50's Supro Lapsteel - Rewiring control cavity, assistance needed...

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by infiniteposse, Jun 11, 2008.

  1. infiniteposse

    infiniteposse Member

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    Hi all-

    I bought a dead Supro 6 string lapsteel a while back and now that I've learned how to solder and have wired up a few guitars, I thought I'd take a crack at it.

    It's not a standard wiring config I've seen before though and I've been unable to find any wiring diagrams on the net, so I thought I'd check and see if anyone had any suggestions on how to wire things.

    I'm guessing I should use 250K pots, use one as tone, the other as volume and then use some cap value in between. Since there's no pup switch, I'm unsure how to wire things since I'm used to all connections going to switch. I'm assuming everything gets wired point to point between all the components, but I'm not sure how it should look.

    The pup has a single wire coming out from below it and there's a ground lug attached to the baseplate of the bridge assembly that I'm thinking functions as ground and perhaps even the pickups ground since I see no second wire from the pup for a ground...

    If anyone has any suggestions I'd very much appreciate hearing them!

    Thanks much.

    ps - dig the old cap in between the pots... Whoa.

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  2. David Collins

    David Collins Member

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    It's pretty much wired up as it is. Just connect the hot pickup lead to the outer lug on the volume pot (where the cap is connected), and the center lug to the jack. Make sure all grounds are good and it's done.

    Why are you replacing the pots, are the originals frozen? If you do have to replace them you could certainly use 250k, but many lap steels like this would have originally used anywhere from 100k-300k, often different for volume and tone. If you do have to replace them, check what the old ones are if you want to keep it original.
     
  3. infiniteposse

    infiniteposse Member

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    Thanks for the feedback David-

    Yes, one of the pots was frozen and I was getting zero signal from the cable, so I decided checking out the innards was the best way to go. I just got everything connected again with a new tone pot (to replace the frozen pot) and there's still no signal. I'm not even sure how to test of the pickup is working. I get resistance readings off the individual poles, but not off of the hot lead from the pup and the ground at the base of the bridge assembly.

    I'm going to keep plugging away at it.

    Thanks!
     
  4. David Collins

    David Collins Member

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    When you say you're "getting resistance readings off the individual poles", are you talking about the poles themselves to ground? That's just testing continuity to ground from each screw, and of course there should and will be direct continuity because they are all screwed in to the same plate. That's just seeing if all the metalwork is well connected, and won't tell you anything about the coils.

    You should be able to check the resistance from the pickup wire you see coming through the plate to ground. If the pickup is working you should get a few thousand ohms resistance. If you get a few ohms resistance then it's shorted somewhere. If you get no contact then it's either a broken coil or a bad connection somewhere inside, sometimes just from the coil to the lead or ground. If that's the case then you would have to pop the cover and check point by point to see where the signal breaks.

    I've worked on a lot of these pickups and see bad connections far more often than broken coils, so they're usually easily repairable. Then when you get the electrical part figured out you'll get to have fun with the magnetic part. The one side magnet they used seems almost like a soft iron, and are often found with a very weakened charge. After recharging the magnet, they can still be a pain to get even response across the strings. One string will sound dead, shift the magnet a bit, now that's fine but two more are muffled. Move it again, now those are fine but the another one is boosted higher than the others. It's a real blast, I tell ya......

    If you have a decent meter and want some help troubleshooting here, the most important thing is to tell us exactly what the reading are, and between what two points you got that reading. Start with checking between the black lead wire coming through the plate and the plate itself. Make sure your meter is on the right setting (20k for most meters). You'll probably be hoping for something in the 3-4k range.
     
  5. infiniteposse

    infiniteposse Member

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    Thanks so much for the great feedback David. I decided to check the resistance of the cable that's wired directly into the laptop and it wasn't reading consistently at all, so I grabbed an old cable, hacked off an end and wired it up. Presto - sound. The volume pot was cutting in and out even after being cleaned and lubed up, so I grabbed another CTS 250k pot and dropped it in just to get all of the "maybe this is what's not working" elements out of the way. I kept the old cap in.

    I strung her up and she makes noise, but your post above reads almost like prophecy. Very good level and response from the E-string. A little worse with the B-string and then the rest are all muffled and weak.

