REALLY WEIRD pickup rewinding question (Tele content)

[ken]

Hello all,

I rewound a dead 1950's Fender Tele rhythm pickup for a friend of mine.

I removed all the dead coil wire, cleaned up the bobbin, redipped the bobbin in nitro, and after letting the bobbin dry for 24 hours I rewound it with 43 gauge PE. Once the coil was full, I potted the coil in nitro and set it aside to dry for 48 hours.

Here's the weird part...

Before potting, the coil was roughly 7.4 K DCR, 7.5K@120Hz and about 9.6K ACR@1 Khz.

After I repotted and the coil was dried, I got roughly 5.3K DCR, 5.5K@120Hz and 5.9K@1Khz.

The solder was really grainy looking for some reason, so I resoldered and the DCR went back up to 7K. After a half an hour or so and the pickup cooled, the meter readings dropped back to the lower level.

I thought the nitro was too hot for the PE, so I removed the coil, recleaned and redipped the bobbin, and rewound the coil again, and repotted the coil with a fresh 'never before opened' can of nitro.

I'm getting the same weirdo readings as I did before, but this time the coil seems to be leveling off at about 6K DCR and 7.2K@1Khz.

My question is... WTF? I never had this happen before, could I have a spool of 43PE with insulation that 'went bad' on me, or is there something else wrong? I have made coils with the same spool of 43PE that all worked OK, and used the same can of nitro for dunking all my pickup coils before winding as I did to treat this one pickup.

Thank you,
ken

[big_teee]

I wonder what results You would get if You wax potted a similar pickup.
I'm sure you checked that nothing was shorted to the poles?
Were all the tests done, at the same Temperature?
B_T

[David Schwab]

Did you check to see if you have any continuity between the coil and the magnets? Sounds like you are getting shorts to me.

Why not tape up the magnets next time. it wont alter the tone, and the pickup will have a longer life. Fender pickups die because the wire is touching the magnets and corrodes in time. It's a flawed design. The lacquer isn't all that helpful. Lacquer creeps unless it's been curing for about at least a month.

[ken]

Here comes the weird part... I have no shorts between the coil and magnets. I unsoldered the coil from the bobbin eyelets, and tested the coil. After I did this I got 7.5K DCR on the coil leads.

I should have said that after I reinsulated the bobbin with the nitro, I baked the bobbin at 140F overnight to dry it. Then I let it sit 48 hours at room temperature before I rewound the coil the second time.

ken

[Steve Conner]

Sounds like your flatwork has become conductive! Try measuring resistance between the eyelets with no wires connected. Also between the eyelets and the magnets, and between the magnets and the coil as Dave suggests.

[David Schwab]

I should have said that after I reinsulated the bobbin with the nitro, I baked the bobbin at 140F overnight to dry it. Then I let it sit 48 hours at room temperature before I rewound the coil the second time.

Instead of doing all that, why not tape the magnets up? What are you accomplishing doing all that work? Since something is not working right, I'd try a new pickup and try doing it a different way this time.

If I had to do all that on every bobbin I'd never get any pickups made! Also, old Fender pickups fail precisely because that's how the magnets were insulated.

[big_teee]

I either wax Pot, or no Potting at all.
The Lacquer is IMO an antiquated method, why bother.
T

[ken]

Sounds like your flatwork has become conductive! Try measuring resistance between the eyelets with no wires connected. Also between the eyelets and the magnets, and between the magnets and the coil as Dave suggests.

Hey Steve, you're right! THE BOTTOM FLAT WAS CONDUCTIVE!

I was rewinding a real 1950's Fender pickup, so changing the 'recipe' by adding tape was simply not an option. To many of my customers, the old way is the best way and I do my best to please my customers anytime I can. Besides, there was no flaws in the magnets that would cut coil wires.

I desoldered the coil from the bobbin eyelets, and the coil DCR went back up to 7.5K like I said. I borrowed a megger from a ham buddy and tested from magnet to magnet. I got nada. Then I tested between the eyelets, and got 5.3K, so the leakage was only between the eyelets. Go fig.

I thought this only happened under certain conditions and with high voltages involved, I never heard of this happening to a pickup.

