Strange wiring in bass

[BigRedX]

I recently bought a Kramer 450B which is extremely good condition for a 30 year old bass. However the volume and tone controls were very crackly and the jack socket intermitent, so I opened up the control cavity and found this:



Not exactly original equipment!

The circuit appears to be the same for each pickup/volume/tone control set:



The bit I've labelled output goes to the pickup selector switch and then to the output jack.

Any idea what the pickup coils in the control cavity are supposed to do?

I don't have any values for the pots as they're hidden under all the solder...

At the moment as well as being very noisy when turned the controls only do something between 7 and 10.

Edited November 22, 2007 by BigRedX

[bnt]

My first thought was that those coils are a kind of humbucking system: no magnets, so they don't pick up the strings, but they could pick up noise and mix it in out of phase. Never seen one like that. though.

[BigRedX]

To cancel out the noise they have to work as pickups which IIRC means they need magnets. I checked and the 'pole pieces' are not magnetic at all. They look like the coils from a Dimarzio humbucker (they've got the allen key slots). I think they're supposed to work as either bass filters or bypass...

[Mr. Foxen]

[quote name='BigRedX' post='93108' date='Nov 23 2007, 12:28 AM']To cancel out the noise they have to work as pickups which IIRC means they need magnets. I checked and the 'pole pieces' are not magnetic at all. They look like the coils from a Dimarzio humbucker (they've got the allen key slots). I think they're supposed to work as either bass filters or bypass...[/quote]

I'm not sure they do need to be magnets, hum is induced by a fluctuating magnetic field, whereas string sound is from the metal string moving in the magnetic field. You can pick up hum in leads and stuff if they aren't shielded, so magnets aren't needed.

[BassBod]

Looks like a version of the Alembic system - using a "dummy" pickup as a noise canceller? Can't imagine that was ever standard on a Kramer?

I've never owned a Series I or II Alembic, but I recall they have trim pots (active gain adjustment) to fine tune the effect of the noise canceller in relation to the main pickup outputs. My guess is that to work well they should ideally be physically close to the real pickups so they "hear" the same noise?

Worth some investigating...

BB

[bnt]

[quote name='BassBod' post='93205' date='Nov 23 2007, 10:10 AM']Looks like a version of the Alembic system - using a "dummy" pickup as a noise canceller? Can't imagine that was ever standard on a Kramer?[/quote]
That's what I was thinking of - it's just that the placement is weird. Alembic at least put them in front, in the same plane as the strings. It's the middle "pickup" in this picture:

mmmm... Alembic... (drool)

Edited November 23, 2007 by bnt

[BigRedX]

However if you look at the circuit diagram with the tone control set to minimum treble cut the coil is essentially by-passed. If it was some kind of hum cancelling device it's opperation would need to be constant would it not?

Edited November 25, 2007 by BigRedX

[bnt]

Well, with the treble down, haven't you cut out most of the hiss, anyway? See, you gotta think about these things...