    BTW - taking a reading off my new output cable on the guitar, I get a reading of 5.75k. I'm using a Fluke 83 Multimeter and used the resistance setting to check.

    How do you go about recharging a magnet? I've got an old email contact with Rick Aiello about getting some of his repro magnets for these laptops. They run about $30. Should I just get a new set and be done with it?

    Thanks!
     
  6. kimock

    kimock Member

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    Those pickups are super sensitive to pole piece height adjustment.
    Some of those guitars only use one magnet. If it's working at all the magnet is probably ok, check pole pieces first. . .

    peace
     
  7. David Collins

    David Collins Member

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    Right, one block under the cover will be the magnet, and the block is usually a chunk of wood painted silver.

    Kimock is right in trying the pole piece adjustments first - there are so many different weird lap steel pickups and I forgot yours were adjustable.

    I haven't seen those aftermarket magnets, but it's a good thing for me to keep note of. If the pole pieces can't even it out, you can often balance the signal just by shifting the magnet around a bit. Loosen up the screws over the magnet and make sure it is pressed up tight and square to the inside. Tighten it back down there, then try again and it may be in range of pole piece adjustment. Sometimes I've had to repeat this and give it a bit of a shift left or right, forward or back. Often the balance can just snap right in perfect when you push it in tight, but others have seemed hypersensitive, having to jostle it around until it seemed to find a magic spot.

    If you had to recharge the magnet these can be tricky because they seem so soft. Usually I use some large neodymiums held in a fixture. Most AlNiCo magnets I can insert in to the field, close the magnets on it, then when I reopen and remove it, it will hold it's orientation perfectly. These things on the other hand, if they tilt or twist even the slightest amount as I'm pulling them from my charging magnets, it will totally twist up and change the poles. They just seem to change much more readily than others, so it can be tricky.

    Kimock's right that if it's putting out a decent signal, the magnet is probably fine. Trying to recharge these ones can open a can of worms, but it's probably not something you have to worry about.
     
  8. GuitarsFromMars

    GuitarsFromMars Member

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    Nice posts about the lap steel-I have a 1938 Supro that is a monster tone and has not required a bit of work,at this point.If I need help figuring this out,I will have it handy-Thanks,gentlemen.
     
  9. infiniteposse

    infiniteposse Member

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    Thanks for all of the excellent info guys. I really appreciate it!

    A couple of things. 2-3 of my pole pieces are cracked at the flathead joint, so I'm not sure what the best way to proceed with these would be. Are these pieces replaceable? Do I need to lubricate the grooves at all to safely do this and lessen the risk of braking off the remaining side of the slot? Can lubrication be safely done without damaging the pup?

    Re: the build of the magnets - these are both actually magnets. No dummy side here. Both still pull little bits and bobs of crud to them and they still together as well... Output wasn't stunning, but wasn't horrid either, so I'll work more with positioning the magnets and do what I can with the pole pieces.

    Thanks again guys. Great thread.
     
  10. kimock

    kimock Member

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    I'm sure you can replace those pole pieces, but you might need a donor guitar to salvage them from. The problem is gonna be getting them out if they're stuck. I've never dealt with a "frozen" Supro pickup; mine are all too loose!
    RE: Output. Are the broken poles also the weak ones? Is it possible that this pickup has been apart in the past? If so, and it has two magnets, one of 'em might be backwards, which would pretty much cripple the pickup.

    Also, be super carefull screwing that plate back onto the body.
    Two of the four screws that run through the top of that "keeper" plate are wood screws that go into the body. The front two if I recall attach to the plate. If the previous owner was wrenching on that pickup hard enough to bust the poles, chances are good that the non-wood screws are ready to have the heads torqued off as well. That would be no fun. Carefull. . .I've done it twice!

    You might want to contact Rick about the pole piece extraction thing, this is now officially over my head.

    good luck!
     
  11. walterw

    walterw Gold Supporting Member

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    if this thing is anything like my oahu with a removable coverplate that the strings pass under held off the coil by the 2 magnets, then that would be my first guess as to the issue. make sure the two magnets are oriented so the same magnetic polarity is "up" on both of them, rather than one up and one down.(touch them together to make sure that repelling faces are both going the same way.)

    +1 to polepiece height having a dramatic effect on output.
     

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