Volume drops to half when pickup blend is centered... HALP!

[Addison]

Hello!

LOOOOONG TIME lurker... This is my first post, so take it easy on me.

I just bought a NOS lefty Modulus Genesis 5 with the Bartolini soaps and the Aguilar OBP3 preamp in it, running on 18V.

It sounds great; however, when the pickup blend knob is at the center detent, the volume drops considerably... it's probably half the volume of my Lakland 55-94 or my JoBo5.

If I solo each pickup, the volume comes up A LOT... if I bring it back to center, and the volume drops WAY down again.

It doesn't sound out of phase... it's just like someone turned the volume down a bunch.

Does anyone know why this might be happening?

Thanks in advance!!!

[dmusic148]

Even though you don't think it sounds out of phase, I'll bet it is. Just switch one set of pickup wires to find out.

[Addison]

I might end up doing that just to see, but I really don't think that's the problem.

The sound isn't all thin and nasally like one would usually associate with them being out of phase. It's a decent sound... just quite a bit softer.

I've also verified the wiring diagrams provided by the Bartolini website, and it seems to be wired properly.

I'm almost wondering of the pot itself isn't the right value... ?

Hmmm... I dunno.

[Addison]

Quick update...

I went in and switched the pickup leads to see if that was it and it was DEFINITELY not the problem.

As soon as I switched them there was an awful buzzing and I played a note and it sounded like I was listening to it through a sink drain.

I have no idea.

Grrrr...

[line6man]

Getting a drop in volume at the center position of a passive blend control is perfectly normal behavior, as the pickups are directly loading each other.

[Addison]

Thanks for the info... and I agree that volume sometimes decreases a little when both p'ups are blended equally.

However, the volume of both pickups is literally half of what it is when each p'up is soloed.

Is that normal?

I also have a Lakland USA 55/94 with Bartolinis and the does not decrease when the blend is centered.

Even my USA Joe Osborne with passive Fralins is almost twice as loud.

It just doesn't make sense.

[fender3x]

I had a similar problem, although the volume only dropped off a little at the center. Try just easing it off center a tad. That pretty much eliminated the problem for me.

[SGD Lutherie]

Something to check out is there is a right and wrong way to wire up a blend pot, if it's an audio taper blend. (marked A, linear is B)

You can test for that by removing the pickup wires. Then park the pot at the center detent, and measure with a multimeter from the wiper (center lug) to either outside lug. You will see a much larger resistance on one side than the other. You want the lug showing the larger resistance to the wiper to go to ground, and the other side with the lower resistance goes to hot. The pickups go to the wiper. Each deck is reversed. If you can't get a good reading you might have to remove the ground wire too.

If it is wired so that the low resistance half goes to ground, you will lose output and it wont blend well.

[Addison]

Ok, cool... thanks for posting that... I think I understand what you're saying.

It is wired exactly like the diagram on the right:

http://www.bartolini.net/instruction...250K_blend.htm

So if I read what you said right, all I would have to do to make the fix would be to reverse everything... or basically "mirror image" it to look like the diagram on the left?

I made this quick YouTube clip to demonstrate exactly how dramatic the volume drop is... I pretty much ran directly into my computer completely dry without any compression or anything.

http://www.youtube.com/watch?v=kRnT3Xbwot0

And I know it's sloppy... I was just trying to play the same lick 3 times and it didn't work well since it was past my bedtime.

Anyway, thanks for all the help!

[SGD Lutherie]

Originally Posted by Addison

Ok, cool... thanks for posting that... I think I understand what you're saying.

It is wired exactly like the diagram on the right:

http://www.bartolini.net/instruction...250K_blend.htm

You see how the left diagram is different from the right? The grounds and hots are switched. That's because of the way those two different brand pots were made. If you have the Noble brand pot, than that is correct. If you have the Alpha, then you need the other diagram.

So if I read what you said right, all I would have to do to make the fix would be to reverse everything... or basically "mirror image" it to look like the diagram on the left?

Yes. I'm not saying this is the problem, but it's something I would check.

I made this quick YouTube clip to demonstrate exactly how dramatic the volume drop is... I pretty much ran directly into my computer completely dry without any compression or anything.

http://www.youtube.com/watch?v=kRnT3Xbwot0

And I know it's sloppy... I was just trying to play the same lick 3 times and it didn't work well since it was past my bedtime.

Anyway, thanks for all the help!

Wow, that is a BIG drop. That's not a matter of both pickups loading each other.

Check to see if the blend pot is wired up right for it's brand. Maybe the blend pot is bad.

[Jazz Ad]

Originally Posted by line6man

Getting a drop in volume at the center position of a passive blend control is perfectly normal behavior, as the pickups are directly loading each other.

It is not normal at all since they are humbuckers, not single coils. It would not make volume drop by half anyway.

[Addison]

Well, I completely rewired it...

I even checked the resistance first and it appeared to be wired correctly... about 250 for the ground and 45 for the blended signal.

But, to keep my sanity in check, I rewired it anyway and it was a huge failure.

So yeah... I've left 3 messages with Modulus and no call back. The one other time I did talk with them about something completely different, they were totally cool.

Maybe they're short-staffed right now... I'll give them the benefit of the doubt.

At this point, I'm thinking about switching to volume/volume instead of volume/blend, but I'm not sure what types of pots I need. Truth be told, I'm partial to that configuration anyway.

SGD Lutherie: Do you happen to know? I think if they're active I need 100K pots, and if they're passive I need 250K... is that right? How do I know if my pickups are active or passive? Do you know what type of Bart soapbars are on Modulus basses?

