Pickups Or Guitar Doing The AC Hum Thing

Discussion in 'Pickups' started by Classicplayer, Aug 22, 2017.

  1. Classicplayer

    Classicplayer Senior Member

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    I think I've had it (honestly) with my 2010 Studio Deluxe. Other than some copper foil shielding added in the control cavity, the guitar is completely stock. I don't seem to be able to get my volume controls below +-8.5 or thereabouts without an annoying AC hum. Is my pickups, or the guitar's wiring, is the question. I plan on visiting a tech not far away who has a good reputation for his guitar. I want to keep the guitar as original as possible and don't want to resort to swapping out the stock wiring.

    Anyone else have or had some similar problem?


    Classicplayer
     
  2. rclausen

    rclausen Senior Member

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    Perhaps none of the above. Dirty power in your house could be the culprit. Are you running an air conditioner or playing in a room with florescent lighting?
     
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  3. Classicplayer

    Classicplayer Senior Member

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    Wes are running the a/c, but none of my other guitars exhibit the sort of hum the Studio Deluxe does. At first I thought it was the uncovered 490R - Burstbucker Pro. combination. I'm not convinced that truly is what's going on. Of course the hum goes away when my hand comes in contact with strings and metal parts. Folks tell me this is normal. Yes, but this seems excessive. I've experienced both with my Micro Dark and my prior amp, a Blackstar HT. Both amps specialize in lots of gain. Do you suppose that would be a big contributing factor?


    Classicplayer
     
  4. DarrellV

    DarrellV Murry Chrirstmers to earl! Silver Supporter Premium Member

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    Gain is amplification, so the short answer would be yes. Gain will always make noises louder. Just like the signal.

    You say you have other guitars you can plug into the same cable and amp and they are not as noisy, is that correct?

    Don't suppose you have a pic of the cavity and shielding...
     
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  5. Classicplayer

    Classicplayer Senior Member

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    IMG_3263.JPG
    Here is a pic of the cavity and controls. That's copper foil in there and on the plate back. Yes, my other LP (a Classic in my avatar) does have some hum, but it's not anywhere near as noticeable. It has shielding of a different sort. In that one, the backs of the controls are grounded by one continuous wire attaches to a screw on the cavity lip. The screw passes through a piece of electrical type adhesive strip. When the aluminum foil on the plate's back contacts the screw it quiets the guitar effectively and is completely reversible. These two jobs were done by different techs.


    Classicplayer
     

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  6. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    It looks like your string ground is going to one of the switch terminals on the neck volume pot. Is that pot otherwise grounded or are you relying on the copper shielding to ground it via contact with the threaded shaft?

    It looks like you might have a yellow ground wire connecting to that pot's chassis from the neck tone pot but I can't really tell from that photo.

    Touch the pickup pole pieces or cover without also touching anything else metal on the guitar, cable, or rig. That includes the strings so you may have to remove them for this test. On either pickup, does the hum increase or does it always decrease?

    In any case, something is wrong if your other guitars don't hum in the same rig. It's not normal.
     
  7. DarrellV

    DarrellV Murry Chrirstmers to earl! Silver Supporter Premium Member

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    It does sound like you have a floating ground for lack of a better term.

    I can see your string ground going to the back of one pot, but then it is a mess of wires going back and forth.
    It also looks like you have 4 wire pups in there, is that correct? Are they coil splitting?
    I see white,green,black and red.

    The reason I ask is because it looks like someone has custom wired it at some point and has grounded the coil splitting leads to the back of the pots.

    Normally, I have seen the two coil leads soldered together and taped and set aside, I haven't seen them both soldered to ground.

    If that's the case here there is the source of your hum.
    It would be tying one lead from each coil to ground and essentially you are running 2 parallel single coils.

    Instant noise!

    My 2 cents.... YMMV.
     
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  8. Classicplayer

    Classicplayer Senior Member

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    Classicplayer
     
  9. Classicplayer

    Classicplayer Senior Member

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    The wiring is sort of a rat's nest...must have been someone with fingers the size of a needle. I did the test Cookjuk asked for. My gain and volume at about 30% and guitar volumes rolled down to 8 to 9; there was no decrease in hum when touching a pole piece with my finger on either pickup. The pickups are tapped....a feature of this Gibson. When I play it and my hands are contacting strings, that hum goes to almost nothing, so perhaps the string ground is OK, but not Ok somewhere else in the circuit.?

    I found a new tech recently who I will consult with. Some years ago, I found a YT video of this particular model being played and the whole video was spoiled by a loud AC hum. I thought it was a jack'd up amp, but what I heard sounded similar to my own situation.

    As a counter test. I maxed the amp's volume and turned the gain knob up,from zero to 1/4 and the hum was present again. Gain at 1/3 was just as loud as loud as gain up and volume down.


    Classicplayer
     
  10. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    FYI - those terminals on the back of the volume pots (like where the string ground is connected) are not grounds and don't count as "grounding the back of the pot" like a typical metal cased non-switched pot would. Those are the switch contacts for the push pull switch inside the pot. You have to ground the pot's metal casing, like a traditional non-switched pot, but on the side. Maybe that's already happening with the yellow wires, but I can't tell.


