Testing phase - The Screwdriver Pull-off Test

[JohnH]

If there is more than one pickup on a guitar we often need to be able to check if the pickups are in phase or out of phase. The phase of a pickup is dependent on the coil winding direction, the order of wire connections and the magnetic polarity. Changing any of these reverses the phase, which can lead to a thin, weak sound in combination with other pickups. You can often hear this difference, but is is sometimes difficult to be sure, particularly when different types of pickups are combined.

The relative phase of two pickups, or of two coils can be definitively tested using the 'Screwdriver Pull-off' test. The required equipment is, a screwdriver, and something to test the output. This can be an analogue multimeter, or more common nowadays, a PC with a sound card.

The Screwdriver Pull-off’ test, using an analogue meter

This is the traditional method, and requires a meter with a needle. The method is as follows:

• Plug a normal jack cord into the guitar, and turn all controls to max.
  
• Connect the meter across the cord, using the most sensitive DC voltage setting (maybe 50mV)
  
• Select a pickup setting that you want to test
  
• Lay the tip of a screwdriver flat on each pickup pole, then lift it up sharply away from the pickup,
  

The meter needle will jump. It may jump up or it may jump down, that does not matter, but if all pickups in that combo are in phase, then the needle will jump the same way when each is tested. If the needle jumps in different directions, then they are out of phase.

Update: Feb 2013: Using a digital meter

Although an analogue meter is clearest, they are not common now and digital meters are cheap. Using the same technique with e digital meter set to a low dc volts setting, the numbers will jump around wildly and be hard to read. But, the '-' sign at the left of the display may flash briefly, or not, depending on phase.

This works with my simple yellow $10 meter and is worth a try. Just swap the leads and test both ways to see if your meter will work in this way, with the '-' appearing one way and not th eother.

If the display gets too jumbled and you can't see the '-' symbol clearly enough in time, then a pc can be used as below:

Using a PC Instead of a Meter

Instead of a meter, it is possible to use a PC instead to sense the pulse.

• Plug the guitar directly into the PC line-in, a standard (eg sound-blaster) card is fine.
  
• The pull-off test is the same as above
  
• Record the signal into a suitable audio program, eg Audacity, which works well and is free
  
• The signal trace, recorded and visible on the screen will initially jump up or down when lifting the screwdriver tip
  

I did this test using two pickups. The top trace has both pickups in phase, while for the lower one, I operated a phase reverse switch on one pickup. You can see clearly how they are out of phase in the 2nd trace, because one signal jumps up and the other down (as would be observed on a meter):



Easy!

John




(Edited to modify a few puncuation marks. On certain browsers, they would display incorrectly.)

[genmce]

Thanks a million on this one, I just confirmed my middle is out of phase with my bridge.
Opps, time to heat up the soldering iron.

[treguiers]

placing the screwdriver on the pup, also causes a blip in the graph.....I would assume you place the screwdriver on the pup with the vol off?

[JohnH]

Dec 11, 2011 19:24:34 GMT -5 treguiers said:

placing the screwdriver on the pup, also causes a blip in the graph.....I would assume you place the screwdriver on the pup with the vol off?

The test is only the part when the screwdriver tip has already been placed carefully on the pickup, and you then lift it up, with one movement, one direction, and it makes a blip up or down. There may have been another blip when you first placed the tip on the pup, but just ignore it, or do it slowly and it will be small.

J

[sumgai]

tres' question points out a need for clarity.

In the above tests, you don't plunk down the screwdriver and then yank it back off the pup right away. Leave it there for several moments, enough time for the meter's needle to return to center (the starting point), or for the PC's display to settle back down to pretty much flat.... then you remove the screwdriver, in a manner most succinctly described as: with authority!

FWIW, you usually get the cleanest results by pulling the tool straight up from the pup's pole piece(s). Dragging across the piece seems to cause noise before finally lifting off, and that can skew your results.

HTH



sumgai

[reTrEaD]

Apr 26, 2010 3:37:16 GMT -5 JohnH said:

• Lay the tip of a screwdriver flat on each pickup pole, then lift it up sharply away from the pickup,
  

Flat is the keyword here. For best results, the screwdriver will have a flat side of the blade in contact with the face of one of the pickup's poles and the shaft of the screwdriver will extend over part of the coil. This will give you the largest impulse when the screwdriver blade is levered upward, away from the polepiece.

One could do this test with the blade moving down to make contact with the pole, but I wouldn't recommend it. The abrupt, violent collision of the screwdriver slamming into the polepiece is not pretty. Much better to lower the blade against the polepiece gently (well as gently as you can while the magnetic field tries to yank it downward), then no matter how much "authority" is used when pulling the blade upward, you won't have to worry about putting dents or scratches in the face of the pole.



Worth noting:

• Not all guitars follow the same convention.
  
• Most guitars are wired so that the output goes positive when the string moves closer to the pickup.
  
• When you lift the blade away from the polepiece, this simulates the string moving away from the pickup, so expect a negative pulse if your pickup follows the standard convention.

