Need help with defective wiring

Discussion in 'Luthier's Corner' started by Worblehat, Nov 19, 2018.

  1. Worblehat

    Worblehat Active Member

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    While switching pickups last weekend I completely rewired my SG. Had some problems with the previous wiring and also wanted to learn more about this stuff. I certainly did, having almost no experience with soldering before. Although the wiring did not turn out as neat as I hoped, everything worked afterwards.

    Today I plugged in again and wasn't able to get any sound from the bridge pickup. So I got my multimeter and tried to figure it out:
    I set the multimeter to resistance measurement (20k ohm). Putting the multimeter leads to the wires of the bridge pickup I get the correct resistance value of 8 point something. The same with the input lug of the volume knob and its back. But the strange thing is: measuring between the back of the pot and its output lug (the middle one) I get zero, having the knob turned up to 10. I am not sure, but this seems wrong to me.

    Here is the wiring I used (hopefully Seymour Duncan doensn't mind that I post it here):

    [​IMG]

    I should probably mention that I kind of treated the pots not very kindly. I had a cheap soldering iron first that was new but didn't really work very well. In the process of desoldering the pots I heated them way to long. I ordered a better soldering iron before continuing the work which worked way better. But might be that the damage has already been done. The strange thing is that it worked yesterday...

    Any ideas?
    Do I need to remove the pot from the circuit to check if it's working?
     

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    Last edited: Nov 19, 2018
  2. Col Mustard

    Col Mustard Well-Known Member

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    I suggest that you take your prized SG to the best
    luthier you can afford, and have him put it right.
    You may have destroyed some of your components, so
    be prepared to throw those away and install good quality
    parts.
    Pay the man for doing you the service you need, and
    then play the hell out of the SG...
    >With an SG, that's actually not possible... play it a lot
    and there's plenty of hell left. *grins

    Then, if you want to learn how to do this kind of
    work, practice on some beater that you pick up at a
    garage sale. Throw away your cheap soldering rig and
    buy a good quality tool. There are lots of online tutorials
    to help you learn this work, and it's a good skill to have.

    I recommend you buy a book called "How to make your Electric guitar
    play great" by Dan Erlewine. It's a great reference to have around,
    because it tells you how to do lots of these tasks, and shows good
    pictures.

    Erlewine also has produced several videos, including "How to wire a
    Gibson guitar" (which applies to Epiphones as well)... also
    "How to wire a Fender guitar" which applies to "fender type" guitars
    as well. I studied this video when trying to learn, and it helped me
    avoid many mistakes.

    It's a great idea to learn how to work on your own guitar.
    Your mistake was in trying to learn by working on your best
    guitar. Luthier's school rule number one:
    PRACTICE ON SCRAP
     
    Last edited: Nov 19, 2018
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  3. cerebral gasket

    cerebral gasket Well-Known Member

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    0 ohms = short
    Back of pot casing = ground
    The output is shorted to ground which is why you have no output.

    Can you post a photo of the control cavity?
     
    Last edited: Nov 19, 2018
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  4. plankton

    plankton Well-Known Member

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    Yes, pics would be helpful.

    If it was working and then it stopped the next day then you probably haven't killed the pots.
     
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  5. Worblehat

    Worblehat Active Member

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    Thanks for the whole lotta good advice, Colonel! I bought Erlewines book before beginning to mess with my guitar based on your recommendation in another thread! It has some very useful soldering tips and helped me to get started.

    As I said, I bought a new soldering iron. Still inexpensive (only 10€ more than my old one), but its a reputable brand, made in germany, works really well and feels much better! Those 10€ made a huge difference.

    I know that a luthier would do a better job and would probably be worth every penny. Some time ago I decided to do as much as possible on my own because I am eager to learn about that stuff and it is a very rewarding experience. I am aware of any consequences.
    It's an Epiphone by the way and, Col, I remember you saying that Epiphones are a great modding platform. :D Well... in fact it's the Iommi signature model and my main guitar which costed me as much as a Gibson Faded would cost. But hey...electronic parts are cheap. An if I become desperate I can still take it to a luthier. Resale value is secondary to me.

    Now I am not sure anymore if it really measured 0. Maybe it was 1 which would mean "no continuity", right? (wasn't aware of that before). I will re-measure when I come back from work today!

    I will also post pics! Actually hoped to solve this issue without pics as I am a bit embarrassed of how messed up it looks :ohno:
     
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  6. Daniel.S

    Daniel.S Well-Known Member

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    If you don't post pics, then it's harder for us to help you.
     
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  7. cerebral gasket

    cerebral gasket Well-Known Member

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    A photo will be helpful and don’t worry if not perfect solder job, not everyone is a solder tech.

    The purpose here is to help, not criticize others.
     
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  8. plankton

    plankton Well-Known Member

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    Yeah, I'm sure we've all been guilty of some poor soldering before. No judgement.

    Another thing I thought of, once I had a similar thing happen to me with a pickup working fine and then not, and I eventually realized that the pot had twisted a bit and one of the pot lugs was touching the shielding paint on the side of the cavity and shorting to ground.

    I was simultaneously relieved but also feeling like a dumb ass for not realizing the problem sooner.

    Not sure if that's your issue, but it's something worth keeping in mind.
     
  9. Worblehat

    Worblehat Active Member

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    Thank you all for your awesome support so far! Unfortunately I won't be able to post pics today. Have to wait until tomorrow evening. I will also check plankton's suggestion. Good idea! It may well be, that I applied to much solder to a lug which could touch the cavity.
     
