Blend pot wiring problem

[stype]

I've concluded this week my blend pot is wired funny. At 1, I get full bridge pickup and at 10 I get full neck. That part is right, but unfortunately its the only part right. From 1-5 I still get full bridge, but the neck is just starting to come in at 5. From 5-10 the bridge pickup fades out smoothly. From 5-8 the neck pickup comes in slowly and from 8-10 is ramps up quickly. The balance point for the two pickups is somewhere around 8. From those tests, it looks to me like the bridge pickup is wired right but the neck pickup is totally backwards.

So I know what the problem is, but I'm not sure how to fix it. I wired it using this diagram: Link Removed
I have checked the wiring many times and it looks right. I'm using a 500K audio taper blend pot from stew-mac, bartolini J pickups.

Does anyone know what sort of wiring mistake could cause this?

 

[stype]

Well, I spent some time with my ohmmeter tonight measuring the curves on my blend pot. The results? I don't think my blend pot is a blend pot. Someone else can correct me if I'm wrong, but isn't a blend pot supposed to be two pots on the same shaft, but with the curves going in opposite directions? The top and bottom pots on mine have the same resistance values at all positions. That means I basically have two identical pots on the same shaft, and wired one backwards assuming it was truly a blend pot. Being audio taper, I get one pickup following a log curve and the other on an antilog curve.

Has anyone else ever had a problem using blend pots from stewart macdonald? I'm pretty sure at this point I need a different pot. Does anyone know other sites that sell blend pots?

 

[fretlessrock]

I gave some thought to your first post but couldn't figure out what you might have done wrong.

There doesn't sound like there is anything wrong with the pot. Just for the record, I've invested a lot of yelling and swearing into different passive blend arrangements and none ever worked the way I wanted. The StewMac blend pot looks like every other blend I have used. They are a pretty common item, but show up mostly along wth preamps kits. Check out guitarpartsresource.com for comparison.

Is this a passive setup or are you using a preamp?
Have you looked at any other VBT circuits to compare?
Did you check out/try the StewMac example wiring?

 

[Searcy]

I have to agree with that. I have tried a few blend pots because I think the idea is great. So far I have not seen a passive blend pot that gave good results. I have seen many active blend systems that work great but never a passive one.

Others disagree but this has been my experance.

 

[stype]

Thanks for the help. This is a passive setup. The wiring I'm using matches the stew-mac diagram and every other blend diagram I've seen. I think I am going to order another blend pot from somewhere and spend some time going over it with my ohmmeter and see if its different from this one.

The way its wired now isn't bad, but it kills me to put all this time into it and not have it perfect. The blend works now, it just hits its midpoint around 8.5 instead of 5. I have a visual picture of the volume curves this pot is giving me and I think I will throw them into excel later to help me visualize this problem.

 

[fretlessrock]

I understand your frustration, but just for the record, the diagram that you supplied isn't the same as the StewMac example diagram, where you have the ser/par switch, the master volume wired differently....

I tried this on a fretless Jazz-style bass that Jack Read built for me and we must have gone back and forth for a month with different schemes, rewires, pots... I never got it to where I would gig with it. The way that the pickups load against each other makes it a nightmare. At least that is what I found. You have impedance issues that are separate from the resisitve values.

In the basic "crossover" wiring you have opposite ends of the pots grounded, and that leaves opposite ends of the pots open, and they are directly mixed via the ungrounded jumper, and the signals come in on the wiper. If both pots are audio (log) tapered in the same direction, then yeah, the pot will not be 50/50 in the center when wired with the crossover, at least not in the way you intended. If both show the same reading in every position, then they are tapered in the same direction.

http://tangentsoft.net/audio/atten.html might be a good reference, though you seem to have the idea under control. Just make sure that you aren't testing the pot with the crossover on it! that wall make it look like they are the same.

Good Luck with this. I wouldn't blame the pots just yet, but it could be the problem.

 

[stype]

Well, I made the mistake of leaving the crossovers on when measuring, so I removed them and checked everything again. The top and bottom pots still have the same readings at all positions, so I'm pretty sure they are tapered in the same direction.

The only good news? I realized with the schematic I used, when in series mode, the blend is bypassed...so even having completely removed the blend pot and having a hole where it used to be, I can still play in series mode. All this and I still have a useable bass.

Thanks for the link and the help. I have limited experience with electronics but I'm learning more every day dealing with this.

 

[stype]

Well, I'm sure you've all been anxiously awaiting the conclusion of this thread. I ordered a new blend pot and guitarpartsresource.com and got it today. Checked it with my ohmmeter and its nothing like the one I had before. Stew-mac sent me a pot that looks like a blend pot but isn't, and that was the problem the whole time. I soldered in the new pot and my blend knob works as expected now, both pickups fully on in the middle position. Case closed.