Help! Carvin PB5 With Mystery Single-Coil Hum

[Jory D]

Hey Talkbass!

I recently acquired a used Carvin PB5 from Ebay with a single coil J pickup in the bridge position and all passive V-V-T electronics. I really like this bass so far, except for this one weird issue with the electronics. For some reason, the split coil P pickup is humming like a single coil when used on its own. This hum sounds the same as the hum that comes from the soloed single coil pickup. However, the bass is hum cancelling with both pickups engaged. This is upsetting, because 90% of the time I only want to use the P pickup, which is supposed to be hum cancelling on its own.

At first I thought it might have been due to poor shielding, since the bass was producing lot of noise unless I touched the strings/bridge with my hand, regardless of the pickup configuration, so I went in and applied a bunch of copper tape to the electronics cavity. This has resolved that particular issue, and the bass is now dead quite with both pickups on, but the mystery hum from the split coil pickup remains.

Basically, I'm stumped. Would it be possible to have this issue repaired, and if so, about how much would that cost me? If it's more than $100 or so, I think I might be better off just getting a new set of pickups.

 

[Killed_by_Death]

Did you also shield the pickup cavities when you did the shielding of the controls cavity?
If you did, make sure there's continuity between the shielding in the pickup cavity & the output jack's sleeve.

 

[Jory D]

Thanks for the reply. I did shield the pickup cavities. This was my first time performing such an operation, so it is possible that I didn't do a perfect job. For example, I couldn't get all the way into the routing between the pickup and electronic cavities. Even so, I would think that the fact that there is no noise when the pickups are both on should rule out a shielding issue, no?

 

[Killed_by_Death]

For the shielding in the pickup cavity to work it must be connected electrically to the controls cavity shielding & that all must be connected to ground & the sleeve side of the output jack.
You can check this with a multimeter set to Ohms.

The reason the noise is reduced with both pickups on is because one of the coils of the split-P is canceling out noise from the J pickup.

 

[Ben B]

For the shielding in the pickup cavity to work it must be connected electrically to the controls cavity shielding & that all must be connected to ground & the sleeve side of the output jack.
You can check this with a multimeter set to Ohms.

The reason the noise is reduced with both pickups on is because one of the coils of the split-P is canceling out noise from the J pickup.

A buzz is a shielding issue. A hum is a single coil issue. Turning on both pickups would not fix a shielding issue.

What I find odd is that you even had to do any shielding on a Carvin bass. They all come with copper shielding in both the control cavity and pickup cavities from the factory. No exceptions and they've come that way since the 1970s if not the 1960s. Since you had to do some shielding, it makes me wonder if this bass isn't either a Carvin kit or someone did some rewiring. If someone did some rewiring, I'm wondering if they did mess something up with the split coil pickup--like maybe only half of it is working.

 

[Killed_by_Death]

I did not state that both pickups on fixes a shielding issue, but I did explain why the noise was reduced with both pickups on

 

[Ben B]

I did not state that both pickups on fixes a shielding issue, but I did explain why the noise was reduced with both pickups on

I was adding to what you wrote, not disagreeing.

 

[Killed_by_Death]

Oops, I'm so accustomed to folks trying to contradict me on here

I guess the OP should check that he's getting sound from G & B strings with only the split-P selected.

 

[Jory D]

Oops, I'm so accustomed to folks trying to contradict me on here

I guess the OP should check that he's getting sound from G & E strings with only the split-P selected.

Yup, the pickup sounds fine across all 5 strings.

A buzz is a shielding issue. A hum is a single coil issue. Turning on both pickups would not fix a shielding issue.

What I find odd is that you even had to do any shielding on a Carvin bass. They all come with copper shielding in both the control cavity and pickup cavities from the factory. No exceptions and they've come that way since the 1970s if not the 1960s. Since you had to do some shielding, it makes me wonder if this bass isn't either a Carvin kit or someone did some rewiring. If someone did some rewiring, I'm wondering if they did mess something up with the split coil pickup--like maybe only half of it is working.

For what it's worth, there was some copper shielding in there, but it wasn't thorough at all. I found this surprising too, given Carvin's reputation for quality craftsmanship.

EDIT: Actually, I just remembered that there was shielding in the control cavity, but none in pickup cavities.