Inconsistant Ohms Reading

[Brian W.]

All of my Strat and Telecaster pickups exhibit this trait....
When I tap the pickup with a screwdriver [or other such object] the ohm reading will jump around to different values [and then return to it's normal reading]. At first I thought this was due to a shorted coil...
I get the same results if I tap on the magnet or the pickup cover.

[Helmholtz]

Tapping the PU with a screwdriver generates a transient voltage which upsets the meter.
It shouldn't do it when tapping with something non-magnetic like plastic, wood, brass or aluminum.

[Dave H]

That's normal. Moving a screwdriver near the pickup has the same effect as a vibrating string. It produces a voltage at the output of the pickup which disturbs the resistance reading.

Edit: simulpost

[Leo_Gnardo]

Nothing unusual going on there. You're generating a voltage spike when you do that - of course it upsets the ohms reading.

[Enzo]

A similar experiment: clip your ohm meter to a speaker's terminals. Now thump the speaker cone with your fingers. Watch the ohm meter swing. Same principle.

[Mick Bailey]

It shouldn't do it when tapping with something non-magnetic like plastic, wood, brass or aluminum.

That depends on the pickup and the degree of microphony. If there can be any relative vibrational movement between the windings and magnet you'll get some disturbance to the meter reading.

[Helmholtz]

That depends on the pickup and the degree of microphony. If there can be any relative vibrational movement between the windings and magnet you'll get some disturbance to the meter reading.

In principle, yes. But I think a PU being microphonic enough to upset an ohmmeter would be rather unusable.

For comparison/reference: Snapping the low E string of a Strat while measuring PU DCR doesn't noticeably trouble my meter's reading, meaning that the screwdriver tapping produces considerably higher voltages than a string ever could.

[eschertron]

as an aside: tapping the pickup with a screwdriver is a way to test the electrical polarity of the pickup. That, and a magnet to test the magnetic polarity is used to identify and wire up unknown winds.

[Helmholtz]

as an aside: tapping the pickup with a screwdriver is a way to test the electrical polarity of the pickup.

THis is best done with a sensitive analog (needle) meter. Approaching the screwdriver produces an opposite sign voltage as opposed to removing it.

That, and a magnet to test the magnetic polarity is used to identify and wire up unknown winds.

Be careful doing this test. As soon as you feel a repelling force, the field is strong enough to partially demagnetize Alnico magnets. It takes a much stronger field to remagnetize a magnet than to demagnetize it.

[eschertron]

Very good points, Helmholtz, thanks for keeping me honest!

as an aside: tapping the pickup with a screwdriver is a way to test the electrical polarity of the pickup.

THis is best done with a sensitive analog (needle) meter. Approaching the screwdriver produces an opposite sign voltage as opposed to removing it.

The 'bump' test requires a ballistic meter movement to clearly see what's going on.

That, and a magnet to test the magnetic polarity is used to identify and wire up unknown winds.

Be careful doing this test. As soon as you feel a repelling force, the field is strong enough to partially demagnetize Alnico magnets. It takes a much stronger field to remagnetize a magnet than to demagnetize it.

I should have mentioned I use a small pupose-built magnet: pea-sized, painted N and S, inside a small plastic tube, so that it flips over to reveal the detected polarity. Stew-Mac sells these.

[Helmholtz]

I should have mentioned I use a small pupose-built magnet: pea-sized, painted N and S, inside a small plastic tube. Stew-Mac sells these.

I also have one of these. Seems to be safe: Small, low weight neo, easily flipping.

The 'bump' test requires a ballistic meter movement to see what's going on.

I typically use a 30ľA or 50ľA needle meter. The ballistic needle defection corresponds to the voltage-time area, which in turn is proportional to flux change caused by the screw driver.

[eschertron]

For anyone who cares: I installed a Gauss-meter app on my phone and went about checking Gauss levels on some pickups. All I can say is DON'T DO IT! I destroyed the detector in the phone, could never use a compass app or night sky app after that.


On my current phone I haven't the faintest desire to add a Gauss-meter app

[Brian W.]

Excellent info..Thanx to everybody!

The first time I noticed this was on a rewind that I just finished and I thought it was due to a short in the coil.

[Mick Bailey]

As an experiment, late last night I ran through my pickup collection - some brand-new, boxed, others used but good and not overtly microphionic. Every single one will momentarily show a resistance change when tapped with a piece of aluminium bar The actual reading depends on your meter acquisition. I have one that does min/max peak readings that are too fast to visually resolve but I didn't need to use that as the effects are clear.

[mozz]

Although using a analog meter will show polarity, i use my digital Fluke when phasing a pair of pickups, works fine. Touch the tip of a small screwdriver to a screw pole piece, pull screwdriver away while watching the meter, it will be either neg voltage or positive, wire accordingly.