Constant hum when blend pot not 100% on bridge (Bartolini Jazz pickups and NTMB preamp)

[andyhaines]

Hi All,

 

Recently had two Bart Jazz pickups and early NTMB preamp (pre-wired harness) moved from an old bass to another -all works fine except that when moving the MN250K blend pot away from 100% bridge towards the neck where there is a steady (ground?) hum. When fully on bridge it's silent (as in no hum). Both pickups work fine and you can clearly hear the neck pickup tone coming through, just with the added hum! I've disconnected the neck pickup from the blend pot just as a test and (as expected) turning the blend pot to 100% neck results in silence (no pickup or hum).

Touching strings and bridge makes no difference so I'm assuming there's no ground problem there (otherwise the bridge pickup setting would also hum) and the ground wire is making contact below the bridge.

 

Any clues as to what's going on? Possible failing blend pot?

 

Thanks!

 

Andy

 

[itu]

Pictures, please.

[andyhaines]

No problem -pic of the blend pot before I disconnected the neck pickup - hot wire goes to centre lug and the ground braid is soldered to the same lug as the bridge ground.
 

2nd pic of the wider control cavity. Hope that helps!

[Downunderwonder]

At 50/50 blend, or somewhere thereabouts, does the hum go away?

[itu]

This is just a guess: the blend is suspect. It has been soldered not from the lugs, but the rivets. Not good. That may disconnect the track from the lugs.

[andyhaines]

2 hours ago, Downunderwonder said:

At 50/50 blend, or somewhere thereabouts, does the hum go away?

No. The hum is not present when 100% bridge. Turning the pot back then gradually introduces the hum, through 50% blend increasing hum as you get to 100% neck

[andyhaines]

1 hour ago, itu said:

This is just a guess: the blend is suspect. It has been soldered not from the lugs, but the rivets. Not good. That may disconnect the track from the lugs.

Thanks - so the yellow and blue wires that cross over behind the lugs should be soldered to the lugs and not to the rivets?

[itu]

Yes, please. The heat may be too much to the rivet-track connection.

[sandy_r]

 

Just checking, are the pickups both the same construction? Thanks

 

[andyhaines]

10 hours ago, sandy_r said:

 

Just checking, are the pickups both the same construction? Thanks

 

Yes, they’re mid 90s bartolini jazz pickups (passive). The undersides are marked N and B so designed for neck and bridge.

[andyhaines]

11 hours ago, itu said:

Yes, please. The heat may be too much to the rivet-track connection.

Thanks, good to know. I’m assuming the blend pot was wired by bartolini (part of a pre wired harness)

[itu]

I had a similar bartolini harness in my MG Quantum 5 custom, and the blend was defective. I actually changed it to the Noll Mixpot after several trials with different pots. Substantial change!

[Doctor J]

Swap the pickups around on the blend pot and find out if the noise follows the pickup or the pot.

[sandy_r]

 

+1 re. @itu's comments about reducing heat trauma to pot tracks

 

If hum still present after blend pot change might be worth checking under pickups that the ground/shielding connection(s) on neck pup match those on bridge pup for both wiring and good soldering - also - difficult to tell from photos above but make sure cavity-shielding encloses the whole volume, with good continuity from lower volume to cover plate(s) - and continuity to Jack shield lug too

 

Edited February 4, 2024 by sandy_r

[andyhaines]

Thanks everyone- really good advice all round! I’ll probably try to swap the pickups on the blend to confirm whether it’s a blend or pup issue, but sounding like a new blend pot may be required.

[itu]

You need a new one, it will be Bourns 250k MN. That has tracks for blend, i.e. starting from the centre.

[andyhaines]

Interestingly, swapping the pickups around on the blend pot reversed the issue - the pronounced hum is now on the opposite side, indicating that the issue follows the pickup.

[Suburban Man]

1 hour ago, andyhaines said:

Interestingly, swapping the pickups around on the blend pot reversed the issue - the pronounced hum is now on the opposite side, indicating that the issue follows the pickup.

Might it be possible that the earth/live connections to the humming pickup have got reversed at the pickup end? Pickup would still work, but shielding would be all wrong?

[andyhaines]

Worth a look - thanks! I suspect not as I didn’t have this problem in the previous bass these pickups/electronics were in, but that was some years ago so I may be mis remembering!

[Doctor J]

Wire the pickup directly to a bog standard jack, find whether the pickup itself is the issue.

[andyhaines]

7 hours ago, Doctor J said:

Wire the pickup directly to a bog standard jack, find whether the pickup itself is the issue.

Good call, thanks!