Code: Select all

push-pull "down" = no reading
push-pull "up" (split) = 0

[DownRight]

A couple additional updates. With the volume up on my amp quite a bit, I've discovered that there is no scenario where I'm truly getting "no sound"... but it is extremely faint to the point where at normal playing volume it is very hard to hear with the push/pull in the "down" position.

I haven't yet attempted to desolder the series connection on the push/pull yet... I figured you might prefer to review my multimeter results before I take that step.

Assuming the worst, I have a Seymour Duncan TB-4 JB "Trembucker" in my cart... I might just pull the trigger on that purchase anyway, to have a backup. I've read that I'll need to remove the tabs, which I'm guessing won't be too bad. I suppose alternatively, I could swap the back plates from the original / replacement.

[mmmguitar]

I haven't yet attempted to desolder the series connection on the push/pull yet... I figured you might prefer to review my multimeter results before take that step.

Thanks for the detailed reply. Based on the info we have, it seems the pickup's screw coil (oriented magnetic south with a red "finish" end and green "start" end) is shorting most of its signal to ground in parallel with the series connection. This could be due to an internal break in that wire, or a short imitating one (e.g., when the shielded jacked for the 4-conductor bundle was clipped at the factory, the insulation of the red or green ends could have been nicked; with an exposed hair of copper coming into contact with the shield when the wires are angled a certain way).

The next thing I'd like you to try is to desolder the red and white wires from the middle lug of the tone pot's DPDT, while making sure they're still connected.

If you get a 16-17k measurement from the black and green/shield wire ends after that, then we've identified the push/pull switching component as faulty; and it will likely need to be replaced with a new push/pull pot (they can be disassembled for servicing - but USM used horrible quality pots and switches, to begin with).

If you're still only getting a null reading, then the push-pull is working - But some more exploratory surgery to do with the pickup wires will be required; with the disclaimer that this may become a rabbit hole with an unsatisfactory ending. You can try repositioning/angling the bridge pickup's wire bundle with the guitar plugged in, to see if that alleviates any potentially intermittent short in the wire run by letting the full series humbucker signal through.

The next step after that would be desoldering the green/bare wires from the common ground lug and separating them to see if you get the full 16-17k reading from the pickup's black and green wires (indicating a cold or cracked solder joint at ground having caused these gremlins). If there's still a null reading, then it's up to you whether you want to investigate the dead coil further, or simply replace the pickup.

[DownRight]

With the red/white wires desoldered from the middle lug of the tone pot's DPDT, (confirming that they're still connected to each other) I am still getting a null reading between them and the common ground.

You can try repositioning/angling the bridge pickup's wire bundle with the guitar plugged in, to see if that alleviates any potentially intermittent short in the wire run by letting the full series humbucker signal through.

I tried this a bit already, but I'll give it another shot. I have not tried removing the pickup from the body to see if there is anything going on underneath it, though... maybe that's worth a shot.

The next step after that would be desoldering the green/bare wires from the common ground lug and separating them to see if you get the full 16-17k reading from the pickup's black and green wires (indicating a cold or cracked solder joint at ground having caused these gremlins). If there's still a null reading, then it's up to you whether you want to investigate the dead coil further, or simply replace the pickup.

I think I'll do this, but I might just go ahead and order the pickup that I mentioned in the edit of my post just previous to yours. Never hurts to have a spare even if between now and when it arrives I figure out the issue.

[DownRight]

The next step after that would be desoldering the green/bare wires from the common ground lug and separating them to see if you get the full 16-17k reading from the pickup's black and green wires (indicating a cold or cracked solder joint at ground having caused these gremlins). If there's still a null reading, then it's up to you whether you want to investigate the dead coil further, or simply replace the pickup.

@mmmguitar Today is the day that the replacement pickup should be arriving, so in preparation I fully removed it from the guitar, and performed this test. Sure enough, I get a ~17.45k reading from my multimeter. I get the same result when then twisting the bare wire back with the green wire.

I've tried twisting the wires around a bit to see if I can reproduce the null reading, but so far I have not been able to. I think I'll solder it back up and see what happens now.

[DownRight]

Original pickup re-installed, all readings in spec and it's working beautifully again. I'll keep that humbucker I ordered as a spare incase the problem comes back one day.

I can't thank you enough for all of the help @mmmguitar.

[mmmguitar]

Original pickup re-installed, all readings in spec and it's working beautifully again. I'll keep that humbucker I ordered as a spare incase the problem comes back one day.

I can't thank you enough for all of the help @mmmguitar.

I can hardly take credit for the wild goose chase those gremlins led you on (We've all been there) - But I'm happy to cheer you on, any time.

For anyone reading this in the future: I suspect a faulty solder joint between the TB-4's green south coil "start" wire and ground was responsible.