1974 Maxon single coil J-bass pickup restoration/repair - how do these things even work ? Very unusual pup design ...

[murgel]

hey everyone .. i'm new to this forum and this is my first post 😊

 

i'm a guitar designer/builder but also a bass player and i'm working on some bass designs and pickups. I've wound a number of my own guitar pups and last year a friend of mine who is an amazing bass player came to me with a problem he had with the pups of his 1974 Ibanez J-Bass. These are pretty interesting pups and there isn't any information on them on the interweb so i thought i'd share this here to maybe gain some insights into this unusual pup design ... and hopefully provide something interesting/entertaining to the pup nerds here.

 

The Story:

The player tours a lot and plays on pretty big stages so the problem he had with these pups was that they were ridiculously microphonic. He loves the bass and the sound but frequently plays on stages where it just wasn't possible to use it because of the microphonics. When i first plugged in the bass i immediately understood why he was in love with these pups .. they just had 'something magic' for lack of a better description. Compared to my Fender J-Bass these pups were so lively and detailed the difference was crazy. So the idea was to pot the pups in an effort to reduce the microphonics, but i also noticed that the pups were mounted super loosely and wobbled around a lot which produced a lot of mechanical noise when moving the bass and/or hitting the pups or pickguard. Also, being single coils they were pretty noisy so i decided to shield the inside cavities and resolder everything cleanly to reduce EMI and hum which both also posed problems on big festival stages.

 

Little did i know what an odyssey this would become 😂 it turned out that i wasn't the first to tinker with these pups and whoever put a hand to them previously did a super poor job to put it mildly .. the pups had been repaired/rewound/soldered numerous times in the 45 years of their life and the materials and components had warped and degraded a lot over time. Just getting the masking tape of the coils took me a couple of days since it had disolved into a kind of gunk which had seeped into the coils. Then the coils had been rewound so loosely they barely held together and the coil wire was hanging out of the coil on all sides and had just been taped into place. When i finally got the tape of i found out that the coils had been not only rewound partially or completely or both but that there were numerous solderpoints that were pressing sharp ends into the coil since they hadn't been isolated. It looked like the coils had been completely rewound at some point (since i can't imagine a Japanese company winding coils so loosely and poorly) but then also partially unwound and rewound a couple of times to fix/solder broken wire (no surprise with such a loosely wound coil). The bobins (if you can even call that flimsy piece of soft plastic a bobbin) were warped in all directions as well as the 'pole plates'. The rewound coils were so uneven and wider than originally intended that the previous repairman had filed down the inside of the metal housing to cram them back in there.

 

After about a week of working on it i managed to fix everything as best i could:

 

• remove masking tapes and clean up the gunk
• unwound about 10% of the coils to get rid of the poor and sharp solder points
• rewound an outer layer to trap the loose coil wires so it wasn't hanging out of the coil anymore
• lapped down the pole plates so they were flush again
• heat pressed the coil back into shape (more or less)
• retaped the coils back together
• potted the pups under a weak vacuum, i wanted to pot hem without vacuum first so not to change the sound too much but also didn't want to leave the coils to long in the hot wax since the bobbins were so flimsy and would deform a lot under heat, so i used a bit of vacuum to speed up the process but didn't wait until all the bubbles came out.
• fixed the mounting of the pups so they sit tight and don't wobble around anymore

 

Also shielded the cavities and resoldered and grounded everything cleanly .. since the tuners were almost completely unusable i also completeoly dissasembled and cleaned them and they are butter smooth now. Then did a general setup where amongst other things i noticed that the truss rod had been cranked almost to the breaking point. Leveled out some high/low frets and now with a proper fretwork and adjusted truss it plays like a dream with pretty low action. The bass is now back touring the world with him and he has had no more problems with microphonics or noise 😎

 

 

The pup design:

SO this is the main reason i'm posting this here and i was hoping someone has a take on this unusual design. There are 3 main things i'm wondering about:

1. Is this a known pup design and does anyone know of another pup/manufacturer who has used this design ?
2. The magnets are mounted with opposing/repelling poles against each other, why ? Is this the intended original design or did the dude who 'repaired' them previously just mount them the wrong way round?
3. The top of the pole plates do not touch the metal covers and are pretty far away from the strings but the magnets touch the mount of the screws so i guess there is some magnetism going through there as well as from the top of the pole plates but from opposite magnet poles ... why? Wouldn't that cancel each other out a bit ?

 

For reference here are some pictures of the teardown and repair:

 

The patient on the operating table:

 

Uncovered:

 

The teardown:

 

Tape gunk removal:

 

This dude took scatter winding to a whole new level:

 

Pole plates warped and barely touched anymore, magnets are mounted with repelling poles facing:

 

Lapped down the pole plates so they sit more or less flush again but some parts were too warped and i didn't want to take away too much material:

 

Measured the resistance before opening them up:

 

Shielding the cavities, the shielding on the pickguard is more to keep the warped pickguard flush with the body so it doesn't produce any mechanical noise:

 

Cleaning up the tuners:

 

I'm glad all worked out in the end and so happy to see this bass touring on big stages without problems now. The player is really an amazing artist and top level bass player and he has a collection of high end basses and is adamant that these pups (especially the bridge pup) are really something special.

 

I'm working on a new bass model together with the player now and of course i am tempted to replicate this pup design for it ... but the one thing i'm truely wondering now is wether these pups sound so great because of the original design .... or because the dude(s) who previously botched the repairs on them introduced some magic to them by accident 😂

Edited June 8, 2023 by murgel

[BigRedX]

I think @Bassassin would be interested in this and may have some additional information.

[tauzero]

I would hazard a guess that the magnets are mounted like that so they reinforce each other when inducing the magnetic field in the pole pieces. Looking for a diagram to try to confirm this, I spotted exactly this configuration used in P90 pickups. While P90s have separate pole pieces rather than a blade type, it's got the back-to-back magnet. More detail: https://courses.physics.illinois.edu/phys406/sp2017/Student_Projects/Spring10/Dan_Carson/Dan_Carson_P498POM_Final_Report_Sp10.pdf

which also mentions that this would be paired with a second pickup with the magnets reversed to give hum cancelling.

[murgel]

hey thx @tauzero that's really helpfull .. so it's some sort of a p90's hybrid .. interesting, the difference being that the induction seems to be generated by both magnet poles, partly through the blades and partly through the screws on the sides. since the coils don't sit in their original position anymore and the inside of thwe covers have been filed down it's difficult to tell wether the blades were supposed to be touching or be really close to the covers. stil quite unusual to use both poles of the magnets for induction i think. from my research it seems that thes pups weren't used on a lot of models .. only on a few models of some Ibanez J-Basses and some Grecos and possibly some other of the Matsumoko low cost brands it seems.