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Thread: P90 woes

  1. #1
    Les Paul Forum Member
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    P90 woes

    Hello All, I have just finished restoring a very abused and neglected 68 small headstock gold top which came to me missing all hardware bar the two pups. Both had an average resistance of between 8 + 9 k when I checked them. i have re wired it using standard braided wire, CTS 500k TVT Lin pots for the volume and Bourne 500k log pots for tone, switch craft switch and jack. The problem is that i cant get any sound from the bridge P90 in the treble sw position. both vol pots work in the centre position and there is a difference in sound from rhythm to centre! i have checked and re checked my wiring (50's) checked switch functionality. Neck pickup works as it should, I am at a loss as to what’s going on! any suggestions great fully received

  2. #2
    Les Paul Forum Member J.D.'s Avatar
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    Re: P90 woes

    Did you verify the magnet pole orientation?

  3. #3
    Les Paul Forum Member MapleFlame's Avatar
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    Re: P90 woes

    You need to flip the magnets, that will bring it back to life.
    Top carves are sexy

  4. #4
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by MapleFlame View Post
    You need to flip the magnets, that will bring it back to life.
    Would that make a difference with only that pickup selected? I could be wrong but I thought that would just make it out of phase and things sound thin when both pickups were selected ? Or are P90ís different?

  5. #5
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by Tosh View Post
    Would that make a difference with only that pickup selected? I could be wrong but I thought that would just make it out of phase and things sound thin when both pickups were selected ? Or are P90’s different?
    You are correct. If the problem occurs when only one pickup is selected then it is likely to be a wiring fault and nothing to do with magnet orientation. One common fault worth looking for is where the braided wire solders to the back of the pots, it's quite easy for the heat from soldering to melt the insulation on the inner conductor and cause a short. This can be very difficult to see visually but if you have a multimeter checking for shorts between the back of the pot and the lug that the conductor connects to should show if this is the problem. (Set the pots to 10 when checking this, if they are on 0 you will get a false reading)

  6. #6
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by PaulD View Post
    You are correct. If the problem occurs when only one pickup is selected then it is likely to be a wiring fault and nothing to do with magnet orientation. One common fault worth looking for is where the braided wire solders to the back of the pots, it's quite easy for the heat from soldering to melt the insulation on the inner conductor and cause a short. This can be very difficult to see visually but if you have a multimeter checking for shorts between the back of the pot and the lug that the conductor connects to should show if this is the problem. (Set the pots to 10 when checking this, if they are on 0 you will get a false reading)
    Thank you I will have a look at that when I get back, itís the cloth inner braided outer cable Iíve used so I guess it could also be feasible that a strand or two of the outer may have punctured the cloth?

  7. #7
    Les Paul Forum Member
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    Re: P90 woes

    Another quick check you could do is measure the DC resistance at the output jack (simplest way is to plug in a known good lead and connect the multimeter probes to the two poles of the jack plug). With the volume controls on 10 check the resistance in all 3 switch positions, it should read something close to pickup resistance in the neck and bridge positions (8 - 9K), and around half of that value in the middle position. If not that would indicate a wiring fault or a faulty switch.

  8. #8

    Re: P90 woes

    Look at the switch, too.

  9. #9
    Les Paul Forum Member musekatcher's Avatar
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    Re: P90 woes

    Quote Originally Posted by mdubya View Post
    Look at the switch, too.
    I've had switches go bad, that look ok visually. I've also had switches that worked intermittently, really confusing trouble shooting. I've also cooked pots with too much soldier heat. And I've had some good looking soldier connections prove to be "cold" or open. Careful removal of each component from the circuit, and of each connection, one at a time, should bracket in on the culprit. Good luck -

  10. #10
    Les Paul Forum Member corpse's Avatar
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    Re: P90 woes

    Those tiny ends of the braid shield can be a problem as well. Check those that they are all "captured" where soldered.

  11. #11
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by PaulD View Post
    Another quick check you could do is measure the DC resistance at the output jack (simplest way is to plug in a known good lead and connect the multimeter probes to the two poles of the jack plug). With the volume controls on 10 check the resistance in all 3 switch positions, it should read something close to pickup resistance in the neck and bridge positions (8 - 9K), and around half of that value in the middle position. If not that would indicate a wiring fault or a faulty switch.
    Hi, I have now checked the switch which is working as it should, I have checked for shorts between the pot lug and corresponding braid, all seems ok, when checking the DC resistance at the jack using a know good lead I’m getting neck pu close to where it should be, half values on centre but nothing on bridge. I have also checked the pu resistance at the pots. Ryt position gives full neck pu resistance with nothing at the bridge pot as you would expect, half values on each pu in the central position but when in the treble position I’m getting half values on BOTH pup’s?! Confused is an understatement just found that in the treble position I seem to have continuity between the two pup hots
    Last edited by Tosh; 01-05-20 at 06:46 AM. Reason: Typo

  12. #12
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by Tosh View Post
    Hi, I have now checked the switch which is working as it should, I have checked for shorts between the pot lug and corresponding braid, all seems ok, when checking the DC resistance at the jack using a know good lead Iím getting neck pu close to where it should be, half values on centre but nothing on bridge. I have also checked the pu resistance at the pots. Ryt position gives full neck pu resistance with nothing at the bridge pot as you would expect, half values on each pu in the central position but when in the treble position Iím getting half values on BOTH pupís?! Confused is an understatement just found that in the treble position I seem to have continuity between the two pup hots
    This all seems to suggest a fault with the switch or the switch wiring so that is probably the area to concentrate on.

  13. #13
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by PaulD View Post
    This all seems to suggest a fault with the switch or the switch wiring so that is probably the area to concentrate on.
    Thanks everyone for the help, much appreciated.Right walk of shame coming up... whoís a silly fella then, neck pickup hot-wired to switch output and output jack hot wired to....yep youíve guessed it, neck pickup switch position ....DOOH!

  14. #14
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by Tosh View Post
    Thanks everyone for the help, much appreciated.Right walk of shame coming up... whoís a silly fella then, neck pickup hot-wired to switch output and output jack hot wired to....yep youíve guessed it, neck pickup switch position ....DOOH!
    LOL yes that would cause the problem Done similar things myself many times and spent a few frustrating hours scratching my head - it's sometimes easy to miss the glaringly obvious!

  15. #15
    Les Paul Forum Member
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    Re: P90 woes

    Quote Originally Posted by PaulD View Post
    LOL yes that would cause the problem Done similar things myself many times and spent a few frustrating hours scratching my head - it's sometimes easy to miss the glaringly obvious!
    Yea your not wrong there, spent hours chasing my tail when perhaps a little less self confidence and 2 minutes with the multimeter could have rectified it, still all working as it should now, and wow! my 2013 standard just doesnít come even close to this beast for sustain or tone!

  16. #16

    Re: P90 woes

    Good news it was something simple and easily fixed without any extreme measures needed!

    IME-it almost always is that way.

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