I have to say, going through this type of garbage on a brand new bass is pretty lame.

Thanks for all your help!

[Addison]

If anyone is interested where this thing stands...

I called an old friend of mine who is smart as a whip, especially with music related electronics, etc. and he walked me through some stuff.

He told me to measure the resistance from the center lug to the output lug while the pot was at center detent. At that position, resistance should be close to zero and any changes while the pot rotated should be nominal... if anything, it should get closer to zero if it read anything above that to begin with.

At center, resitance was 16.65K, and he immediately said there was a problem. He said that the closer to zero, the better.

To verify this, I measured the blend pot on my Lakland 55/94 and it showed about 4.5K at center detent.

He then asked me to hold the probes where they were and rotate the knob in either direction. When I did this, the reading lowered to zero as I turned the knob.

So, somehow, he thinks I got a bad pot. According to him, a good blend pot will have little-to-no resistance in the center position, allowing for a full signal from both pickups.

I hope my above interpretation of his expertise is ok... I've spent the brunt of my 20 years with bass learning how to play, and not nearly enough learning the back end. If my translation of what he was trying to tell me seems off, I welcome any corrections.

Anyway... I ordered a new blend pot from StewMac and I also ordered two mini 250K pots from AllParts (Why doesn't StewMac carry these?) if I decide to rewire it to VVT if the new blend pot doesn't work as I hope.

Still haven't heard from Modulus... hopefully today.

I'll keep y'all updated if interested.

[SGD Lutherie]

None of the pots go to zero at the center detent. They should, but they don't. They all vary as well.

But I've used some that read about what you have, and they didn't have that drop in the center.

You just have to make sure that the low resistance side is connected to hot, and the high resistance side is connected to ground.

[Addison]

Ok, so I completely removed the pot in anticipation of the arrival of the new ones... and to be honest, I really wanted to get a nice solid resistance reading on the pot without any connections, etc..

Picture the pot with lugs facing you, shaft pointing up.

Here's what I found...

Center detent, top lugs:
One probe on left lug, one probe on center lug: 39.77 K
One probe on right lug, one probe on center lug: 205.6 K

Fully clockwise, top lugs:
One probe on left lug, one probe on center lug: 242.0 K
One probe on right lug, one probe on center lug: 0.0 K

Fully counter-clockwise, top lugs:
One probe on left lug, one probe on center lug: 0.0 K
One probe on right lug, one probe on center lug: 242.0 K

Center detent, bottom lugs:
One probe on left lug, one probe on center lug: 221.9 K
One probe on right lug, one probe on center lug: 49.2 K

Fully clockwise, bottom lugs:
One probe on left lug, one probe on center lug: 267.5 K
One probe on right lug, one probe on center lug: 0.0 K

Fully counter-clockwise, bottom lugs:
One probe on left lug, one probe on center lug: 0.0 K
One probe on right lug, one probe on center lug: 267.4 K

Is that normal?

Also... to be sure, I marked ground and it was connected to the top right lug, and crossed around the back of the pot where it was soldered to the body, and then connected to the bottom left lug.

I'm excited to get the new pots so I can measure those and see what I get for results.

Or...

If anyone has a known good blend pot lying around not hooked up to anything that they would like to measure for me, that would be fantastic...

[SGD Lutherie]

Originally Posted by Addison

Is that normal?

Yep. And you can see that a 250K pot is never 250K.

Personally I would have used a 500K for blend.

So make sure it's wired up so the lower resistance half goes to hot, and higher resistance half of the pot goes to ground, and the pickups are connected to the wiper.

[3506string]

I had the exact same problem. When the blend was centered the volume was significantly lower. When I would blend to the neck or bridge the volume would rise almost as if it were a volume knob. IIRC I had the output and the ground of the blend reversed but i'm not 100%. I reversed the connections and now the volume is much more stable when blending. The centered volume is slightly lower but not nearly as much as it was.

[Addison]

Originally Posted by SGD Lutherie

Yep. And you can see that a 250K pot is never 250K.

Personally I would have used a 500K for blend.

So make sure it's wired up so the lower resistance half goes to hot, and higher resistance half of the pot goes to ground, and the pickups are connected to the wiper.

Thanks for all of your help.

Frankly, if the new blend pot doesn't work as it should, I'm switching to VVT.

I have to say, though... I'm seriously bummed about this problem, and the fact that I still haven't received a callback from Modulus after leaving 3 messages.

It's unacceptable for a bass that retails for almost four grand.

[Addison]

Originally Posted by SGD Lutherie

Personally I would have used a 500K for blend.

I forgot to ask... is there a specific reason or reasons you recommend this?

I did order a few, just in case... but I do really like the tone the bass gets with the 250K pot.

If there's a technical reason, like they help balance pickups better or something, maybe I should switch.

[SGD Lutherie]

Originally Posted by Addison

I forgot to ask... is there a specific reason or reasons you recommend this?

I did order a few, just in case... but I do really like the tone the bass gets with the 250K pot.

If there's a technical reason, like they help balance pickups better or something, maybe I should switch.

A few reasons. The first is that you will get a smoother range with the 500K. Also, since the blend has two 250K pots in parallel, that's 125K load on the pickups. Then of course you have the volume pot added to that. So using a 500K bland with a 250K volume will be more like the tone of two 250K volume pots. I just find that the 250K blend dulls the tone too much. I can use my tone control if I want a darker tone.

It's the same reason a lot of people don't care for the tone of the stack knob Jazz basses. There's too much loading going on.