Can a cap go bad and cause a short (humming)?

[Cagey]

Unfortunately, you and I aren't in the same room, so I can't see what DiMarzio sent you.

 

[AutoBat]

Unfortunately, you and I aren't in the same room, so I can't see what DiMarzio sent you.

the picture in is OP's first post on page 2

 

[Cagey]

Sorry. Forgot it was there.

Ok. So, they have you taking the hot lead to ground through a variable RC pair. That should function as a high-pass filter. That is, high frequencies will pass to ground, but low frequencies will not. If that circuit opens up, nothing will happen. It's a series circuit. It would essentially be a "no load" pot turned all the way up. Even a cold solder joint would manifest itself as an intermittent or non-existent tone control.

 

[lafromla1]

Sorry. Forgot it was there.

Ok. So, they have you taking the hot lead to ground through a variable RC pair. That should function as a high-pass filter. That is, high frequencies will pass to ground, but low frequencies will not. If that circuit opens up, nothing will happen. It's a series circuit. It would essentially be a "no load" pot turned all the way up. Even a cold solder joint would manifest itself as an intermittent or non-existent tone control.

So?  ???

If it should act as a no-load pot when turned all the way up, shouldnt that allow the entire sound to come through as opposed to killing the sound altogether?  Is there a cold solder joint on the tone pot somewhere thats causing to kill the sound?

 

[drewfx]

Your idea of jumping wires gave me an idea and I started playing around a little more this morning and this is what I found; when I jump the open tab on the tone pot to a ground, the humming goes away entirely for all pickups.  However 2 things occur, 1) when I touch the screws to the 2 single coils (not that there would be any reason to do so when playing), there is a slight hum and, 2) when I turn the tone knob to 10, the sound cuts out entirely.  From 1-9.5, the sound is exactly where it should be.  I am wondering if this relates to the fact that the original schematics had me putting in a no-load pot instead of using a regular pot (as the shaft was too short on the Fender no-load) or should that not matter?

When you turn it to 10, you have no resistance between your signal (center lug) and ground, so all of your signal goes to ground. You only want the tone pot connected to ground through the cap (so only the high frequencies go to ground).

As to why jumpering this would get rid of the hum, I'm not sure.

 

[lafromla1]

When you turn it to 10, you have no resistance between your signal (center lug) and ground, so all of your signal goes to ground. You only want the tone pot connected to ground through the cap (so only the high frequencies go to ground).

As to why jumpering this would get rid of the hum, I'm not sure.

Ok, so the first part makes sense to me.  As to the second part, this is my dilemma.  All the wiring diagrams in Erlewine's book show the third tab on the tone pot kept unattached as well.  I am thinking of replacing the cap with a new one and resoldering all the grounds.  Does this sound like a good idea or am I wasting my time? I am really getting frustrated with this humming and not being able to find out where its coming from.

 

[lafromla1]

OK, so I went through and removed all the grounds and resoldered them to a main ground pole.  Same hum as before.  When I touch the strings all the hum goes away. 

Could it just be a bad jack?

 

[drewfx]

Did you check the ground to the bridge (meaning the actual bridge itself - or the trem claw - and not the bridge PU)?

 

[lafromla1]

Did you check the ground to the bridge (meaning the actual bridge itself - or the trem claw - and not the bridge PU)?

I drilled a hole through the back of the trem claw and wrapped the string wire through the hole, up and over itself then soldered it down.  I am using a Wilkie trem, anything special I need to know about it grounding to the strings?

 

[lafromla1]

When I was screwing around with the volume knob I noticed that the humming decreases to what I would consider acceptable noise some of the time (when turned up to 10) and then I'd turn it down and up again, and the humming would come back.  I replaced the volume pot with a new one and the humming pretty much stays low level all the time (an acceptable level of hum). So it was a bad volume pot (I am assuming) was my problem all along.

The only difference is I had a 300k pot in there and replaced with a 250k that I had and I can tell the difference when the guitar is turned up high.  I'll keep it with the 250k until im ready to build my next one and place an order for a couple 300k's just in case.