I'm going to ask the owner if I can replace the bottom flat with one of mine, or should I try to save the original flat somehow...

Is this weird or what?

Thank you,
ken

[David Schwab]

I either wax Pot, or no Potting at all.
The Lacquer is IMO an antiquated method, why bother.
T

He was doing that to insulate the magnets.

[David Schwab]

I was rewinding a real 1950's Fender pickup, so changing the 'recipe' by adding tape was simply not an option. To many of my customers, the old way is the best way and I do my best to please my customers anytime I can.

It's not a "recipe". Lacquer does nothing to change the tone, and just insulates the magnets. Tape will do the same thing. Tape will also last longer. History has shown this to be true. Of course pickups that are built to possibly fail in the future is a good for rewinders.

It also has nothing to do with flaws on the magnets. It's due to "Inner Coil Pole Corrosion."

[ken]

That's just it... there was no corrosion or other flaws anywhere on the magnets at all. I have seen other pickups with casting flaws and other nasties on the outside of the magnets, but not these. They were nice and shiny all over, nicer than I expected them to be.

The owner said he bought the pickup to put in a 1958 Tele, but the pickup seemed to have little power so he pulled off the cover to see what was wrong.

While he was removing the cover he accidentally poked a screwdriver into the coil, breaking the windings.

ken

[David Schwab]

While he was removing the cover he accidentally poked a screwdriver into the coil, breaking the windings.

That'll do it! I keep screw drivers away from coils!

Now the question is, how did the bottom flat become conductive?

[ken]

Dave, that is a seriously great question.

According to the research I've been doing supposedly Forbon's natural color is light gray. Carbon black (powdered carbon) is mixed into the Forbon 'dough' during manufacture to make it black. Then the Forbon dough is extruded into sheets and vulcanized (baked under pressure) to make it tough and flat.

Supposedly, under certain conditions the powdered carbon in the mix can conduct from one granule to another, actually forming a leakage path or resistor. If a leakage path exists between two terminals, this can add parallel resistance to the circuit it's part of just like it did in my Tele pickup. I'm told that when this happens the whole piece of Forbon doesn't become conductive all at once, and you can have an entire circuit board with only a very small area gone bad.

I do see how this can work, but I can't understand how this stuff was used in high voltage transformers for 100 years or so without this happening more often.

Did you know that Forbon is actually considered a 'green' material? It's recycled cardboard... a cousin to MDF.

ken

[big_teee]

If you ever want to check an insulator for Conductivity, just put it in the Microwave.
If it warms up, then it is conducting. If it stays cool, then it is fine.
So does the grey have carbon it it? It has some black specs in it?
T

[David Schwab]

Dave, that is a seriously great question.

According to the research I've been doing supposedly Forbon's natural color is light gray. Carbon black (powdered carbon) is mixed into the Forbon 'dough' during manufacture to make it black. Then the Forbon dough is extruded into sheets and vulcanized (baked under pressure) to make it tough and flat.

Supposedly, under certain conditions the powdered carbon in the mix can conduct from one granule to another, actually forming a leakage path or resistor. If a leakage path exists between two terminals, this can add parallel resistance to the circuit it's part of just like it did in my Tele pickup. I'm told that when this happens the whole piece of Forbon doesn't become conductive all at once, and you can have an entire circuit board with only a very small area gone bad.

I do see how this can work, but I can't understand how this stuff was used in high voltage transformers for 100 years or so without this happening more often.

Did you know that Forbon is actually considered a 'green' material? It's recycled cardboard... a cousin to MDF.

ken

That makes sense. This same thing was happening with certain black tapes wrapped around humbuckers. But now this begs the question as to why it happened now, and not when the pickup was made?

Maybe the lacquer and oven? Could be the solvents in the lacquer drove the carbon to the surface. You'd have to try some new black pieces to see what happens.

So how did you stop it from being conductive?

MDF is green, except for the binder, which is urea-formaldehyde, and is considered a probable human carcinogen. But we all know how nasty that is when you make it into dust! I use it all the time, but you have to wear a mask.

I'm starting to use forbon more and more. Now that you can have it laser cut, it becomes a great material for pickups.