G&L L-1500 pickup question

[MarkusBass]

I'm trying to solve a hum problem of a 5-string G&L bass with one humbucker pickup. I used copper foil to shield the bass but this did not help. The bass is humming both in passive and active configuration. What I found is that the steel pole pieces in the pickup are not grounded. Is it normal, or they should be grounded? I checked several other basses with humbucker pickups and they have the pole pieces grounded. Could this be a reason for the hum? I tried to ground them with a copper foil but it worked only for two pole pieces (out of 5) so it is not a good method. I'll try to ask on the G&L forum but I thought that it could be worth asking experts here.

In the evening I will post some photos.

Mark

[bbsailor]

There is one more thing to check.

See if the strings are properly grounded by using an ohm meter to test continuity from each string to the jack ground.

Joseph J. Rogowski

[David King]

The pole pieces and any other exposed metal should absolutely be grounded. Copper tape, even if it has conductive adhesive will need a positive pressure either from springs or compressed foam under the pickup to get a reliable connection. Since I think these pole pieces are adjustable it might be complicated due to the moving parts but I've never looked at the bottom of one. Another secure way to ground pole pieces and magnets is with nickel conductive paint. The nickel particles are attracted by the magnets as the paint begins to harden in place. Copper tape can be applied and burnished to the paint and then soldered to your drain wire.

All that said, it's possible that the coils are unequal and can't effectively cancel. You need to deal with two different sources/types of radiation; magnetic and electrical. Shielding/grounding can only help with the electrical, the magnetic must be canceled out using a CMR scheme.

[MarkusBass]

There is one more thing to check.

See if the strings are properly grounded by using an ohm meter to test continuity from each string to the jack ground.

Joseph J. Rogowski

This was the first thing I checked. And of course they were not correctly grounded but I fixed it. This didn't help.

[MarkusBass]

The pole pieces and any other exposed metal should absolutely be grounded. Copper tape, even if it has conductive adhesive will need a positive pressure either from springs or compressed foam under the pickup to get a reliable connection. Since I think these pole pieces are adjustable it might be complicated due to the moving parts but I've never looked at the bottom of one. Another secure way to ground pole pieces and magnets is with nickel conductive paint. The nickel particles are attracted by the magnets as the paint begins to harden in place. Copper tape can be applied and burnished to the paint and then soldered to your drain wire.

All that said, it's possible that the coils are unequal and can't effectively cancel. You need to deal with two different sources/types of radiation; magnetic and electrical. Shielding/grounding can only help with the electrical, the magnetic must be canceled out using a CMR scheme.

Thanks for the info. I may try with the nickel conductive paint but I'm not sure where to buy it.
The pickup looks like this:







The bass is at least 15 years old but it is hard to believe that the pole pieces were somehow grounded when the bass was new. You can see that the magnet does not touch the pole pieces. Is it possible that the pole pieces were grounded inside the pickup but now the connection is broken (due to pickup age)?

Mark

[MarkusBass]

I asked about the problem on the G&L forum and I've got confirmation that the pole pieces in the pickup are not grounded. But I still would like to test what happens when I ground them.

Mark

[Helmholtz]

I asked about the problem on the G&L forum and I've got confirmation that the pole pieces in the pickup are not grounded. But I still would like to test what happens when I ground them.

Mark

As the strings are (now) properly grounded, all you need for testing is sqeezing some metal parts between strings and pole pieces.

[David King]

I have to wonder what the purpose of the copper plated steel plate on the back of the ceramic magnets is. I suppose it's a part of the magnetic circuit. I'm guessing that the plate is glued to the magnets and that the magnets are glued to the pole pieces. Since the magnets are non-conductive the plate isn't grounding the poles.
For starters try conductive tape across the tops of the poles to ground them and see what happens. If that gives you an improvement then acquire a can of MG Chemicals Nickel conductive paint and mask off and spray a narrow stripe running around the copper plate that passes over all the pole piece edges and up the sides of the magnets and onto the copper plate.

Test the coils by tapping on each row of poles with a small screwdriver while the bass is plugged in to make sure all the coils are active. If one has gone bad that that's the most likely explanation. You might be able to fix a dead coil by simply reflowing the solder connections where the coil wires terminate. If that doesn't help then rewinding the coil or replacing the pickup would be the next logical steps.

If you want to test whether additional shielding would help in any way simply wrap the entire body of the instrument in aluminum foil while connecting the foil to ground (via the jack.)