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Shielding/Star Grounding Troubleshooting Help Needed

S

samuelzero

May 7, 2008
13
0
#1
Hello,

I just finished re-wiring a noisy-as-hell Fender MIM Jazz bass, and the results are not satisfactory. It's a model from the 90's, when they used the cheapo mini pots.

Here's what I did:

*Lined the entire control cavity with conductive-adhesive copper foil, with a lip around the edge to contact the control plate and pickguard, the back of which was also covered with copper foil.
*Replaced mini pots with CTS 250K audio taper pots. Replaced cheap ceramic cap with Orange Drop. Replaced output jack with switchcraft.
*Rewired bass using unshielded cloth-covered wire for all connections, including the wire to bridge ground and the wire connecting the shielded bridge pickup cavity to ground. (Pickup wires are super thin unsheilded.) All grounds connected to a single point, which is a to a ring terminal under the bridge volume pot. Once this was done, I buttoned her up and installed a single string to test it out...

And here's the problem:

*I get signal to the amp, but there is a constant hum/buzz, regardless of the position of volume and tone knobs. (hum gets darker when tone is rolled off, but it's still there)
*Turning both pickups all the way up doesn't have any hum cancelling effect.
*Touching the bridge/strings doesn't change or reduce the hum.

And here's what I've done to troubleshoot so far:

*Confirmed the ground connection from bridge with multimeter.
*Removed the wire from the output jack ground to the ring terminal. No difference.
*Tightened down the bridge volume pot to get better connection to ring terminal. No difference.
*With bass plugged in, I tried pressing on the pots and the output jack to see if the hum changed in any way. It didn't
*With bass plugged in, I removed the control plate and pickguard and touched and pressed on everything I can find, yet the buzz/hum remains absolutely constant.


Any ideas? Could one or more faulty pickups cause this?

Could having only one string installed somehow be the culprit (I doubt it)?

I own another almost identical MIM jazz (with Duncan Antiquities), and it is dead quiet into the same amp with the same cable (with both volumes fulll up). It has no cavity shielding, and it's also wired up with unshielded wire, for what it's worth.

I appreciate any ideas or suggestions.
 
#4
"*Turning both pickups all the way up doesn't have any hum cancelling effect."

It sounds like the pickups are wired in parallel; they need to be in series. Try reversing the black and white wires on one of the pickups (at the pot). This will probably fix it.

Can you post clear pics of your wiring?
 
#6
Craig_S said:
"*Turning both pickups all the way up doesn't have any hum cancelling effect."

It sounds like the pickups are wired in parallel; they need to be in series. Try reversing the black and white wires on one of the pickups (at the pot). This will probably fix it.

Can you post clear pics of your wiring?
Click to expand...

Thanks Craig. I'm happy to post some, but I'll have to do it later from home.

I re-wired it the same way it was before, and though it was noisy, setting both volumes all the way up did make a difference if I recall.

Though this is certainly worth a try.
 
#7
When you turn up each pickup, does the hum change? If the hum is still there when you have the volumes turned down, that means it is not a problem of the pickups being out of phase, since they shouldn't be making any noise at that point.

If that is the case, try this: Un solder the pickups completely, but leave everything else wired up. See if it still hums. If so, start checking with a multimeter to make sure your pots are good, and that all of your grounds are correctly connected. Make sure that they do make a final connection to the sleeve of the output jack.

If the hum goes away when the pickups are removed, put only one back in, and see what happens. Then remove that one and attach the second one. See what happens.

The important part of troubleshooting is to remove variables, so if you can remove the pickups from the situation, that makes it a lot easier to diagnose.
 
#9
markblasco said:
When you turn up each pickup, does the hum change? If the hum is still there when you have the volumes turned down, that means it is not a problem of the pickups being out of phase, since they shouldn't be making any noise at that point.

If that is the case, try this: Un solder the pickups completely, but leave everything else wired up. See if it still hums. If so, start checking with a multimeter to make sure your pots are good, and that all of your grounds are correctly connected. Make sure that they do make a final connection to the sleeve of the output jack.

If the hum goes away when the pickups are removed, put only one back in, and see what happens. Then remove that one and attach the second one. See what happens.

The important part of troubleshooting is to remove variables, so if you can remove the pickups from the situation, that makes it a lot easier to diagnose.
Click to expand...

Thanks for this advice. The bass is silent with the pickup volumes down.

I want to make sure I understand you correctly. When you say, "if that is the case..." at the beginning of the second paragraph above, are you referring to if they do hum with the volumes down, or if they are quiet with the volumes down?

Regarding the output jack, the bass was formerly wired without a connection to the jack's ground lug, and this is also how my other Jazz is wired. What's the benefit of wiring it (or not) to the star ground?

Thanks again for responding.
 
#11
If you test with a multimeter, and find that the sleeve of the output jack is connected to ground, than you are fine.

If turning down the pickups individually causes them to be silent, than you have narrowed down the possible problems.

Now, If it is quiet when you have nothing turned up, and then when you turn up a single pickup you get a buzz, this is normal. Each pickup should cause about the same amount of buzz when turned up individually. If you turn one of them up, and then when you turn the second one up, the buzz increases, you should try taking one of the pickups and switching the ground and hot wires. This will correct the problem if the pickups are just wired out of phase. If this doesn't fix the problem, than either your pickups do not match, or there is something else wrong with your wiring (although I don't know what that would be)
 
#12
Fajah said:
I had a similar problem and it was the pickups being out of phase.

