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Missing Ground Lug in Control Cavity (is it a problem?)

Discussion in 'Tele-Technical' started by dberlind, Nov 29, 2020.

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  1. dberlind

    dberlind TDPRI Member

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    Hey all, first time poster here.

    I have a 2014 Am Deluxe Tele. The original owner bought it new from Sweetwater and Sweetwater installed a Bigsby before the owner took delivery. The strings wouldn't stay in tune whenever I used the Bigsby and so I upgraded to a Mastery bridge to allow for better string glide. After the Mastery went in though, I had a new hum problem that I didn't have before. This guitar has the S1 switching system with the N3 noiseless pickups. I took it back to the luthier who originally installed the Mastery bridge and said, hey, while you're in there, lets paint the cavities with shielding paint. It came back worse than before. This time, the neck pickup worked and I got nothing from the bridge pickup (and something in-between when both were selected when the switch was in the middle position). Being an amp tech, I decided to fix it myself this time. The first thing I discovered is that the hot lead on the backside of output jack was making contact with the conductive paint (thus grounding the audio). Suddenly, the neck pickup was sounding better. But I still have a bridge pickup issue. I also noticed that every wiring diagram for the American Deluxe shows a ground lug in the cavity itself. The lug is connected to ground on the back of the control plate (which leads to ground on the jack) and another wire leads to the bridge. My guitar does not have this ground lug. I'm curious if anyone else has seen this and if it matters.

    Thanks in advance

    David
     
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  2. LutherBurger

    LutherBurger Poster Extraordinaire

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    Only one of my guitars has shielding paint in the control cavity. There's no ground lug screwed into it, but the guitar is pretty quiet anyway. Your hum is probably coming from somewhere else.
     
  3. dberlind

    dberlind TDPRI Member

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    Thanks @LutherBurger. I thinking there's a hot audio lead that's making contact with the shielding paint somewhere. But, if there's supposed to be a grounding lug in the control cavity and the bridge metal is connected to it, then the absence of those things might explain part of my problem. I am thinking that significantly dampened audio from the bridge pickup could mean some of the audio is bleeding off to a ground short somewhere. Would greatly appreciate guidance on that ground lug though. If it's missing from my guitar, I can easily install one.
     
  4. LutherBurger

    LutherBurger Poster Extraordinaire

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    First make sure that the bridge is grounded and the + leads aren't.
     
  5. WalthamMoosical

    WalthamMoosical Tele-Holic Ad Free Member

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    Welcome to TDPRI!
     
  6. old wrench

    old wrench Tele-Afflicted

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    If you decide to add a ground lug, pay very close attention to the thickness of the body at the point of attachment, as well as the length of the screw :).

    Awhile back a neighborhood player brought his guitar over for "a little repair". Yep, sure enough he added a grounding lug and ran the screw tip through the back of his Strat. I'm sure you'll be wiser than he was.

    We got it fixed up for him, but every time I hear "grounding lug" I think about that particular instance ;).


    .
     
  7. Vizcaster

    Vizcaster Poster Extraordinaire

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    Have you had the bridge off yourself or only in the hands of the repair shop? Some Teles have a shielding paint lug in the bridge pickup cavity (hopefully that paint is continuous with the paint in the control cavity so only one screw lug is necessary; when painting cavities I use a cotton swab as a paintbrush to cover the wire tunnels).

    Another issue is that you don't really need a separate wire from the control cavity to the bridge plate if your aftermarket bridge pickup has a metal baseplate. There's usually a jumper from the ground eyelet to the baseplate that allows for a connection (via the pickup mounting screws) to the bridge plate.

    Also I would make sure to test out the shielding connection to ground with a multimeter.

    And good catch finding the jack touching the paint on the side of the tunnel. That was a pretty elusive one when it happened to me.
     
  8. dberlind

    dberlind TDPRI Member

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    Thank you for the welcome!
     
  9. dberlind

    dberlind TDPRI Member

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    this is great advice. Within the control plate cavity, there is an "elevated" area that isn't as deep as the rest. In the schematics that I've found, it is this area that the ground lug is screwed into. So there's a bit more wood -- maybe 3/4 inch, to work with. That's a lot longer than any ground lug needs to be.
     
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  10. dberlind

    dberlind TDPRI Member

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    @Vizcaster I have not had the bridge off yet myself. I plan to take it off this weekend. I don't know about the continuity of the paint but have read about that before. I think I have found at least one other problem. Some of the wires going to the 3 way switch have the paint on them and when you put the control plate back into the cavity, these wires end up between the three way switch at the side-wall of the cavity. I think the conductive paint on the insulation is shorting a connection between the bare metal on the 3-way switch and the sidewall of the cavity (which is painted as well).

