It apparently doesn't know the words...

We are fortunate to count Paul Gagon as a member here, so it seemed natural to give him a home to stretch out in and relax a little. Maybe exercise the grey matter and present a little history of the guitar and bass (amps too!) from his perspective as one of the eminent designers of our time.

Got questions? Great! Good questions might be, "What are the EQ mappings for the M Series preamps?" Or, "What was the thought process behind the MJ-4?" Troubleshooting questions should remain in the regular forums (he reads those too). Finally, please be mindful of how I feel about tech questions via PM or email.

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It apparently doesn't know the words...

Postby jglunt2112 » Wed Feb 10, 2021 4:26 pm

I'm changing my SB-2 from 2 volume controls to master volume/blend controls. The blend pot I ordered is a name brand, either an Alpha or CTS, and it blends the pickups fine. But man, what a hum. I have literally triple checked all my wiring using a diagram I got from Ken a while back, and it is all correct with no loose grounds or cold solder joints. I can only think of two possibilities: (1) The blend pot has no room to ground the chassis on the back, so I soldered a ground to the side. Could that have damaged the pot? Does that blend pot chassis even need to be grounded? (2) The control cavity has no shielding except at the pickguard. However, the bass didn't hum when it had two volume controls. One other quick note: The hum is louder when the blend pot is at center detente position than in either the neck or bridge position, if that matters...Jim
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Re: It apparently doesn't know the words...

Postby Ken Baker » Wed Feb 10, 2021 5:00 pm

I'm an old fart with a 10 minute retention capability. This is the circuit you used?

Image

jglunt2112 wrote:(1) The blend pot has no room to ground the chassis on the back, so I soldered a ground to the side. Could that have damaged the pot? Does that blend pot chassis even need to be grounded?


Got a pic of the pot? The back of the SB-2 pickguard should have shielding where the pots & jacks are mounted. If the threaded shank of the pot is metal, the case of the pot should be grounded by benefit of the physical connection to the pickguard's shield. It is typical that pot cases are grounded in this manner. Without seeing the pot, I can't tell if it could be damaged.

(2) The control cavity has no shielding except at the pickguard. However, the bass didn't hum when it had two volume controls. One other quick note: The hum is louder when the blend pot is at center detente position than in either the neck or bridge position, if that matters...Jim


You've already done this, but re-check your connections to make sure your leads go where they're supposed to go. Carefully wiggle and flex all your leads looking for both loose/bad connections as well as broken leads.

Edit to add a little explanation about broken leads. We wiggle and flex wires when we suspect that a conductor inside its insulation might be broken. It doesn't happen often, but it does happen.

Ken...
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Re: It apparently doesn't know the words...

Postby deltafred » Thu Feb 11, 2021 3:11 am

I had a pot where the back was not making good contact with the body, the grounds all connected to the pot back and the pot body made the connection to somewhere else (long time ago, can't remember which bass it was now).

It hummed pretty bad and I was tearing my hair out (what little I have left) because the wiring looked correct but the multimeter said otherwise.

Eventually I checked the continuity from the pot back to the body and found there was none so soldered an extra black wire to the pot back, backed the pot nut off, trapped the stripped wire end under the pot mounting bush and tightened the nut back up.

Silence at last! :D
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Re: It apparently doesn't know the words...

Postby jglunt2112 » Thu Feb 11, 2021 8:44 am

Great advice from both of you. It will be a day or two before I can get back to this project, but I'll keep you posted. Thank you!...Jim
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