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How to test a wiring harness?

Discussion in 'Pickups & Electronics [BG]' started by nsmar4211, Apr 22, 2010.

  1. nsmar4211


    Nov 11, 2007
    My original 5er ended up with awful humming issues a while back. I spent $150 on a new input jack and a cavity reshielding and some other stuff at a local luthier, when the noise was still there he told me it was the preamp and I was looking at $200 to get part and install, plus 4-8 week wait time. I was gigging and needed bass NOW, gave up and bought new bass, threw original in closet.

    Long story short, I like that bass. I started eyeing it again with the idea to get it fixed (but not for $200....only was 350 new!). Called Ibanez, got part number and a price for a new preamp i.e. wiring harness. $112 plus tax n shipping. 4-6 weekish wait (sheesh). Okay, $150.....and I could probably put it in myself (I've since learned how to solder). However, before I do so, I'd like to somehow test and make sure the preamp is indeed the issue.

    So, How do you test a wiring harness? Can you plug the bass in, poke a voltmeter on certain spots and look for a certain reading? And if so.....how do I find those magic spots and readings? All the info I can find is on how to replace one, not how to test one and make sure that's the issue......Help?

    This is the wiring diagram (so I was told): myBasswiringdiagramSoundgear.

    Edit: Just found bartolini harness online that's the same configuration......in stock (so no 4 week wait) and same price.....ohhhhhhhhhhh
  2. There really is no way to test a circuit to identify the cause of noise.
    Make sure everything is grounded properly and the shielding is good.

    You could check the resistance between all your ground points to be sure it's all at zero ohms.
  3. nsmar4211


    Nov 11, 2007
    Pretend I know nothing about this.......when you say check between ground points, where do I place the probes?
  4. Touch one lead to the sleeve terminal of the output jack, then touch the other to every grounded point on the bass.
  5. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    You'd need some technical data from Ibanez that they are not likely to give out. The voltage and resistance to measure at various test points will be different from one preamp to the next.

    What I'd do in your situation is make a passive Vol/Vol harness between pickups and output jack, bypassing the preamp completely. That way you can find out how much of the hum or noise was due to some grounding or shielding problem unrelated to the preamp. Also, a professional tech could hook the preamp up to an oscilloscope (and a test tone) to look for noise and distortion.
    Will_White likes this.
  6. nsmar4211


    Nov 11, 2007
    Ibanez wasn't talky about info , kept telling me to take it to local luthier (duh, already did) so yea I'm not gonna get any more out of them.

    However, I pulled the control cover off to start poking around with voltmeter and immediatly noticed the black wire coming from the top pickup had a some point *melted* to the white wire coming from same pickup! The copper is exposed, they weren't stuck together but the white wire has melted plastic on it. Pic below :). You can also see the wire by my thumb is partially melted, the way they were all stuff in the cavity it was probably in the same area (but no exposed copper on it) Now, that means that at some point one of those two wires got hot enough to do that. This bass has always been kept in climate controled environments (besides, electric wires shouldn't melt in normal temps). wth?!?! Now what......bad pickup!?

  7. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I'd put a fresh battery in the preamp, plug in a cable (to turn it on), and test the voltage and current at each of those exposed points.
  8. nsmar4211


    Nov 11, 2007
    ok, with my analog multimeter (etek 10729 model) dialed at x10k Ohms, I plugged in the bass (just one end of a cable to activate the switch) and placed the black probe on the ground leg of the input jack. I then poked the following: the exposed wire where in insulation was melted, the place on the volume pot where there is 5 (yes 5) ground wires soldered on, and all three black wires on the little board-each time the needle went to 1-2 ohms. Isn't it supposed to go up to 0? (starts at infinity on my meter, 0 is all the way to the right).
  9. 1. Basses don't have currents that will melt wires. Looks to me like someone messed around with a soldering iron.
    2. On your multimeter, touch the two probes together. That's your zero reading. On a 10k scale, I wouldn't be able to pick out 1-2 ohms.

    Without knowing the design voltages for the preamp, not sure poking around will be very enlightening.
  10. You can measure voltage, but to measure current would require desoldering to put the ammeter in series with the lead in question. (Unless you have an ammeter clamp on hand, which the OP probably doesn't have.)
    I don't see any reason to measure the current flowing through the wire, the reading (Probably on the order of a few hundred micro-Amperes to a Milliamp?) won't give you any relevant information to the problem.
  11. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    You're right about the difficulty of testing, but if the current was bizarrely high then that would explain the melted wires.
  12. stflbn


    May 10, 2007
    That there wire was melted by a soldering iron.

    Shouldn't really be source of noise unless it touched something metal though.

    If someone's been snooping around in there with a soldering iron I'd first question if everything was configured correctly as it currently is.

  13. Come on, it's totally obvious that those wires melted from someone burning them with the soldering iron.

    Wire doesn't melt that way when it overheats from passing more current than it's rated. It would heat from the inside out.
    It also doesn't melt in one random spot in the middle.
  14. nsmar4211


    Nov 11, 2007
    Well, if ya'll think it's from an iron...... :) True though, it is only in that one spot and not on the backside of the wire at all.... I did realize when I started to pack everything back in there it looks like maybe that bare spot could be touching a connection on the circuit board....seeing as how it's a ground wire maybe that's part of the problem, and that might explain the intermittancy.

    This after I just spent 40 minutes color coding and then tracing all the wires. Everythings according to the diagram except the grounds! Instead of one ground coming from the tip going into the circuit board and from there going out to each pot, they've got all the grounds tied together in one place on the volume pot (the group of 5 wires, but each of the other ends goes to the "diagram" place). However, they don't appear to have been resoldered at any point, all the soldering except the input jack (it was replaced) looks factory-same color, same shiny, etc.

    Out of a "what the heck" I plugged her in with her innards all lying around.....and no noise?!?!?! However, it was an intermittant issue when it started (and worked it's way up to an all the time issue) so my hopes aren't quite up yet....

    I'm thinking maybe I'll cover the exposed spot,pack everything back in while seeing if anything else is touching as I'm putting it all in there..and play her for a while and see if the noise comes again? If it doesn't...... :)... then again, I just don't know at what point I'd trust her enough to gig on :(

    I do have electrical tape, but could I use a drop of Plasti Dip to cover the exposed spot instead (the stuff you coat tool handles with, also used to coat piezo mics for underwater use)? I know it takes a day or two to dry... doable? Or just stick with the bulky black tape?

    Edit: And if anyones wondering how I could've missed that before....I never opened it up after it came back from shop and I didn't know then what I know now :).
  15. Shannon


    Sep 17, 2016
    Yep, looks like a soldering iron mistake to me too - not from time working on basses, mind you, but I use to do soldering back in my Navy days. Tape or the Plasti-Dip, should be no issue either way.
  16. BadExample


    Jan 21, 2016
    They sell a brush on insulation, available at walmart automotive 5 years ago. Dries fast.

    NAVAIR rocks! :D
  17. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    the right way (besides running new wires) is to unsolder one end of the errant wire and slip some heatshrink tubing up over the exposed spot.

    barring all that, a little electrical tape will be fine.
  18. Shannon


    Sep 17, 2016
    Of course, you'll have to restring the bass with tapewounds... :roflmao:

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