Whether or not your guitar is wired to follow the standard convention, the most important factor is that the results of the test are the same for all pickups. If all pickups give a negative pulse when the screwdriver is lifted, the pickups are in-phase with each other. If all pickups give a positive pulse when the screwdriver is lifted, the pickups are in phase with each other.


Sorry if the above was a bit wordy. Verbosity seems to be my strong suit at times.

[megi]

I have an LCD digital multimeter, and still find I can use the pull-off test - I can clearly see the reading momentarily drop or rise after lifting the screwdriver. I have been laying the length of the screwdriver shaft along all the poles of one coil, rather than the blade over one coil. I don't know if that helps to give a bigger signal (maybe helps with a digital display) - but just thought I should report my findings, cheers guys.

[reTrEaD]

Dec 24, 2011 4:10:38 GMT -5 megi said:

I have an LCD digital multimeter, and still find I can use the pull-off test - I can clearly see the reading momentarily drop or rise after lifting the screwdriver.

Success with a digital meter varies. If the display has a fast enough response time and you are careful about interpreting the readings, you could get good results.

But it is possible to misinterpret the results, so use with caution!

[JohnH]

I just updated the first post to describe more on using a digital meter for this. It seems to work with my cheap one, looking at whether or not the '-' symbol appears when the display jumps.

John

[Deleted]

Cool man, this has helped a great deal of ppl.

[dannyhill]

I don't know about you guys but I can hear a difference if tapped carefully for a pup forward and reverse wired.

[sumgai]

Danny,

Thanks, you brought a genuine smile to my lips this morning. ;D

Now, when you can snatch this pebble from my hand, grasshopper, only then it will be time for you to leave.

Sincerely,




sumgai

[eman]

I tried the test but the signal blip was both up and down like a typical signal.  not up or down.  using Mixcraft 7 recording software.

[reTrEaD]

Time is of the essence.

I tried the test but the signal blip was both up and down

It can't be up and down at the same time.
Which comes first, as the screwdriver is quickly lifted up from the polepiece?

[ashcatlt]

Dec 12, 2011 12:11:22 GMT -5 reTrEaD said:

...you won't have to worry about putting dents or scratches in the face of the pole...

Sorry, but this just made me LOL this morning. "What kind of anal retentive whackjob..."



eman - Pics or it didn't happen!

[newey]

What kind of anal retentive whackjob..."

I worry more about the anal expulsive whackjobs. . .

[sumgai]

^^^^ I simply cannot go there, it would result in my immediate expulsion!

But if know my upside-down derailers, I'll bet that g-f-b could give this a fair go.....

[angelodp]

Having just completed this test on a few pickups, is it correct to say that with a humbucker that has four leads, you can determine the start and finish of the bobbins. Or you could nominate them plus and minus. So any unknown pickup could be reverse calculated. Any bobbin that jumps positive would have the plus lead on the red meter lead and the other lead wire that is on the black meter lead is negative. Make sense?

[JohnH]

Dec 8, 2016 21:31:55 GMT -5 angelodp said:

Having just completed this test on a few pickups, is it correct to say that with a humbucker that has four leads, you can determine the start and finish of the bobbins. Or you could nominate them plus and minus. So any unknown pickup could be reverse calculated. Any bobbin that jumps positive would have the plus lead on the red meter lead and the other lead wire that is on the black meter lead is negative. Make sense?

I'd say that we usually need to check all pickups even if only one is unknown. We can completely sort out any set of unknown humbuckers and singles, to an extent that we can successfully wire them into a guitar, using three types of tests:

1. Resistance tests to check which leads form pairs to connect to coils, and also whether or not any wires are connected internally to baseplates.

2. Relative magnetic polarities of each coil, using a compass or other magnet. We need to assign 'north' and 'south' to each. It doesn't matter if we get all this reversed so long as all coils are assigned consistently. Fundamentally, no matter how you wire them, in-phase humcancelling pairs must always have opposite polarity.

3. The Screwdriver pull-off test.

We cant acuallly pick 'start' and 'finish', which is a hidden physical feature that we cant see, but can assign '+' and '-' to each coil wire across a pickup set. Again, it is often ok if we reverse this so long as it is consistent across the set. Sometimes there is a clue such as a coil wire which is also the shield or braid.

We usually need to test the full set of pickups even if only one is unknown, to get all the relativities right. We can deal with a totally unknown set, but if there are some known pickups, we can use their makers data, or other features like coils connected to baseplates, to make the other assignments.

[angelodp]

Thank you John, very concise.

A

[newey]

This link has a tutorial on ascertaining the color codes for an unknown HB. Add in the magnet test for N or S and you have all the info you'd need (Also note what JohnH said, that N and S are relative only, as mfrs differ on how they assign it)

www.projectguitar.com/tutorials/electronics/how-to-determine-the-color-code-for-a-humbucker-r35/

(And, thanks to RT for resurrecting the link in the other thread here.)

