Repair of Swedish Kjell-pickup

[Peter Naglitsch]

As required in another thread, some photos of a repair of a vintage Swedish pickup; Kjell mikrofonen (mikrofonen meaning pickup in swedish...). This pickup was created by swedish Jazz and session player Kjell Sjölund in the early 50's. This is what it looks like:

The pickup is made to be attached to the neck, hovering above the top of the guitar. The problem was that the screws were rusted stuck and I had to be a bit too violent to them to get them out. New screws had to be made (unfortunately no pics of the finished pickup). The pickup itself was also totally dead and the circuit open.

Carefully prying it open (it as obviously been worked on before...) this is the inside:

It is at first glance something similar to a P90 in construction, but more on that later. Is also has an air coil wrapped in cloth for insulation. Now I got really nervous. Lets hope the problem isn't within the coil itself. That would be bad.

The magnet/keeper bar assembly came out pretty easily:


Peeling of one small extra piece of cloth insulation around the connection lead wire shows the joint between the lead and the coil:


I was fortunate enough to find a bad solder joint in there. The coil itself was OK:


When examining the magnets I noticed that they were not oriented like I first thought (like in a P90, same poles facing inwards). Probably because they would have been to thin to hold the strength if they were oriented like that. They were instead oriented with north/south pointing outwards towards the treble or bass side of the strings. That was a surprise to me. I had to play around a bit to orient them to get the most out of them as their magnetic fields were not exactly symmetric either. In the end I got the oriented in a way that still produced a lower-than-average-on-a-modern-pickup magnetic pull. The pole screws in the end measured from 150 Gauss at the E string poles to around 80 Gauss at the D and G strings (forgot to document this).

Hope this was interesting.

[jonson]

Gotta admit that coil would have been fun to do as at 11.24k that's gonna be 45/46awg in there. Not as difficult as some think to wind with and with ease once mastered.

[David Schwab]

That's a weird pickup!

So, wait, the magnets had the poles on the long ends?! Like this? N=======S

[Peter Naglitsch]

Like this? N=======S

Exactly! It took me a while to understand why the magnets didn't behave like I expected them to do...

Tried to re-orient the poles, but that didn't work (and maybee that was good, as it would have made this a different animal from the original). So I just tried to boost them up as high as possible (didn't work either) and ended up flipping then 1/4 turn at a time to find the combination that gave the highest gauss readings at the pole screws as they were also a bit asymmetrical and after running through all 16 combinations I found one that gave the pickup a decent output. The sound is warm and still clear. a bit low output in my test guitar but still nice. I guess that in an arch top with thick strings it will give a bit more output.

[Mike Sulzer]

Did you finish with the two Ns pointing to the opposite sides of the guitar?

[Peter Naglitsch]

No I ended op with them positioned in the same direction. I tried all the 32 combinations (I think it ends up with 32, 2 parts in 8 different position, should be 32) and the end positions were the combo that gave the highest, and most even result along the pole screws.

[David King]

Very strange pickup indeed. At first I figured the magnets must be ceramics but I judge these are actually AlNiCos?

[Mike Sulzer]

Very strange pickup indeed. At first I figured the magnets must be ceramics but I judge these are actually AlNiCos?

I do not remember having ceramic magnets available until the 60s.

It would appear that the strings are mostly magnetized directly from the magnets rather than magnet-pole-string as in a P-90 or regular humbucker, although it is not clear what was really intended. This gives a wide aperture, perhaps effectively wider than double aperture of the normal humbucker. Thus the higher strings harmonics will be filtered out, noticeably so on the bass strings, giving a jazz sound similar to a humbucker (but probably not exactly the same).

[Mike Sulzer]

That is what makes the most sense, but I was not sure with the additional asymmetries that you mentioned.

Magnitzing the string in the vertical direction (with the guitar sitting flat on a table) is most effective because this maximizes the changing flux through the coil. In this design most of the magnetization occurs in the other direction, but there is a component in the vertical, more so in the outer strings, rather than the interior ones.

No I ended op with them positioned in the same direction. I tried all the 32 combinations (I think it ends up with 32, 2 parts in 8 different position, should be 32) and the end positions were the combo that gave the highest, and most even result along the pole screws.

[Peter Naglitsch]

Very strange pickup indeed. At first I figured the magnets must be ceramics but I judge these are actually AlNiCos?

Yeah, should have mentioned the materials... The magnets are probably Alnico of some type (at least they are electrically conductive, who knows what they used in the early 50's). The keeper bar is some type of soft iron and so was the screws. The original screws didn't survive (they were to rusted) so I had to make new screws. I used some metric pole screws and cut of the heads/cut an adjustment slot so that they looked more like Mini HB screws. No, wait, the customer did that part, but he initially got the screws from me. The cover looked like nickel-silver and the bottom was plated brass.

To clarify: If I flipped the magnets 1/4 of a turn (keeping the poled oriented in the same direction) I could measure a slight difference in the magnetic field at the screws. This must come from some type of irregularity in the magnetic material or how the material is oriented inside the magnet (maybe the magnet was'n cut exactly along the magnetic orientation???) or something like that. So I tried all the combinations to get the best out of the magnets, measuring the gauss levels of all screws for all combos.

Adjusting the pole screws did really change the output of the strings, although not as much as on a modern pickup. So some of the magnetic field were indeed going through the screws up against the strings. How much, I dunno. But I did manage to smoothen the output from string to string with a drastically elevated D string screw.

[Mike Sulzer]

Y

Adjusting the pole screws did really change the output of the strings, although not as much as on a modern pickup. So some of the magnetic field were indeed going through the screws up against the strings. How much, I dunno. But I did manage to smoothen the output from string to string with a drastically elevated D string screw.

The second purpose of the poles pieces, entirely independent of guiding the permanent field to the strings, is to guide the field from the vibrating strings through the coil, increasing the changing flux. Yes, there is some permanent field in the screws, but even if not, changing the screw height would change the output some because the screws guide more field down from the vibrating string when they are closer the strings. This pickup is a good example of the basic principles of pickup operation, applied in a different way.

[David King]

So this is almost like a horseshoe pickup without the horseshoe?

[Mike Sulzer]

So this is almost like a horseshoe pickup without the horseshoe?

If the two magnets are to be like the two ends of a horseshoe magnet, they should have opposite magnetic polarity in the direction of the strings, that is, one N the other S. They are the same, however.

[fieldwrangler]

Is there a way to see the pictures that seem to have been a part of these posts originally?

[Audiotexan]

Sadly, no. They (among countless other files) were lost in a server issue a year or two back

Some things have been reposted as available/requested (by those OP's who are either still active, or other members who managed to save a copy) so there's always a chance. But I wouldn't hold my breath!

All most of us can do is keep our fingers crossed, ask, and hope!
I know ya have two out of three done, but did you remember to cross your fingers!?
GL