Blend Pot Trouble.

[Mitch]

I bought a blend pot from stew mac and I wired it into my guitar. I definatly wired it correctly to the stewmac diagrams (twice, because i thought it might have been a wiring fault so i desoldered and rewired).

The problem is when I have the knob at 10 it plays only the bridge pickup (this is how it is supposed to be) and when it is at 0 it plays all neck pickup (as it should do). But when I turn down from 10 (bridge pickup) it just decreases the volume till it gets to 5 and the guitar is silent then i go below 5 down towards 0 the volume starts going back up but on the neck pickup.

0=full neck pickup

3=neck pickup quiet but no bridge pickup

5=no sound at all

7=bridge pickup quiet but no neck pickup

10=full bridge pickup

What i want from the blend pot is a fade between the 2 pickups.

I also noticed (when i took the pot apart, couldn't resist ) that half the carbon track on both tracks was white. This must be some kind of isulator. Have I missed something on the stewmac website? Is this supposed to be an alternative to 2 volume knobs?

I'm considering just getting a switch or doing this diagram:

would this work? (thats a potentiometer by the way)

Thanks is advance

[billm90]

so you did this one?

Edited March 25, 2009 by billm90

[Mitch]

so you did this one?

yep.

[MES_C]

Maybe stew-mac sent you a center detent off instead of a center detent full blend pot. You could try a new blend pot, or test the one you have.

[Keegan]

Yeah, they gave you the wrong kind of pot.

Also I don't think you can do it with a normal pot. It'll just give you a similar problem, except that the middle will be 50% volume instead of 0%.

Edited March 28, 2009 by Keegan

[j. pierce]

If you search for blend pot in the electronics forum here, you'll find a couple of posts from folks (myself included) having trouble with Stewmac blend pots. I ordered several from them, and ended up with standard dual gang pots, but with a center detent. Which give symptoms similar to what you've got. They were awesome about mailing new ones out to me - but I think I went through three on one return cycle, and gave up.

I would have assumed by now, they had their supply issues sorted out. I will say this is the *only* item I have had any problems with from Stewmac, and they've always been awesome about returns and such. I don't think they even had me send anything back. I tried to explain the problem with these pots to one of their techs, but they weren't getting it. It's a little weird to explain, and I'm not the best at these things, so that doesn't help.

First thing though, before you start doing too much trouble shooting - your symptoms sound like you may have reversed the hot and ground coming out of this pot. Maybe you've got it right and the pot is backwards, or you were simply looking at the pot upside down when you wired it. Who knows. But if you leave the pickup connections to that pot normal, and swap the wires going to ground and output from that blend pot, you might get things working. Although it sounds like you pulled it out, so I don't know.

So back to the possibility of supply issues:

So the best bet might be to try measuring what you've got. Normally, I don't suggest looking at bad parts as the first point of failure - it's almost always a simple mistake that we all make and miss. But knowing that these particular parts have come to me mistaken before, I'd say check it out. And worst case, if you write down how it works, you can figure out how to connect it yourself.

First - the principle. Basically, what you've got is two pots with one shaft. One is a reversed so that it goes "backwards" of normal - turning the pot counterclockwise makes a pickup connected to this half louder, and without the volume jump you get from a backwards wired pot. Each of these pots also is only supposed to work over half the rotation, so that turning the pot from center should make one pickup quieter and leave the other one alone. That's why you see what's probably a fully conductive trace on half the pot - once the wiper makes it past the halfway point, the measurements aren't supposed to change on each wafer. The conductive trace should be different sides on each wafer. This is what makes me think you have a blend pot, but it's wired wrong.

Anyway, you should try measuring the pot with a multimeter. You only need to be able to measure resistance.

Disconnect the pot from it's connections.

With the rotation in the center, you should have full resistance (probably 500K, unless you got a 250K pot) between the center lug and one of the outside lugs, and one of the outside lugs should be 0 resistance - that is, continuity. The other half of the pot, should be opposite that. That is, if the top half of the pot is full resistance between center and left, and continuity between center and right, the bottom half of the pot should be full resistance between center and right, and continuity between center and left. Reversed.

As you turn the pot clockwise away from the center position, you should see one half of the pot (top or bottom) shouldn't change measurements, while the other half should work like a normal pot - resistance starts to increase where there had been continuity and decrease where it had been at it's full measurement. If you return to the center position and then start to turn counterclockwise, the half of the pot that changed when turning clockwise should now stay the same, while the other half of the pot changes in measurements.

If it's not behaving like this - for instance, turning it away from the center detent, the measurements are changing on both the upper and lower half of the pot, then you haven't got a functioning blend pot.

FWIW, I ended up getting the blend pot through Brian at Universaljems, and that worked perfectly. (Go to "pots", click on 250k or 500k, and scroll to the bottom or search on the page for "blend" and you'll find it.) It's also an Allparts item, I believe, so most music shops should be able to order you one.

Also, your single pot idea isn't really going to work. Volume pots work by decreasing the resistance to ground until all the signal bleeds there. You may get some reduction adding a series resistance like that, but it's not really going to work as you'd like.