OBP-3: Grounding issue?

Bassic83 · #1 08-11-2008, 05:21 AM

Put an OBP-3 in my Squier Active 5. It sounds massive, the mids are wonderful, the highs are glassy, everything was wonderful...until I went to change volume. I started picking up a Mexican radio station (1400, I think...) and it buzzes incredibly loudly whenever I touch any of the knobs. I had a ground issue before, where a wire had snuck under the edge of a pot when I tightened it down. That has been fixed, and the bass works fine except for the grounding issue. What to do? When I tried grounding the pot cases, it doesn't work. I've been told it doesn't need to be star-grounded, as the pre does that internally. So does anyone have anything for me to check?

Revmizery · #2 08-12-2008, 10:39 AM

Did you insulate the cavity for the electronics?

Bassic83 · #3 08-12-2008, 10:47 AM

Quote:

Originally Posted by Revmizery

Did you insulate the cavity for the electronics?

Yeah, it's painted, the cover is foiled.

DavidRavenMoon · #4 08-13-2008, 12:49 AM

Quote:

Originally Posted by Bassic83

and it buzzes incredibly loudly whenever I touch any of the knobs.

It sounds like either the pots cases are not connected to ground, or worse yet, are connected to the hot!

Even though you are using the preamp, which has the ground, I'd wire the pot cases to each other, and then run a wire to the ground at the jack. That's helped me in similar cases.

Also double check all your wiring and make sure something isn't touching that shouldn't be touching.

That preamp has a slew of wires, so go over each one and make sure they are where they belong.

Bassic83 · #5 08-13-2008, 05:01 AM

Quote:

Originally Posted by DavidRavenMoon

It sounds like either the pots cases are not connected to ground, or worse yet, are connected to the hot!

Even though you are using the preamp, which has the ground, I'd wire the pot cases to each other, and then run a wire to the ground at the jack. That's helped me in similar cases.

Also double check all your wiring and make sure something isn't touching that shouldn't be touching.

That preamp has a slew of wires, so go over each one and make sure they are where they belong.

Yeah, that pre has more wires than the Space Shuttle! My guitarist and I have gone over it several times, and one thing we found was that grounding the pot cases causes a severe drop in the output...to zero. It won't make a sound if we do that. The cavity is rather small, as well. I actually wired it using Nino's method- I took a thin sheet of plywood and traced the cavity dimensions on it, drilled it for the pots, and wired it up, then dropped it into the real cavity, soldered the pups in. There was a wire coming off the volume pot that went to the ground that got pinched when I tightened up the pot, but we set it free, rerouted it slightly, and wrapped the pinched point. It had a small break in the insulation, but the tape wrap took care of that. If I really have to, I can tear it all out and start over, but I already trimmed the wires to fit.

If I had to just do that one wire, I could do it if I just took the volume pot out.

If I isolate the knobs, like using the corner of my shirt, or a piece of plastic bag when I turn them, this doesn't happen.

Maybe I should dip the knobs in epoxy to isolate them?

DavidRavenMoon · #6 08-13-2008, 08:01 AM

Quote:

Originally Posted by Bassic83

My guitarist and I have gone over it several times, and one thing we found was that grounding the pot cases causes a severe drop in the output...to zero.

OK, then the inside of the control cavity is somehow "hot", which is why you are getting the hum when you touch the knobs, and why you lose signal when you ground them.... you are shorting the signal to ground.

Check the output jack to make sure the hot side is not touching the shielding paint, or that the jack is wired up the right way. You can try to wrap some electrical tape around the jack. Also look for the wire that should be grounding the conductive paint, and make sure that's actually connected to ground. Then check each of the pots to make sure that none of their lugs are touching the paint. A piece of electrical tape under the connections helps.

Look for something simple.... that's why you don't see it. Somehow the hot and grounds are swapped. If you have a multimeter, try doing a continuity test between ground and various points, such as the shielding paint, and back of the pots, and then the hot and the same spots. You can get a cheap meter in places like Radio Shack, and they are very handy to have.

Bassic83 · #7 08-13-2008, 08:10 AM

Quote:

Originally Posted by DavidRavenMoon

Check the output jack to make sure the hot side is not touching the shielding paint, or that the jack is wired up the right way. You can try to wrap some electrical tape around the jack. Also look for the wire that should be grounding the conductive paint, and make sure that's actually connected to ground. Then check each of the pots to make sure that none of their lugs are touching the paint. A piece of electrical tape under the connections helps.

Look for something simple.... that's why you don't see it. Somehow the hot and grounds are swapped.

Look for something simple- boy, ain't that the truth! Hiding in plain sight, so to speak...

Great tip about the electrical tape under the pots- I think I'll try that first to isolate the pots. Should be the easiest thing on that list, and if it works, quickest. Somehow, I have a feeling it's something small like that. The jack is not touching anything, we took it out and double-checked with a dental mirror (side-mount, looked down the "tunnel"), but the volume pot had turned when I tightened the pot, and it was just touching the paint, resulting in the "no sound" condition. I think I might put some elec. tape on that corner of the cavity to prevent it from happening in case it works it's way loose.

Thanks for the tips, I'll let you know if that's it.