    The reason I was asking about if the hum increased when you touched the pole pieces or covers is because Gibson pickups that come stock in push/pull guitars (I don't know where your pickups came from but since they are mixed, I assume they didn't come in your guitar) have different wiring and internal construction for the neck and bridge pickup, in recent years. They do this so that when both pickups are split into single coil mode, and the selector switch is in the middle position, you have a hum canceling effect, like you'd get in the 2 and 4 position of a modern Strat with a RWRP middle pickup. Sounds like you've covered that, but I wanted to clarify, just in case that helps. You can't just go by the wire color code charts that are all over the internet regarding these pickups, anymore.
     
    Last edited: Aug 22, 2017
  11. Classicplayer

    Classicplayer Senior Member

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    The pickups come as stock on this model Studio. They are the 490R and Burstbucker Pro. They are coil split and operate as push-pull to activate that feature.

    I think the string ground is on the neck volume casing.
    When I turn my volumes to 10, no AC hum is present. I read that volume pots are "voltage dividers". There was an article about this aspect of volume controls and how they can produce a hum when set just so.....
    I saw it years ago on the net....should have saved that.

    Classicplayer.
     
  12. chasenblues

    chasenblues Senior Member

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    It doesn't look like that to me..

    Captureol.JPG
     
  13. Classicplayer

    Classicplayer Senior Member

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    IMG_3263.JPG I double checked and and the string ground is soldered to to a metal post going through the plastic into the back of the neck volume pot. There are two terminals. here is A blown up version of the pic.

    Classicplayer
     
  14. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    Those terminals on top of the black switch section of the pot are not metal posts that connect to the back casing of the pot.

    Those are the internal switch's contact terminals. When you pull the knob, it connects those two terminals together. They are, otherwise, completely isolated from all other pot contacts and the pot casing, unless you manually connected them.
     
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  15. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    Those are regular CTS 450 series pots, that have an optional SPST switch mounted to the back that is activated mechanically by pushing/pulling the pot shaft. Fender and Gibson both use them in some products.

    [​IMG]

    The two solder terminals on the back of the black switch casing are completely isolated from all other pot terminals and the pot casing (that's why it's plastic, to be isolated). You need to ground these pots on the actual metal side of the pot's chassis, as if they did not have the black plastic switch assembly attached.

    You also need one of the outer lugs of each pot connected to this metal chassis for the volume controls. The outer lug that doesn't have the pickup or switch connected. That's how you send your signal to ground when you roll the volume down, and if you aren't doing this it would explain all of your problems.

    There are some schematics for the optional switching on the back of these pots on the CTS website here: https://www.ctscorp.com/wp-content/uploads/450.pdf ...though they may be a bit confusing to someone not familiar with the products.
     
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  16. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    Though this is a different brand and type of push/pull switched pot, you can see how I have had to ground to the side of the pot's metal casing/chassis. This is EXACTLY what you need to do for both of your volume pots AND connect the outer of the three standard pot lugs (the one without the switch or pickup connected to it, you could call it the "top" lug from your photo), on each volume pot, to these points, which is what I have done with the cream colored insulated wire. It connects that outer lug to the same grounding point on the metal casing.

    [​IMG]
     
  17. Classicplayer

    Classicplayer Senior Member

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    Thanks for you all taking time to educate me. It's much appreciated. The ultimate question for me is: would a competent tech be able to sus all of this out and verify that the factory wiring is done right, and if not, will they be able to rectify it?

    Classicplayer
     
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  18. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    A competent tech should, yes. Make sure of that competent part before you hand over your instrument. :D

    ...but if your tech installed that copper shielding to fix this problem, I would not return to him for this because the problem still exists and that obviously wasn't the proper solution (though, it also shouldn't hurt anything if done right) which he should have figured out when testing the instrument before you took it back.

    These guitars are so expensive and important to us, it's worth paying more to see the best guy in town for something. Especially, if others have failed you on the same problem, already.
     
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  19. Classicplayer

    Classicplayer Senior Member

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    I no longer use that same tech. Although he does great work that I have asked him to do, I feel that sometimes he doesn't like getting too involved in anything he senses could be time consuming. He did state that the wiring was a good factory job. The one I recently used is a store owner and their reputation is pretty solid. I feel that Gibson may not take the time to properly inspect their guitars before shipping. I bought this Studio Deluxe as a used guitar, so my investment is not as great as the original owner who purchased it from a Maryland Guitar Center, but got rid of this guitar not long afterwards..... I have the store's original sales slip.
    Except for the pesky hum situation, this guitar was in almost perfect condition.

    Classicplayer
     
    Last edited: Aug 23, 2017
  20. cooljuk

    cooljuk Transducer Producer Premium Member MLP Vendor

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    Was there a metal plate in the guitar originally, which was replaced by the copper foil? Those metal plates provide the grounding. They can come out but one must re-do the grounding in a more traditional way when they are removed. This isn't all stock is it?

    Are you in Maryland, yourself? If so, you would be wise to take your guitar to Phil Jacoby at Philtone in Baltimore. He's one of the best on the East Coast, and a Master Luthier with a GREAT deal of build, repair, and service experience from several of the biggest guitar companies in the US. He will never steer you wrong. If you are in Maryland, that's the place to go.
     

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