  10. Worblehat

    Worblehat Active Member

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    Picked the guitar up yesterday and the pickup magically worked again. Today I only get fading/crackling sounds from it. The multimeter measuring resistance between the volume pots output and its back shows 1 with the volume on 10! So no continuity here.

    So here we go...pictures! Don't know if they actually help.

    The wire I bought was useless as it was thin, very inflexible and kept breaking. So I needed to use old leftover wires from some electronic experiments I did on a breadboard years ago. Therefore the colorful look :D Next time I will get stranded wire!
    And as you see I gave up on removing the old solder from the pots. But I tried to put the new connections on clean spots.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    The last pic is a close-up of the volume pot. The output lug I mentioned above is the one with the blue wire (and some molten insulation :rolleyes:). Its hard to see on the photo, but there are quite large blobs of solder on the lugs. But I don't think solder touching the cavity is the problem as it persisted after loosening the knob and raising it bit away from the cavity.
     

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  11. cerebral gasket

    cerebral gasket Well-Known Member

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    Lift the blue wire on the volume pot up a little so that it is not touching the lug next to it that is wired to the pot casing, which is ground. If the blue insulation is molten enough to where bare wire is exposed underneath it and touching the lug next to it that is ground, then that is why the output is shorted to ground and there is no sound.

    Also check the pickup leads to that same volume pot. Make certain none of the outer braided shielding which is connected to ground is touching the center conductor (hot wire) inside the braided shield or the lug that it is connected to. The black cloth insulation should be keeping the braided shielding and center conductor separated from each other to avoid a short circuit.

    Also I just realized the braided shielding on that pickup does not appear to be soldered to the pot casing and needs to be so that it is grounded. If it is, reflow the solder.

    Pictured below is the cavity on one of my SG Specials that I redid...

    [​IMG]
     
    Last edited: Nov 21, 2018
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  12. Worblehat

    Worblehat Active Member

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    Checked it...they are not touching. It just seems so due to the perspective.
     
    Last edited: Nov 21, 2018
  13. Worblehat

    Worblehat Active Member

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    You just don't see it on the picture. The solder is hidden behind the wire. I just checked the grounding of all parts with my multimeter.

    Good suggestion! Now I pushed back the the braided shield a bit and made sure it does not touch anything else. Plugged it in and it seems to work now. :thumb: I will test it extensively tomorrow!

    Thanks for the photo! Your cavity looks really neat :smile: Its 50s wiring, right? I might want to try that, too.
     
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  14. cerebral gasket

    cerebral gasket Well-Known Member

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    Thanks!
    Good to hear you got it working again. The braided shield was probably causing the intermittent problem. Have seen that happen before.
     
  15. Worblehat

    Worblehat Active Member

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    Thanks again for your awesome support!
    I finally got around to testing it thoroughly. Sounds good! But I noticed two things:
    1. The output volume is a bit quiet. But that might be just normal as I have put a neck pickup in the bridge position.
    2. Measuring the bridge pickup with the multimeter gives about 18 ohms even though this pickup should be about 8 ohms. I measured at the ouput cable as well as the output lug of the volume pot, with volume and tone on 10. How can that be?
     
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  16. cerebral gasket

    cerebral gasket Well-Known Member

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    DCR of pickups should be kilo ohms, so it would be 8k ohms instead of 8 ohms. Is the multimeter set to the correct range if it does not have auto range function?
     
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  17. Worblehat

    Worblehat Active Member

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    Sorry, forgot that "k" in the post above. The range is set to 20k an I measured 18k ohms.

    The neck pickup gives the expected result of 15k (it's the Gibson Iommi humbucker). The bridge pickup is a P90 sized humbucker which should be about 8k. I had the P90 in the neck position before rewiring.

    Edit: Oh...and note that measuring directly at the pickup wires I get the expected 8k ohms. So the pickup is ok.
     
    Last edited: Nov 28, 2018
  18. 4wight

    4wight New Member

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    Not a wiring expert by any means, but logically if the pickup measures correctly when not connected, but measures incorrectly when wired in, then there is only one conclusion - the wiring is at fault somewhere. Try disconnecting the neck pick up and measure the P90 again - it should read 8k ohms as expected. If it reads incorrectly again when you connect the neck pick up back in then clearly you are getting a leakage between the two pickups somewhere. It looks to me as if the ground wire is running from the switch to the bridge volume pot and then directly to the jack, rather than snaking from the switch through all the pots in turn before hitting the output jack. As I say, I'm not an expert but none of my SGs are wired that way. (And I can't really see what the tailpiece ground is connected to - is it to the switch? Most diagrams I've seen show it connecting to the first pot in the ground chain.)
     
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  19. plankton

    plankton Well-Known Member

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    Can you measure the resistance of a single component when it's wired in a circuit?

    I would have thought that if you place the probes on the pickup leads when it's wired in then you will be reading the resistance of the entire circuit.

    Can any electronics experts clarify this?
     
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  20. cerebral gasket

    cerebral gasket Well-Known Member

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    It depends where you place the meter probes.

    Resistance checks are always done with power OFF when troubleshooting circuits. You place the meter probes directly to each end of the component you wish to measure.

    If you were to place the black meter probe on ground and the red probe to one end of the component, then you would be measuring in reference to ground and be picking up resistance readings from all other components that may or may not be connected in parallel or series with component that is under test.

    Measuring resistors in circuit is no problem. You place the meter probes on each end of the resistor.
     
    Last edited: Nov 28, 2018
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