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SOLVED: New pots scratchy/dirty, old pots NOW scratchy/dirty & grounding question...

N

NCD

Mar 19, 2011
167
5
#1
Hey all,

Short version, I put quarter pounders in a bass that had a crappy single PU. I had my reasons. Anyway...

A) Wiring diagram used: [Invalid or Expired Link Removed]

Both pots are new 250k, cap is .05uf and PU's are SPB3's from SD. I used 20ga solid copper because it's what I had on hand, is beefier than stock and the solid should be a better conductor anyway. Wiring is exactly like the diagram, despite my seeing something amiss with it. (later)

B) I'm an electronics technician, formally trained in the service with over 25 years field experience but not in audio. This is making things especially frustrating because I'm not about to allow a super-simple bass wiring harness to kick my a**, despite having no specific audio experience. That experience is why I think there may be a mistake in the diagram.

C) With both new pots the volume was horribly scratchy and would cut out, as if there was a loose solder joint. tapping the pot helped and at times putting pressure on it did too. I checked for a loose joint with the wiggle/tap test repeatedly, no joy. So I put the old volume pot back in, thinking the new one was defective and had bad connections internally.

D) Then the tone pot started the same thing. Additionally, though having the old volume pot in there did help some, it still does the same thing but to a lessor extent. it never did this before I redid the harness.

Worse, the issue now is constant when plugged into an amp, yet intermittent when using a small battery powered practice amp for headphones (it's a Vox). Might that be a clue? Not sure...

E) Diagram error?? During testing I got a mild shock off the nut that secures the volume pot to the pick guard. This is what caused me to re-examine the diagram. Notice that the tone pot is grounded to the outer sleeve of the jack but the body of the volume pot is not. Considering that the bridge ground goes to the volume pot instead of the tone, which is actually grounded via the amp, shouldn't these grounds be ganged to prevent getting a shock off the strings? I mean, if we're to ground this thing properly shouldn't the bridge, volume pot body, tone pot body and outer sleeve of the jack all be the same electrical point?

In closing:

I've wiggle tested this thing to death, even beating on components with a screwdriver handle. When the cover is open and the bass is on it's back it seems fine but when I close her up and sling the strap over my shoulder it's got intermittent issues.

And you experienced guys all know how frustrating it is to have an issue that goes away when you open the hood, only to come back when you close it. Been there, did that, burned the t-shirt in frustration.

Thanks for any help.
 
#4
duffer426 said:
hello: there should be a ground wire from the top of the volume pot to the top of the tone pot. your diagram does not show that wire
Click to expand...

Bingo, that explains the shock and may have something to do with the static (scratchy).

As a trained tech I thought the grounds needed to be ganged together but I didn't want to just go and do it in case there was a reason that there had to be a floating ground for the volume or bridge.

I've always hated floating grounds... necessary at times but they always get you zapped.

Thanks!
 
#5
First off, if you are getting electrical shock from your bass, that's an issue with your amplifier and/or your houses' wiring. If your amp has a 'polarity' switch, flip it the other way. Use a multimeter to check for voltage from the amp chassis to ground.

duffer426 said:
hello: there should be a ground wire from the top of the volume pot to the top of the tone pot. your diagram does not show that wire
Click to expand...

That's correct. On a jazz bass that wire isn't needed because the metal control plate acts as a common ground connection. Maybe your P-bass (or copy) has foil shielding inside the pickguard? Normally that would be enough to ground it, but solder a ground to the back if there isn't.

Another thing to check - is the control cavity shielded? Loose wires could be grounding out on the shielding when you "close the hood" as you put it.
 
#6
just cut a length of wire, solder it in with the ground wire from the pup, and ground wire from the bridge. the other end solder to top of tone pot with the cap,and ground wire from jack. hope this helps
 
#8
Clarification:

I ran a wire from the body of the volume pot to the out shield of the jack. This caused the bridge, shield of the jack, bodies of volume and tone pots to become "the same electrical point" (what voltage you get at one, you get at all).

Getting shocked by the volume pot was not an amp problem it was the fact that the pot wasn't grounded properly in the first place. That said, if you get shocked when the bass wiring is proper then yes, you have an amp problem.

So if you're wiring SPB3's then know the diagram is wrong and you need an extra ground wire to the volume pot body.
 
#9
This is what I keep saying. Don't reply on the mechanical ground connection to a chrome plated piece of metal. You should always solder ground wires to the backs of pots. Some people are worried about ground loops, but you can't have a ground loop with only one ground, and the instruments are always quieter with real ground connections.
 

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