    How did I diagnose this? With the control plate OUT of the cavity and volume set to max, I hooked up my DMM to the output of the guitar, set the DMM to A/C and set the DMM's display to show the max voltage observed. I put the 3-way switch onto the bridge pickup and did several "controlled" single-stroke strums across the whole string set. The range of outputs was 48 to 60mv (the variation reflecting my inability to perfect control the strum pressure). Then, I did the same with the 3-way switch flipped to the bridge position. If there's a short somewhere under the bridge, then I'm thinking that the bridge pickup is going to yield a lot less voltage with the same approximate controlled strum. But it didn't. I was still getting in the high-40s to 50s. At this point, I wondered if maybe I'd see different observations with the control plate IN the cavity (which would suggest that there was a short taking place when it's all back together). So, I re-inserted the control plate back into the cavity holding it there with one finger and did the same DMM test and controlled strum with the other finger and sure enough, the output was a lot lower for the bridge pickup and noticeably lower for the neck pickup.

    I started poking the wires to see if any were loose. For example, maybe the insertion of the control plate back into the cavity was putting tension on a connection in a way that was causing it to short with something else (or the sidewall of the cavity). And that's when I noticed that you really can't reinsert the control plate without some of the wires settling into position between the 3-way switch and the sidewall of the cavity. And then I looked at the insulation on the wires and they have conductive paint on them. So, right now, I am very delicately cleaning the paint off all of the wires with thinner on a Q tip. I am going to pull the other pickups as well to look for similar issues under them. The last thing I was noticing is that the conductive paint is worn off the bottom of the control plate cavity in the exact position of some solder-bulbs on the bottom of the 3-way switch. I think some contact is being made there so I am going to line the bottom of the cavity with some thin rubber as a form of insulation. I will report back to this group once I've completed all these tasks.

    ps: I also wrote to Fender about the the grounding lug. They sent me the schematic for my guitar and agreed is shows the grounding lug but implied that some guitars may not have it and that it is nothing to be concerned about. The actual quote was "It does show a ground lug in the control cavity, however the actual ground is not necessary. whereas the bridge ground is."
     
  11. LutherBurger

    LutherBurger Poster Extraordinaire

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    I think you've found the problem. Stick a piece of electrical tape to the floor of the cavity under the switch and see if that helps.
     
  12. dberlind

    dberlind TDPRI Member

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    Yes, for the floor of the cavity, I've cut a fat piece of electrical heat-shrink to match the width of the cavity floor and am gluing that down to the floor with contact cement. It has a tad more thickness than electrical tape and if one of those solder bulbs has a sharp edge to it, I think the rubber of the heat shrink will do a better job resisting compromise. However, based on the feedback from Fender, I am curious about grounding the bridge. In Fender's reply, they noted the bridge needs to be grounded. In the schematic the supplied for my Am deluxe, I do not see a dedicated ground wire coming from the bridge. I haven't removed the bridge yet. If I do, will I find a wired connection from the bridge to the grounded core of the bridge pickup? Not having removed a bridge pickup on a tele before, I wasn't sure exactly what they were referring to. Or maybe they just meant to be sure the bridge pickup was grounded?
     
  13. SuprHtr

    SuprHtr Tele-Afflicted Silver Supporter

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    I've had the exact same switch lug issue after installing copper shielding. In addition to the insulation (tape or heat shrink) on the bottom of the cavity, I bent the switch lugs sideways to increase the clearance.
     
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  14. LutherBurger

    LutherBurger Poster Extraordinaire

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    You'll probably find that the pickup grounds the bridge by means of its metal base plate. Or that there's a ground wire mashed under the bridge plate. Or maybe both.
     
  15. dberlind

    dberlind TDPRI Member

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    Eek. I'm terrified of doing any bending. I could see myself busting the switch and having to order a new one (hey, in which case I get a 5 way and rewire the whole cluster for some other configurations!.. LOL).
     
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  16. dberlind

    dberlind TDPRI Member

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    Thanks LutherBurger. I will find out. I'm going to make a video out of the whole thing. I'll share it back here once I've edited it. But you might see me resurface with other questions. Something tells me though that once I'm done applying my OCD/TLC to this, she's going to one quiet tele.

    ONE LAST QUESTION THOUGH: This tele has a Bigsby on it. Are the guitar strings enough (given that they all touch the bridge and the Bigsby) to inherit the ground from a properly grounded bridge? I'm assuming yes.
     
  17. Vizcaster

    Vizcaster Poster Extraordinaire

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    Use the VOM on continuity to see if the bridge plate is grounded.
     
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  18. dberlind

    dberlind TDPRI Member

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    For sure. Thanks.
     
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