unexplained hum...bad pot?

[dmraco]

I just finished wiring a guitar with a single p90 in the neck and humbucker in the bridge.  I used a push/pull tone knob to split the humbucker to single coil. 

I am getting a HUGE amount of intermittent hum that I cannot explain.  Everything works as it should but the hum comes and goes when I just handle the tone knob.  It also completely goes away when I roll the tone all the way off and comes right back as soon as I engage the tone, even just a bit.

Do I have a bad pot?  This sucker cost me $15 and I am going to be ticked if I wired everything up right and the tone knob is busted.

Thanks

 

[BigSteve22]

I take it that its not just in single coil mode?

 

[dmraco]

Both.

 

[AirCap]

Grounding problem, perhaps?

 

[Logrinn]

Could you post an image of the wiring?

 

[dmraco]

Grounding problem, perhaps?

That is what I thought.  I check for lose wires but nothing.  That thing that gets me is it is intermittent.  I will try to post a photo

 

[weezingthejuicebodhi]

Well the buzz going away when the pot is off might make sense. If there is a grounding issue on that pot (or that the pot introduces to the circuit when engaged) then that could explain the buzzing and it wouldn't be considered intermittent. The buzzing is apparent whenever the tone control is on, yes?

Confirm your wiring against the schematic, no mix-ups in color coding? All grounds connected? Good solder joints? No crossed wires?

What kind of pot are you using? DPDT? What manufacturer? Sometimes the references for these pots can be confusing. I recently ran into new black CTS DPDT pots that didn't transpose obviously to any old schematics.

 

[mayfly]

Are you using a metal knob or a plastic knob?

Did you solder to the back of the pot (Might have warped the disk if so)

 

[BigSteve22]

Like Mayfly stated, you could have damaged the pot if you soldered to the back of it. Might try re-flowing the solder connections to the terminals. Especially if any of them have a white-ish cast or a grainy appearance, that would indicate a cold joint.

Also check the schematic for multiple paths to ground. The difference in potential between them could also cause that hum. The use of solder lugs helps to avoid both of these potential pitfalls.

 

[dmraco]

I soldered to the pot side.  It was a real PIA because that metal does not like to take solder, even after scuffing.  See attached

 

[Cagey]

Soldering to pot bodies is poor practice. They're not designed to take that much heat. The typical pot used in a guitar is a 2 watt part. You put a 40 watt soldering iron on it, chances are pretty good you're gonna have a negative impact on its integrity. Solder ring terminals are a much better way to go.

They're easier to solder to than pot bodies, plus it makes it a bit easier to change the pot when it inevitably fails. They cost very little, so it's not going to hurt your pocketbook. I used to get mine from here, but there are other places. Just do it. You know you want to.

 

[Verne Bunsen]

I go back-and-forth on the ring terminals. On the one hand, "easier to replace the pots" is irrefutable. On the other hand, things are often quite tight between the contours of the pot case and the contours of the route. Many's the times that a single wire alongside a pot prevented a control plate or pickguard from seating correctly. There's also the argument that sometimes pot mounting hardware comes a bit loose, which in this case would lead to losing your path to ground. With the right iron and technique, soldering to the case is generally pretty uneventful. You mention a 40w iron being a risk, but it's been my experience that lower wattage irons are the bigger risk as they take too long to get you up to temp, cooking the pot before flowing the solder. I use a 40w adjustable iron on full, with a big chisel tip, for soldering to the back of a pot case. It's usually about 2 or 3 seconds from applying the iron to removing it. I flow a bit on to the back of the pot, then tin the ground wire/s thoroughly, then let things cool down for a few minutes. After that, joining the tinned wires to the pool of solder is easy and fast.

All that being said though, the location marked in yellow on the picture is not the side of the pot but is rather the side of the switch, the innards of which can and will melt quite readily. I'd say that's an excellent application for a ring terminal, or at the very least the side of the actual pot case is preferable to the switch. The smaller diameter pot case makes fit issues with the route walls a bit less of an issue.

 

[dmraco]

someone sent me some rings one time.  I think it was you Cagey.  I ran out.  They are nice.

 

[mayfly]

I soldered to the pot side.  It was a real PIA because that metal does not like to take solder, even after scuffing.  See attached

Yea - all that heat might have warped it.  Best to swap it I think.  May as well get some 'Cagey Rings' while you're at it.

 

[Cagey]

I go back-and-forth on the ring terminals. On the one hand, "easier to replace the pots" is irrefutable. On the other hand, things are often quite tight between the contours of the pot case and the contours of the route. Many's the times that a single wire alongside a pot prevented a control plate or pickguard from seating correctly. There's also the argument that sometimes pot mounting hardware comes a bit loose, which in this case would lead to losing your path to ground.

I've not had any fitting issues, although I agree they're possible and having used them exclusively for years on more guitar builds than I can remember, I could even add one or two items one might complain about. But, even winning the lottery has its drawbacks. That doesn't mean I'd throw away a winning ticket. Nothing is 100% upside, but for some things, the pros outweighs the cons pretty dramatically. Also, with some things you don't even know you received any benefit because you don't eventually suffer the ramifications of engaging in a poor practice. Changing your engine oil or brushing your teeth regularly would be good examples of that sort of thing.

 

[Verne Bunsen]

Yeah, I can definitely see applications for them. Bonding all the pots in a Les Paul comes right to mind, push/pulls as in this case. I have to catch myself sometimes getting stuck on "But this is how we've always done it!" Because that doesn't always mean you're doing it the best way.

 

[Cagey]

Even MilSpec pots are only rated for 25,000 turns, and most pots aren't anywhere near that good. Plus, most pots have plastic resistor strips in them these days instead of the hard carbon strips the old units used. Then, most pots aren't sealed like MilSpec parts are. So, you know a typical pot in a guitar application is going to die young mechanically, and while overheating it might not kill it immediately, it's liable to shorten it's already short life. A few extra minutes and pennies spent mitigating that at installation is time and money well-spent. You never know - you might get to jam with Joe Satriani on the day Eric Johnson loans you his Dumble, and that will almost certainly be the day your volume pot decides to die because you burned its guts up when you installed it :laughing7:

 

[weezingthejuicebodhi]

You never know - you might get to jam with Joe Satriani on the day Eric Johnson loans you his Dumble, and that will almost certainly be the day your volume pot decides to die because you burned its guts up when you installed it :laughing7:

Whoa - easy there Ozzy Lucifer.

 

[Cagey]

Yeah, I know. As my sainted martyr mother used to say - "I've told you a million times...never exaggerate!"

 

[Verne Bunsen]

You never know - you might get to jam with Joe Satriani on the day Eric Johnson loans you his Dumble, and that will almost certainly be the day your volume pot decides to die because you burned its guts up when you installed it :laughing7:

That's just wonderfully awful, haha! Would have made a great verse in that Alanis Morisette song about tragic irony  :laughing3: