Grounding issue - please help!

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by TJT79, May 22, 2015.

  1. TJT79

    TJT79 Member

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    Hi

    Rehearsed with the band tonight. At the start, everything was normal. We played for a while. I tried a friend's SG - no problem. I plug mine back in and play a couple of songs, then I notice a buzz when not playing...

    Symptoms:
    - was NOT there previously
    - goes away when I touch the bridge or strings. Not sure if touching pickups or metal pots made a difference. I can try this tomorrow.
    - the SG into same amp/cable doesn't have the problem so I'm sure it's my guitar
    - I tried different guitar cables, and also a different wall socket
    - buzz is the same on both pickups (firebird style mini-hum bridge, P90 neck)
    - I took off the pickguard and bridge. Everything looks OK (solder joints). Nothing has obviously come loose
    - With my multimeter I have continuity from ground at the guitar jack all the way to the tuners, as well as to the mini-hum cover and mounting ring, and the two P-90 height screws.
    - the bridge is a screw on hardtail strat style. The ground wire from the cavity is just pressing up against the underside of the bridge, held in place by the pressure of the bridge against the body. Is this normal?

    My guess would usually be that there's not a great contact between the ground wire and the bottom of the bridge. Is it possible that the DMM shows continuity but the electrical contact is a bit flaky?

    Perhaps when I took off the bridge I re-established a good connection (hence the good DMM connectivity) - it's late now but I'll plug in again tomorrow and see if the problem remains.

    I repeat: this issue was NOT present at the start of the rehearsal!!

    Guitar is one of these:
    http://www.musicradar.com/reviews/guitars/divill-by-italia-b100-594665

    I can post photos of the inside of the pickguard tomorrow, if that helps.

    Any help very welcome.

    Thank you in advance!
    TJT
     
  2. TJT79

    TJT79 Member

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    PS. Hum goes away when I turn down the guitar's volume knob.
     
  3. mc5nrg

    mc5nrg Supporting Member

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    Nothing wrong judging by your description. Keep your hand on the strings or metal parts connected to strings.
     
  4. TJT79

    TJT79 Member

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    Hi

    I understand why you say that, but it was definitely not making the buzz noise previously. It started at some point during tonight's session. Normally it's whisper quiet.

    Cheers
     
  5. TJT79

    TJT79 Member

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    mc5nrg - I just re-read my post from last night. It sounded a bit blunt - not my intention. Sorry if I sounded rude and thanks for your input!

    I have good continuity from guitar strings/bridge/pickups all the way through cables, amp, AC lead to the wall.

    I'm wondering if the problem was environmental. Maybe something else was plugged in which caused interference? Perhaps the SG was just better shielded than my guitar, hence the difference??

    The baby is asleep now so I can't make noise but once he wakes up I'll put the guitar and back together and see if anything has changed.

    Cheers.
     
  6. SHOOTOUT!

    SHOOTOUT! Member

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    Last edited: May 23, 2015
  7. TJT79

    TJT79 Member

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    I think you're possibly right, although I still can't explain the difference between the completely silent guitar at the start of the session, and the suddenly very buzzy guitar at the end. Same guitar/cables/amp/wall plug etc.

    I've had friends here for the weekend so today has been mainly drinking beer and playing cards. :) Tomorrow I'll plug in here at home (where buzz has never been as issue) and see what happens. And in the evening we're playing again where the buzz incident occured, so I'll see if anything changed there.

    Cheers
     
  8. SHOOTOUT!

    SHOOTOUT! Member

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    So you're getting a different kind of buzz when using the beer? ;O)

    Cheers!

    Marc, Director
    SHOOTOUT! Guitar Cables, UK
     
  9. TJT79

    TJT79 Member

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    The buzz is definitely worse when I plug the beer into the amp. :)

    I tried at home and it sounded OK but there's nothing here which is likely to introduce noise. We should be rehearsing again this evening so I'll see if the problem persists there.

    Correct me if I'm wrong: when using a humbucking pickup, and with the whole system earthed, there should be no noise, even with fluorescent lights etc? If there was some change in the environment where we rehearsed (eg. something being switched on), as long as there's earth and the pickup is hum-cancelling, I understood that I shouldn't hear any difference?

    The only thing I can think of is to get some aluminium tape and line the inside of the cavity (it currently has that black paint they use). But with a humbucker and good ground connections why is this necessary?

    Sorry if these are stupid questions.
     
  10. Otto Tune

    Otto Tune Member

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    You won't really know until you go back to the rehearsal and recreate the problem.
    It could have been a fluorescent light, microwave, or any odd things on the same power as you (not necessarily the same wall plug).
    Sometimes coincidence is mistaken for causation.
     
  11. The_Whale

    The_Whale Member

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    Some humbuckers don't noise-cancel as well as others. Sometimes the two coils in a humbucker are wrapped, intentionally or unintentionally, with unequal amounts of wire. Humbuckers made this way can put out some noise.
     
  12. swiveltung

    swiveltung Member

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    Something in the building came on. Flourescent lights, refrigeration will do it.
     
  13. TJT79

    TJT79 Member

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    Hi

    Rehearsal again last night at the same place. Noise issue still there. I put a metal earth rod into the ground outside and connected that to the ground on my AC extension lead. Noise almost completely gone.

    Conclusions:
    - The grounding where we play isn't great!
    - The SG wasn't bothered by this, so I guess it's just better shielded than my guitar.
    - I guess my mini-humbucker isn't fully humbucking. Maybe the coils have different number of winds.
    - There is a fridge there on the same circuit, so if that happened to be running or not at a given point might affect things?

    Anyway, the problem isn't with my kit. I will try to buy some copper/aluminium tape and line the cavities, but with the earth rod connected problem is negligible.

    Thanks for all your help. Much appreciated.

    TJT
     
  14. swiveltung

    swiveltung Member

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    I wonder if your input jack rotated or is partially grounding on something inside, shielding maybe ?
    Yes, refrigeration can be an issue at times, although usually at a gig where the unit is large..
     
  15. TJT79

    TJT79 Member

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    You mean the live wire (i.e. the signal) being grounded inside the cavity? What effect would this have? Apart from the buzz which goes away when connecting to a good earth, the guitar sounds normal.

    Question: the metal cover over the Firebird-style mini-humbucker is grounded. This seems weird to me - why does it not block the signal being picked up by the pickup??

    Thanks.
     

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