Check this out:

Click to expand...


Holy freaking Clamato!!!!!!

My ADJV with hum-cancelling SCN pups has been giving me endless noise grief since I bought it, and the (otherwise very good) tech I brought it to was not able to figure out what was wrong. I have tried various high-end preamps in there and they never sounded quite right or worked quite right. I shielded the holy heck out of all cavities and channels per the instructions in this forum, and it made no difference. So in a fit of frustration I recently pulled all the electronics, wired the pickups in series directly to the output jack, and twisted all wire pairs for maximum rejection, and kept the wires as short as possible.

No improvement. How is it possible? I have been pulling my beard out trying wrap my head around how it is even possible for such a heavily shielded, twisted-pair, humbucking pups situation to still have so much hum.

Guess what, they are the same polarity. I had no idea to look for that until seeing that vid, so thanks for posting it. Now I have to figure out whether it makes more sense to buy some magnets and DIY the polarity reversal, or send the pups off to someone else, or say screw it and buy some new pups.
 
#13
bongomania said:
Holy freaking Clamato!!!!!!

I have been pulling my beard out trying wrap my head around how it is even possible for such a heavily shielded, twisted-pair, humbucking pups situation to still have so much hum.

Guess what, they are the same polarity. I had no idea to look for that until seeing that vid, so thanks for posting it. Now I have to figure out whether it makes more sense to buy some magnets and DIY the polarity reversal, or send the pups off to someone else, or say screw it and buy some new pups.
Click to expand...

I'm glad this helped. It can be so frustrating, especially when you know that the cavity is shielded properly, grounds are good, and wires are connected correctly. The easiest and fasted fix for me was to post a WTB ad in Craigslist for a pair of single coil jazz pickups (which is what is on my bass). Within an hour, I had no less than 8 responses from people who had all sorts of nice pickups ranging in price from $20-$80.

I ended up buying a pair that was previously on a Geddy Lee jazz bass for $20. When I went to pick them up, I brought my compass to make sure that they were in phase. The seller, who happened to work in a music store, was quite intrigued when I pulled my compass out. I installed only one in the neck position and of course, what was in phase with my existing bridge pickup.

I plugged it in, mild hum at first until I evened out the volume controls. It's quiet as a mouse now.
 
#14
gforce9 said:
I've done this on two different guitars.......Is your pickup return and cavity 'ground' isolated via a mylar capacitor? One of the star grounding ideas is to isolate the two.
Click to expand...

Interesting. I read about that step, but skipped it because I understood it to be a purely shock-preventative measure (not that that's a bad thing).

Do you think this could affect the hum?
 
#15
samuelzero said:
Regarding the output jack, the bass was formerly wired without a connection to the jack's ground lug, and this is also how my other Jazz is wired. What's the benefit of wiring it (or not) to the star ground?

Thanks again for responding.
Click to expand...

Star grounding is unneeded in a bass and more often than not causes these problems. Soldering all your grounds to one point serves no purpose except making wiring more difficult, thus making wiring errors more prevalent.

The constant hum sounds lie you are missing the ground connection to the output jack or a ground connection somewhere in your signal chain. You can't hear if the two pickups hum cancel because the ground hum is louder.

Do you have a wire from the ground lug on the jack to the controls? Run a wire from the jack's ground lug to the common ground connections on the volume pots. Forget the star ground, it's obviously not working or miswired. Also is the foil grounded, or is that just from touching the pots? The foil needs to be grounded.
 
#16
samuelzero said:
Interesting. I read about that step, but skipped it because I understood it to be a purely shock-preventative measure (not that that's a bad thing).

Do you think this could affect the hum?
Click to expand...

It is merely for shock protection, and having a cap between the shield and ground will make the shield only function above a certain frequency, while letting low frequency noise in.

Not a good idea if you ask me!

I've said this over and over, that whole star grounding thing at GuitarNutz uses flawed logic and makes a bunch of assumptions that are not based in fact, such as having ground loops in your bass. That doesn't happen. None of the major builders of basses use star grounding because there is no reason to do so. In fact I've never seen it in any bass.
 
#18
bongomania said:
Just FWIW the Fender ADJV I mentioned above was star-grounded (stock from the factory).
Click to expand...

OK, I haven't worked on one of those...

And he said:

My ADJV with hum-cancelling SCN pups has been giving me endless noise grief since I bought it
Click to expand...

I have never star grounded and my instruments are dead quiet. :)

The thing the star grounding is supposed to fix, ground loops, doesn't exist in basses. That's an AC powered gear issue.
 
#19
I must admit that I am considering bagging the star ground. This is a kick-around bass, and I did this whole thing just as a project to see how quiet I could make it. So far it is exactly as noisy as it was before.

This being said, I checked out that youtube video posted above, which led me to pick up a compass, and guess what? Both pickups have the same polarity. So this should explain the hum not going away when both volumes are all the way up, right?

Here's another weird thing. I have another set of stock j bass pickups laying around, and I checked those with the compass. The polarity of the magnets is the same on those too...

Which leads me to another question: I was under the impression that j basses come stock with reverse polarity pickups to cancel hum when both are full up. Is this not the case?
 

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