pickup misshap

[groover]

Hey guys,
I was dremmel-ing off the corrosion from the pickup rivets on my '67 sparkle top and the dremmel skipped off and hit the pickup winding on one pickup. I've taken off some of the green insulation but i'm still getting a reading on the multimeter when i measure for resistance on that pickup. Its jumping around alot so i'm not 100% confident in the multi-meter or the pickup. Its moving around between 180ohms and 165ohms. Just wondering if i've buggered the pickup when i took off some of that green insulation from the winding, obviously i was lucky enough not to break the wire.

Thanks for any advice

[pnoboy]

Look at the pickup under strong magnification--using a microscope would be best.  Look to see if the lost insulation is just right on the top of the wires, or if it goes down to the sides of some of the wires such that the bare metal from one turn could touch the bare metal from another.  An even worse possibility is that the damage went below the outer layer into the next layer.  If either of the last 2 possibilities is true, you'll have to unsolder the wire from the terminal that connects to the outside layer, and unwind the pickup until you hit undamaged wire.  If you only have to unwind a little bit, it's probably OK just to solder the good end of the wire to the terminal.  If you have to unwind quite a bit, the pickup should be rewound, or if you're really good with a soldering iron, you could potentially splice in a piece of new wire to make up for the wire that was lost. 

[David Aubke]

Look at the pickup under strong magnification--using a microscope would be best.  Look to see if the lost insulation is just right on the top of the wires, or if it goes down to the sides of some of the wires such that the bare metal from one turn could touch the bare metal from another.  An even worse possibility is that the damage went below the outer layer into the next layer.  If either of the last 2 possibilities is true, you'll have to unsolder the wire from the terminal that connects to the outside layer, and unwind the pickup until you hit undamaged wire.  If you only have to unwind a little bit, it's probably OK just to solder the good end of the wire to the terminal.  If you have to unwind quite a bit, the pickup should be rewound, or if you're really good with a soldering iron, you could potentially splice in a piece of new wire to make up for the wire that was lost.

But what would be the problem if some of the insulation was damaged? Seems to me, it would just short a couple of wraps and lower the pickup's overall resistance by an immeasurably small amount. It wouldn't ruin the pickup's circuit.

[pnoboy]

Unfortunately, shorted turns do much more than reduce the resistance, and do ruin the pickup's circuit.  The moving tine, because it changes the magnetic flux in the core of the pickup coil, generates voltage in the pickup windings.  If there are shorted turns, that voltage will induce current in the shorted turns by means of mutual inductance.  That current will reduce the output of the pickup.  The situation is similar to what happens in a transformer.  Imagine you connect the primary of a transformer to an a.c. voltage source.  All is well.  Now short the secondary--that will create high current in the secondary and in the primary as well.  Think of shorted turns like a shorted secondary. 

[David Aubke]

Well, I don't know nuthin' 'bout no "mutual inductance" so I'll take your word for it.

I just assumed that shorting a single wrap was equivalent to doubling the thickness of the wire for that one wrap but I know very little about the behavior of electrons.

[groover]

Well the wire actually ended up breaking. I've wound it back to the break and its about 40cm from the bobbin. Can i just solder that to the post or is that too much lost wire? If it is i guess i could try to join the wire at the break but i don't know if i have the skills for such delicate work....