Easy P-90 hum cancelling mod

[ECDavis]

I just applied the modification as described by the OP to the bridge pickup on my 2012 LP Special, and it worked perfectly: no hum when the switch is in the center position and both pickups are "on."

 

On my pickups, there is a hole in the corner of one bobbin and an adjacent hole in the brass base plate through which the wires are routed. Consequently, flipping the bobbin required de-soldering the black and white lead wires that join the pickup windings to the shielded 2-conductor hookup wire. It also required a new hole to be created in the corner of the bobbin directly opposite the original hole. It also required flipping the base plate to align with the new bobbin hole. The base plate was bent slightly along the row of holes to hold the magnets in place, so the base plate needed to be bent a bit in the opposite direction. Finally, two new holes needed to be put in what is now the bottom bobbin to accept the base plate mounting screws. The bobbin material is easily drilled by twirling a No. 11 Exacto blade to locate the hole. This is sufficient for the mounting screws. For the hole in the corner of the bobbin, just use successively larger drill bits twisted by hand.

 

Because I needed to desolder anyway, I thought I'd experiment. I flipped the bobbin, flipped the magnets, and I also reversed the wires: originally, the white lead wire was connected to the braided metal shielding wire which was soldered to a grounding lug attached to the base plate with a screw, and the black lead wire was connected to interior signal wire covered with black push-back cloth. I just swapped the attachment points of the black and white lead wires.

 

No surprise, the hum was not cancelled in the middle position. By reversing the wires as well as the magnets, the rotation of the coil relative to the magnetic poles was unchanged. If you're familiar with the Right Hand Rule (fingers coiled like you're making a fist represent the coil, thumb pointing straigh up to represent North) it makes intuitive sense that all I did was rotate the whole pickup (i.e., the whole right hand) upside down.

 

Just for fun, I left the wires where they were and flipped the magnets back to their original position. As expected, this put the pickup out of phase in the middle position (reverse wound, matching polarity), which sounded really awful.

 

 

Lastly, I turned the magnets back around to the OPPOSITE of their original position. (BTW, both magnets were marked on one side with stripes of blue ink to represent the matching poles. The sides with the blue markings were not orignally visible because they were placed against the metal bar through which the pole piece screws are threaded.) I also resoldered the lead wires back to their ORIGINAL locations.

 

Success! The bridge pickup sounded no different by itself than it did originally, the bridge and neck pickups are in phase, and when the selector switch is in the middle position there is no hum at all. I never really found the center postion on my humbucker equipped guitars to be very interesting, but the middle positon with RWRP P90s does provide a very musical and useable sound - the hum cancelling is a just a bonus.

[mihcmac]

On 9/30/2010 at 7:32 AM, Brickster86 said:

Forgive me if this has been answered 1000 times, but I bought a '56 Goldtop with P-90's this summer. I love the guitar in every way, except that the 60 Hz hum is excessive (I am unlucky to have power lines close enough to my house that seem to get picked up quite well by the pups). Anyway, after searching for all sorts of fixes on the web, it seems that converting one of the pups to a reverse wired, reverse polarity was the answer. However, every post talked about swapping wires (i.e. cutting and/or resoldering) on the pickups. I was not interested in that so I figured there had to be a better way. There is, and it's easy. I did it last night, but unfortunately, I didn't take pictures. I think you'll be able to follow it anyway. It took me about 15 minutes (minues the re-do because I had the magnets wrong; see Tip).

 

1. Take off the strings by any method you want (I just loostened them and removed the tailpiece from the bolts). Remove the cover screws on the bridge pickup (doesn't matter which, but the bridge seemed to have a bit more wire slack. If not, open up the vol/tone cover and carefully snip a cable tie to free up the cables)

2. Carefully lift the pickup out of the hole. It should be waxed together, so nothing should fall out. Take a Sharpie and mark the outside edge of the two magents above the bottom plate. Also mark the top of the bobbin. This is very important as magnet polarity is key.

3. Unsrew the pole pieces (six screws on top) and the two screws holding the bottom plate to the pole plate (thin metal strip that sits between the magnets). Carefully take out the magnets. Since everything is waxed together, gently pry apart with a small screwdriver.

4. Flip the bobbin over and arrange the plate and pole plate between the bobbin bottom (used to be the top) and the plate. Put the magnets back in between the bobbin and the bottom plate, making sure that the edge that you marked is now on the inside against the pole plate. If you have a compass handy, make sure that the pickups now have different magentic polarity (one should attract N, the other S as you hold the compass perpendicular to the pup face).

5. Re-attach all the screws and re-install the pup and cover. Congratulations, you now have humbucking P-90 axe when playing in the center position.

 

Tip: If the center position is hum-free, but sounds thin and nasaly, the magnets are not installed correctly.

 

Why does this work? Because the 60 Hz hum only interacts with the coils, so reversing one causes a phase inversion that cancels out the hum. The strings interact with the entire coil/magnet system, so reversing the magnets reverses the phase once again, resulting in a zero phase reversal, just like the original. Neat huh?

I have performed Bricksters mod a few times his instructions are very accurate, performed on my neck P90 and would work perfectly in either the neck or bridge position.. 👍

[mihcmac]

Just as a note, in a reverse coil pickup the actual polarity is reversed, the coil (bobbin) and magnets are reversed creating an opposite magnetic field. The wiring is the same, if changed it will take the pickup out of phase creating an undesirable effect.

If you place a compass next to your pups the normal pup will swing the south end of the needle closest to it, on either end of the pup. The reversed  pup will swing the north end of the needle to it, on either end..

If you take a P90 apart and remove the magnets, you will notice that the magnets are held together in opposing fields trying to push each other apart creating  a single magnetic field at both ends of the pup.

On a P90, if your compass swings to the same  pole on both pups, then they are in the same polarity.. Normal pups in the same polarity will normally swing the south end of the needle toward them..

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If anyone should try to reverse a Gibson P90, it is one of the most difficult because it is very substantially built. You will need a good soldering gun to desolder the cable shielding from the chassis so you can flip the coil, while being very careful not to damage the leads going to the coil. Also need to mark the magnets with something like nail polish so you know the original orientation.

Usually much easier to just acquire an aftermarket reverse polarity P90, most manufacturers are building them this way and in sets..

Epiphone P90's are easier to reverse with no desoldering required, just mark the magnets, disassemble requiring the pole screws to be removed, flip the coil and reverse the magnets in an opposite opposing position, check  the polarity with a compass and finish reassembling.

There are a few other posts in the Epiphone thread explaining in more detail.  

The following link will return you to the original article at the beginning of this post.....    Easy P-90 hum cancelling mod

Edited August 17, 2020 by mihcmac