Advice on rewind: 1974 J Bass

[chad h]

A friend from a local shop has asked me to rewind a pickup for him. I want to make sure I do a good job for him.

This is a NECK pickup from a 1974 Fender Jazz bass.

Two important questions.

1. The pickups have a thick layer of black gum on the bass, and to this is attached some foam. I can easily replace the foam, but I don't know what this thick black gum is. My first thought was to scrape the whole lot off. But then I started worrying about whether the owner would want the instrument in a condition as close to original as possible. I believe I can rewind the pickup without having to remove the gum. So I guess I want to know what others would do. Scrape the gum off, make the pickup look nice and clean, rewind it and then attach a new strip of foam? Or preserve as much of the original junk as possible, while restoring the coil to its original condition?

2. Is there some kind of convention for the direction of the wind on the J-bass pickups? The poles on the neck pickup have south facing up. The poles on the bridge have north facing up.

Thanks for your advice.

Chad

[jonson]

You should be able to see the wind direction from start finish wires but north clock and south counter clock is common. Either keep existing gum and stick the foam back with ca or scrape off and use black silicon sealant to mimic it. Wind with PE.

[chad h]

Many thanks.

[ken]

The gum is what is left of the glue on the foam, the glue rots the foam with age. I have a '75 JB pickup here with the same problem. :P

Just clean off all the gum (lighter fluid might work), and first try to resolder the pickup. If that doesn't work then rewind.

ken

[David King]

Also important would be to match the turns of the original (which will be less than ideal). Figure about 7600-7800 turns for 6.8 k or so. Check the DCR of the Bridge and subtract about 5% for the shorter bobbin. Of course your dcr will depend greatly on the actual diameter of the copper. Try using Coil Estimator calculator.

[David Schwab]

I rewound a set of 1976 Jazz bass pickups. I first weighed the bobbins, which were 62grams for the neck, and 68 grams for the bridge. I ended up winding 8,000 turns of 42 PE.

If you are looking at the bottom of the pickup, with the hookup wires facing down, the neck's start wire is on the right eyelet, and the bridge's is on the left.

[David King]

David's weights listed above would indicate more like a 10% difference in DCR between neck and bridge.

[David Schwab]

The neck and bridge pickups are wound the same, but the bridge pickup is longer. So it has a little more wire on it, and it weighs more even empty.

[chad h]

Thanks to everyone for the great insights.

This gummy stuff on the bottom is truly gross. I kid you not - it looks like tar from the road, and it's almost 1/4 of an inch thick. This can't just be glue. Anyway, it's super sticky, but in a crumbly sort of way. I'm sure I'll be able to get it all off.

At this stage, I gather the coil is damaged. I can't get a DCR reading using the leads, or testing at the terminals. And somehow, the coil looks kind of beaten up anyway.

One thing that confuses me is this:

My friend played the bass for me, and it produced amplified sound from both pickups. But the neck pickup output had no body at all. But if the coil has a short, shouldn't it have given NO output? How do you get some (limited) output from a shorted coil?

[Audiotexan]

This gummy stuff on the bottom is truly gross. I kid you not - it looks like tar from the road, and it's almost 1/4 of an inch thick. This can't just be glue. Anyway, it's super sticky, but in a crumbly sort of way. I'm sure I'll be able to get it all off.

What Ken said in post 4. That's what it is. Deteriorated foam/rubber. (Similar to the way that rubber belts that are in unused turntables would turn to 'semi-liquid' goo after 10-20years)

They used to use them as "spacers" years ago to keep the pickup from wobbling around loose in the cavity.

Personally the best way I've found to get rid of it is Goo-Gone. Then clean the oily residue left by that with a quick pass of rubbing alcohol.

[David Schwab]

Thanks to everyone for the great insights.

This gummy stuff on the bottom is truly gross. I kid you not - it looks like tar from the road, and it's almost 1/4 of an inch thick. This can't just be glue. Anyway, it's super sticky, but in a crumbly sort of way. I'm sure I'll be able to get it all off.

It's what's left of the foam. After a while they plasticize and turn to sticky goo. Try naphtha.

At this stage, I gather the coil is damaged. I can't get a DCR reading using the leads, or testing at the terminals. And somehow, the coil looks kind of beaten up anyway.

Try heating up the eyelets with a soldering iron, and maybe flow a solder on. Often the solder connections oxidize. I might wake the coil up.

One thing that confuses me is this:

My friend played the bass for me, and it produced amplified sound from both pickups. But the neck pickup output had no body at all. But if the coil has a short, shouldn't it have given NO output? How do you get some (limited) output from a shorted coil?

If you tried reheating the solder joints and you are not getting a reading, it means there's a break in the coil. A pickup with an open coil will still produce sound, because the windings are capacitively coupled. The tone is usually very weak and thin sounding.

If you have to rewind them, and it sounds like you might, I'd wind the bridge pickup a little hotter (or the neck pickup a little cooler). They will balance better that way.

[chad h]

Thanks everyone for the advice. I successfully cleaned up both pickups, rewound and potted the broken coil, and presented the finished results to my friend. He was thrilled with how the bass sounded.