andyhaines Posted February 3 Share Posted February 3 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 Quote Link to comment Share on other sites More sharing options...
itu Posted February 3 Share Posted February 3 Pictures, please. Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 3 Author Share Posted February 3 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! Quote Link to comment Share on other sites More sharing options...
Downunderwonder Posted February 3 Share Posted February 3 At 50/50 blend, or somewhere thereabouts, does the hum go away? Quote Link to comment Share on other sites More sharing options...
itu Posted February 3 Share Posted February 3 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. Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 3 Author Share Posted February 3 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 Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 3 Author Share Posted February 3 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? Quote Link to comment Share on other sites More sharing options...
itu Posted February 3 Share Posted February 3 Yes, please. The heat may be too much to the rivet-track connection. 1 Quote Link to comment Share on other sites More sharing options...
sandy_r Posted February 3 Share Posted February 3 Just checking, are the pickups both the same construction? Thanks Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 4 Author Share Posted February 4 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. Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 4 Author Share Posted February 4 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) Quote Link to comment Share on other sites More sharing options...
itu Posted February 4 Share Posted February 4 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! Quote Link to comment Share on other sites More sharing options...
Doctor J Posted February 4 Share Posted February 4 Swap the pickups around on the blend pot and find out if the noise follows the pickup or the pot. 1 Quote Link to comment Share on other sites More sharing options...
sandy_r Posted February 4 Share Posted February 4 (edited) +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 by sandy_r 1 Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 4 Author Share Posted February 4 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. Quote Link to comment Share on other sites More sharing options...
itu Posted February 4 Share Posted February 4 You need a new one, it will be Bourns 250k MN. That has tracks for blend, i.e. starting from the centre. 1 Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 4 Author Share Posted February 4 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. Quote Link to comment Share on other sites More sharing options...
Suburban Man Posted February 4 Share Posted February 4 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? Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 4 Author Share Posted February 4 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! Quote Link to comment Share on other sites More sharing options...
Doctor J Posted February 5 Share Posted February 5 Wire the pickup directly to a bog standard jack, find whether the pickup itself is the issue. 2 Quote Link to comment Share on other sites More sharing options...
andyhaines Posted February 5 Author Share Posted February 5 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! Quote Link to comment Share on other sites More sharing options...
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