OT - Guitar electronics question...

[spkirby]

Acquired a used guitar recently (its not an ebmm hence the OT in the title ) and it has a problem with the volume control whereby the guitars amplified volume only decreases about 30% through the entire travel of the volume knob. I.e. the sound through my amp never gets close to 0 even though the knob is zeroed! I can't say for sure but its possible the pot may have seen some resoldering in the past.

Does this sound like a faulty pot or faulty wiring problem? (Ones easier to fix than the other!)

Thanks
SteveK

 

[beej]

It's most likely a bad pot- you're not getting the correct resistance out of it when the wiper is all the way off. Excess heat or physical stress can cause this.

Easy fix, just solder in a new pot.

 

[Adwex]

Agreed. Sounds like a bad pot.

 

[uvacom]

Not necessarily. A resistor connected between the wiper of the pot and ground would effectively do the same thing, depending on the value of the resistor. Maybe the previous owner was trying to smooth the taper of the pot, or install a treble-retention RC series? there may be other odd wiring combinations that would cause this. If the pot does alter the sound throughout it's range, to me that would indicate that the pot might be fine, there's something funky with the wiring. I've been wrong before, but I'd look at the wiring first before you replace a pot.

 

[beej]

An odd resistor could change the taper, but still the pot should be able to ground the output (zero resistance) and the resistor shouldn't change that.

But either way ... you'd have to take off the pickguard and have a look. So same amount of work.

Steve- I'd call CS and get a wiring diagram for your guit. Then you can quickly see if everything looks normal or not.

 

[Spudmurphy]

Boy, I wish I could understand these guys !!!!

 

[mbgreene]

Yes, much like Arthur, King of the Britons, beej is wise in the ways of science.

 

[Spudmurphy]

Yes, much like Arthur, King of the Britons, beej is wise in the ways of science.

ot: any sparkly news to tell us ?

Back on topic- oh wiggle it about a bit and squirt some of that cleaning "stuff" in there and it should be ready to go!

 

[uvacom]

An odd resistor could change the taper, but still the pot should be able to ground the output (zero resistance) and the resistor shouldn't change that.

But either way ... you'd have to take off the pickguard and have a look. So same amount of work.

Steve- I'd call CS and get a wiring diagram for your guit. Then you can quickly see if everything looks normal or not.

You're thinking if it's wired in a way that makes sense. But if the potentiometer isn't directing the current flow directly to ground at any point in it's travel (i.e. a resistor somewhere where it shouldn't be, like between pin 3 and ground) then it won't work as it should. I'm not saying that's what's happening, but you can't just assume the potentiometer is the problem because that's where the symptom is showing up. Especially if the potentiometer is altering the signal throughout it's travel without any discontinuities, I would look elsewhere first.

One easy way to test the potentiometer if you have an ohmmeter is to desolder it, place your probes between pins 1 & 3, make sure the resistance is constant (probably 250k or 500k depending on the guitar), measure between pins 1 & 2 and make sure the resistance varies between some low resistance (anything under 100 ohms is probably fine but it should be even lower than that) and the maximum value, and do the same for pins 2 & 3. It's kind of obnoxious to do that, but most likely you'd have to do some desoldering no matter what so it's not too big a deal.

 

[beej]

Sorry guys, I hate to go on, this is with the best intentions.

I do think it's wired in a way that makes sense. Pin 1 & 3 (the outside lugs) are wired to the pickups and ground, respectively. The total resistance of the pot (e.g. 250k, 500k, etc.) is between them. This way, the pickups always see this total load (really the combined parallel resistance of the vol and tone pots).

Pin 2, the wiper, connects to the guitar's output jack- fully on, it connects the pickups to the output jack.

Fully off, it does the opposite and connects the ground on pin 3 to the output jack.

So what I'm saying is that even with a resistor soldered across the lugs of the pot, when off the wiper (output) should be in contact with pin 3 (ground) to produce a zero-volume output (the path of least resistance ... literally). So if the pot doesn't turn all the way off, it's probably a bad pot and was overheated/abused at some point.

Regardless, someone is gonna have to take this puppy apart and have a look anyway. Agreed that the best bet is to check the wiring, then unsolder the pot and see if it's working right.

 

[uvacom]

Sorry guys, I hate to go on, this is with the best intentions.

I do think it's wired in a way that makes sense. Pin 1 & 3 (the outside lugs) are wired to the pickups and ground, respectively. The total resistance of the pot (e.g. 250k, 500k, etc.) is between them. This way, the pickups always see this total load (really the combined parallel resistance of the vol and tone pots).

Pin 2, the wiper, connects to the guitar's output jack- fully on, it connects the pickups to the output jack.

Fully off, it does the opposite and connects the ground on pin 3 to the output jack.

So what I'm saying is that even with a resistor soldered across the lugs of the pot, when off the wiper (output) should be in contact with pin 3 (ground) to produce a zero-volume output (the path of least resistance ... literally). So if the pot doesn't turn all the way off, it's probably a bad pot and was overheated/abused at some point.

Regardless, someone is gonna have to take this puppy apart and have a look anyway. Agreed that the best bet is to check the wiring, then unsolder the pot and see if it's working right.

The big assumption is that it's wired that way, and since we haven't seen the guitar, we don't know that. For example, we don't know that on this guitar, pin 3 goes to ground. I'm not saying standard guitar wiring doesn't make sense, it makes complete sense. But you can wire a guitar in any number of non-standard ways that will change the behavior of the electronics. Actually, maybe the previous owner knew what he was doing, and he wanted to wire it in such a way that the volume knob would only adjust the volume by 30%. In that case the wiring would have made sense for him.

But it doesn't matter - the point is, it's very, very possible to wire a guitar such that the volume will only be adjusted by ~30%. The easiest way would probably be to put a 500k or 1Meg resistor between pin 3 and ground.

 

[fbecir]

Finally acoustic guitars are great !!! No need to play with a soldering iron