[reTrEaD]

thanks to RT for resurrecting the link

Not sure how useful it really is. It does have pictures, but I think John's verbal explanation a couple of posts back was far more useful.

[gumbo]

I'm waiting for someone to propose floating the pickup on a piece of styrene foam in a bowl of water and waiting to see how it aligns with North & South...

...not me, you understand....

[reTrEaD]

If you have big enough tires, you don't need styrene foam.

[gumbo]

Love it!

You win!!!

Welcome to The Wonderful World of Derailing.. ...your membership card is in the mail. (Unless of course, the Postal Authorities in Washington State confiscate it en route)

[gumbo]

Just to get this back on track (so someone else can derail it again...)

...how about this little gizmo


www.amazon.com/gp/product/B00OQRG0HA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

"CMS Magnetics Magnetic Pole Detector"  

[ashcatlt]

Jan 12, 2017 8:02:04 GMT -5 gumbo said:

...how about this little gizmo

If you ever really need to know the absolute magnetic polarity of anything, all you need is another magnet whose absolute magnetic polarity is known. This thing does that, I suppose, but it looks kind of silly.

There's no real good reason we ever need to know the absolute polarity of our pickups. Even if you're one of those guys with piles of pickups they swap in and out and around all the time, you just pick one as a reference. Relative polarity is all that matters.

[sumgai]

Ya know, ash, you're absolutely correct - relativity is the name of the game, absolute certainty as to how a magnet compares to the planet is a non-starter.

But look at the bright side - gumbo once again fooled us all by going against the grain.  Not only did he not derail this thread (for a second time), but actually trotted out an on-topic idea/device.

Where's my diary, I need to make a note of this!

[gumbo]

" Relative polarity is all that matters."

...right...
...I've always thought that all my relatives were VERY polarised..


..except maybe one uncle who..       ...never mind.

[thedoc735]

Apr 26, 2010 3:37:16 GMT -5 JohnH said:

If there is more than one pickup on a guitar we often need to be able to check if the pickups are in phase or out of phase. The phase of a pickup is dependent on the coil winding direction, the order of wire connections and the magnetic polarity. Changing any of these reverses the phase, which can lead to a thin, weak sound in combination with other pickups. You can often hear this difference, but is is sometimes difficult to be sure, particularly when different types of pickups are combined.

The relative phase of two pickups, or of two coils can be definitively tested using the 'Screwdriver Pull-off' test. The required equipment is, a screwdriver, and something to test the output. This can be an analogue multimeter, or more common nowadays, a PC with a sound card.

The Screwdriver Pull-off’ test, using an analogue meter

This is the traditional method, and requires a meter with a needle. The method is as follows:

• Plug a normal jack cord into the guitar, and turn all controls to max.
  
• Connect the meter across the cord, using the most sensitive DC voltage setting (maybe 50mV)
  
• Select a pickup setting that you want to test
  
• Lay the tip of a screwdriver flat on each pickup pole, then lift it up sharply away from the pickup,
  

The meter needle will jump. It may jump up or it may jump down, that does not matter, but if all pickups in that combo are in phase, then the needle will jump the same way when each is tested. If the needle jumps in different directions, then they are out of phase.

Update: Feb 2013: Using a digital meter

Although an analogue meter is clearest, they are not common now and digital meters are cheap. Using the same technique with e digital meter set to a low dc volts setting, the numbers will jump around wildly and be hard to read. But, the '-' sign at the left of the display may flash briefly, or not, depending on phase.

This works with my simple yellow $10 meter and is worth a try. Just swap the leads and test both ways to see if your meter will work in this way, with the '-' appearing one way and not th eother.

If the display gets too jumbled and you can't see the '-' symbol clearly enough in time, then a pc can be used as below:

Using a PC Instead of a Meter

Instead of a meter, it is possible to use a PC instead to sense the pulse.

• Plug the guitar directly into the PC line-in, a standard (eg sound-blaster) card is fine.
  
• The pull-off test is the same as above
  
• Record the signal into a suitable audio program, eg Audacity, which works well and is free
  
• The signal trace, recorded and visible on the screen will initially jump up or down when lifting the screwdriver tip
  

I did this test using two pickups. The top trace has both pickups in phase, while for the lower one, I operated a phase reverse switch on one pickup. You can see clearly how they are out of phase in the 2nd trace, because one signal jumps up and the other down (as would be observed on a meter):



Easy!

John




(Edited to modify a few puncuation marks. On certain browsers, they would display incorrectly.)

this is really confusing, not sure what I am testing or the result I am looking for? e.g. am I testing the wires, the coils, the pickups, positive, negative, winding direction, phase and/or out of phase etc?

My G&B 85/15 B/T NU-Z pickups have a reg/green and white/black pairings, the red/green are connected to the screws, and the black/white are connected to the slugs; but I don't know which wires are plus, and which are minus? Also I don't know which coils are positive and which are negative OR whether they are forward wound or reverse wound?