P90 phase problem

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by kingsxman, Aug 18, 2006.

  1. kingsxman

    kingsxman Silver Supporting Member

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    I have an early 70's Gibson SG Pro that I just got that I've posted on before. The guitar has P90's in it and I'm getting a very weak, out of phase (quacky) kind of sound out of the middle position of the toggle switch. I have checked my guitars wiring and it appears to be ok.
    The pickup wire is braided ground so I cant swap ground and hot around to see if the problem goes away.

    What could be the problem? I've got the standard Gibson wiring shown here.
    http://guitarelectronics.zoovy.com/product/WDUHH3T2202

    I'm not sure if its just the way the original pickups sound or not. I checked the pickups themselves as someone suggested the magnets may be not "in" properly. I cant see how you'd take the magnets out so I dont think thats the problem. I'll include a link to a bad pic of the back of one of the pickups.
    http://users.goldengate.net/~jgermann/pickup.JPG

    any ideas?
     
  2. jmadill

    jmadill Gold Supporting Member

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    A quick check to see if the polarity is reversed would be use hum as a tool. Hook your guitar up to your amp, and find somewhere to stand where you get the most noise thru the pickups. There should be some hum from which ever pickup you select. If the pickups are in phase, the "both" pickup position should net you twice as much hum than either individually. If the pickups are RWRP (Reverse Wound, Reverse Polarity) then there should be no hum in the "both" position, as the "out of phase" cancels the sound common to both pickups, which is the hum.

    Are the pickups stock, or aftermarket? If stock, I have a set of P90s here from a 70's era Gibson I can use as a reference. If your pickups are the same as mine, the braided cable at the pickup is accessible, and you might be able to change the polarity there if necessary. If someone changed the electrical polarity, that's where they did it, I suspect.

    The "quack" might be from changing the electrical polarity, and not the magnetic polarity. RWRP pickups have both, so they cancel hum, like a humbucker, but don't create an out of phase condition and quack.

    If the pickup electrical polarity has been changed, you have two choices.

    1) change the electrical polarity back
    2) reverse the magnets, leaving the wiring reversed

    Number 1 will give you original, and number 2 will give you less hum in the "both" position.

    Hope this helps.

    -jm
     
  3. kingsxman

    kingsxman Silver Supporting Member

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    Thanks Jmadill,
    I'm sure these are stock pickups. Or pretty sure. I tested the guitar aagin. Actually...its quite different than what you might have expected. I have no hum in the bridge position. Some hum in the middle..and alot of hum in the neck position.

    I'm just thinking about ordering a pair of Vintage Vibe P90's and being done with it. I think this maybe is something with the pickups themselves.
     
  4. jmadill

    jmadill Gold Supporting Member

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    Interesting. Could be something else in the wiring. How does everything check against the schematic?

    If you know P90s, you'll know that they hum. If you're going to replace the pickups, consider a set like the Lollars that are RWRP and you'll at least have a safe haven in the "both" position where there's no hum.

    -jm
     
  5. kingsxman

    kingsxman Silver Supporting Member

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    I've wired it 2 different times now to 2 different schematics. I'm just like a stock wiring for a Gibson now. Very wierd.
     
  6. John Phillips

    John Phillips Member

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    (edit) - just looked at the pic... that's not a P90, it's a stacked humbucker, maybe a P100 although I can't see it clearly enough. The lower coil is below the baseplate and magnets.

    If it's out of phase, you still need to take one of the pickups apart to reverse either the magnet or the coil wiring.
     
  7. kingsxman

    kingsxman Silver Supporting Member

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    I'll have to try to get a better pic of it. I'm not sure why this one was so blurry. I think its a P90. If I remember it looked the same as the top pickup on the back. I'll take them apart again and check the polarity. (I really liked your last explanation that I didnt save....can you give me that again about how to check the magnets?)
     
  8. John Phillips

    John Phillips Member

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    It should still be here somewhere...

    Take both pickups out and hold them face-to-face. If they attract, the magnets are opposite polarities; if they repel, the same polarity.

    A P90 doesn't look like that on the back - it has a metal baseplate, which is the back of the pickup and mostly what you see. The magnets are above this (towards the front of the pickup), and the coil on top of that. A P100 is basically the same but has a second coil underneath the baseplate - the idea is that this (which is the hum-cancelling coil) is shielded from picking up the string signal by the baseplate, so you still just hear the 'true' single-coil P90 sound. (It nearly works :)).

    If this is a P100 rather than a P90 and is sounding weak and out-of-phase with the other pickup, and the magnetic polarity is OK, it's possible that the top coil is damaged - this gives a much quieter output. Have you got a multimeter or any way of measuring the pickup resistance? If one coil is damaged you'll get about double the resistance of the good pickup, since the two coils are in parallel.
     
  9. kingsxman

    kingsxman Silver Supporting Member

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    John, you are the man. Your correct. They are different pickups. It looks like this maybe is a p100 and a p90. Here's the particulars:

    Bridge pickup measures resistance of 6.2k
    Neck pickup measures resistance of 7.9k
    They attract each other when placed faced to face.

    Here's a link to some more pics of the pickups side by side. Good call. now...what to do...? In looking at the P90 (neck) it does look like someone messed with the wiring as it enters the pickup back. Doesnt look like its an easy fix..but I'll try and take it apart and see...

    http://users.goldengate.net/~jgermann/SG_Both_pickups_1.JPG
    http://users.goldengate.net/~jgermann/SG_both_pickups.JPG
     
  10. kingsxman

    kingsxman Silver Supporting Member

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    Well, I took it apart and it doesnt appear that the wires are reversed. They have been messed with..but appear to be right. I thought about reversing them but then that would make the shield "hot" so to speak..and since these 2 pickups cables run right next to each other.....they would short out as the sheilds would be opposite. That wouldnt work would it?
     

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