Shorted turn detector

[R.G.]

It occurred to me that the same trick for finding a shorted turn in a transformer will find shorted turns in a pickup.

I've read through the archives and seen the ACR versus DCR comparison for shorts. This is an alternate method.

Obtain a neon bulb, ideally an NE-2. Hook up the bulb, a series resistor of some size, maybe 47K to 100K, and a battery. Hook the bulb in series with the resistor and place this in parallel with the pickup. Connect the battery across the pickup and then remove it.

The light will flash if the pickup is not shorted. It will not flash if there is an internal short. The pickup acts like an inductor, ramping up a DC current. When you open the battery, the inductive kickback will flash over the bulb when the voltage hits 80V or so. This should not punch through intact insulation, especially since the voltage between adjacent turns will only be a fraction of the generated voltage.

The inductive loading and kickback will either reinforce or oppose your magnets. I don't know whether this is significant on Alnico or not.

Some thoughts:
1. Do this before magnetizing the magnets, or re-up them afterwards
2. Do this with soft iron/steel slugs in the magnet holes
3. Measure gauss before and after and decide if it's worth worrying about.

It's a quick test, probably better suited to youse guys who make a lot of pickups.

[David Schwab]

That's very cool. What size battery is needed?

[R.G.]

Probably a 9V will work. Neons fire on very small energies. The neon discharges the energy in the inductor (E = 1/2 L*I^2) at its firing voltage of about 80V, so the less energy in the coil, the shorter the flash is. The more resistance in series with the neon, the lower the current. You just want to guard against there being a flash that's too short or too dim to see.

Give it a try - I don't have any pickups I can get to very easily. NE-2s are cheap if you can find them at all.

[David Schwab]

I probably have one of those neon bulb outlet testers around in my gig bag. I used to use them to test the ground between my bass' strings and the microphone. If it lit up there was a shock hazard!

[tbryanh]

I think the neon bulb could still flash if some of the turns are shorted. It probably depends on the number of turns that are shorted. The more turns that are shorted, the less likely the bulb will flash.

I don't think this method can be used as a reliable go-no-go test device.

[R.G.]

I think the neon bulb could still flash if some of the turns are shorted. It probably depends on the number of turns that are shorted. The more turns that are shorted, the less likely the bulb will flash.
I don't think this method can be used as a reliable go-no-go test device.

Your opinion is noted.

However, it works on all transformers of whatever ratio I've ever tested it on, including some with very large number of turns. In fact, it works if you snake ONE turn of copper wire through the transformer and short it.

I suggest that you wrap one turn of copper wire tightly around a pickup, short the ends and try it.

If I had a loose pickup I'd try it, and that will happen soon enough anyway. So instead of hypothesizing it won't work, give it try.

[Possum]

cool maybe...

Well I'd like to see someone try this out who has the time to mess with it, sounds interesting.

[R.G.]

Me, too. That's why I posted it.

It works great on transformers. I've had grizzled amp techs flatly tell me there was no way you could test a transformer for internal shorts this way, then have to pay up on the bets.

The only reason I'm not more dogmatic is that I'm a beginner at pickups. It is possible that there is a condition where high coil resistance could cause a single-turn coil which is not as tightly magnetically coupled to the rest of the coil to let it flash a neon. From all my experience I don't think it's likely, but I'll admit that it might be rarely possible.

What I'd really like is for someone who winds or repairs to test a known-good and a known-shorted pickup and verify.

[Steve Conner]

I've got a loose pickup and will try it and report back

[tbryanh]

So what it the theory of operation?

Does a shorted turn convert the pickup into a transformer?

If so, then is the secondary of the transformer (the shorted turn) heavily loaded (zero ohms) and draws all the energy out of the coil?

[R.G.]

Any coil of wire is an inductor. If you apply a load to one or a subset of the turns, that section constitutes a "secondary" winding. The rest of the coil is in effect a "primary".

All coils linked by a magnetic field are a transformer. All that changes is how you connect up or disconnect the coils.

The shorted turn is indeed a heavy load and clamps the volts/turn on the magnetic core to a low voltage. In most transformers, this is too low to fire the neon bulb.

The only uncertainty I have is whether there are so many turns on a typical transformer that even the clamped voltage times several thousand will be more than a neon's threshold.

[tbryanh]

It can also depend on how well the turns are shorted.

*Heavy currents typically flow through power transformers, and when the turns short, there is probably enough current to weld them together to make a good connection.

This is probably not the case with pickups.

Cases where insulation is missing along lengths of the wire during winding will probably show up, the turns are definately shorted together well.

*But cases where the insulation is thin or has deteriorated over time might not show up. Tiny currents flow through pickups, so if the insulation starts failing, there probably will not be any heavy currents to weld the short together. This might cause a resistive short rather than a dead short.

If the bobbin was metal, a megger might help to find some cases of shorted turns, but probably not most cases. A megger does not appear to be useful here.


*I am not sure if these statements are correct or not.

[tbryanh]

The only uncertainty I have is whether there are so many turns on a typical transformer that even the clamped voltage times several thousand will be more than a neon's threshold.

Yeah, this is probably the issue.

[Steve Conner]

OK, well I tried more or less what R.G. suggested. I took a cheap potted humbucker that I removed from my old guitar, connected a neon bulb across it, (leaving out the 100k resistor because I reckoned the pickup DCR would be enough) and tried prodding it with a 9V battery.

Nothing happened.

So I decided to use some brute force: I put a 2-turn coil of heavy wire on the poleface of the pickup, and touched this on the terminals of a 7 amp bench power supply set to 12V. (Which I happened to know had a 1000uF capacitor on the terminals internally, to give it even more wallop.)

That made the neon flash all right! Unfortunately, when I introduced a simulated "shorted turn", the bulb would still flash anyway.

I guess this makes sense: look at pickups with nickel silver covers. Isn't that cover just one big shorted turn? And yet they still work with just some damping of the resonant peak.

I wouldn't recommend trying this experiment with any pickups that have value: it might screw up the magnets. (I could feel them vibrating when I did it.)

[Joe Gwinn]

OK, well I tried more or less what R.G. suggested. I took a cheap potted humbucker that I removed from my old guitar, connected a neon bulb across it, (leaving out the 100k resistor because I reckoned the pickup DCR would be enough) and tried prodding it with a 9V battery.

Nothing happened.

This was my suspicion when I read the original post, as this is pretty crude. (The 100K is not required.)

To be sure the pulse isn't going through the battery first, I would alter the circuit to include a 1N4007 rectifier diode in series with the battery, forcing any inductive kick to go through the neon tube. The specific circuit is put the neon tube across the coil. Connect the battery negative to one end of the coil. Connect one end of a normally-open pushbutton switch to the plus end of the battery. Between the other end and the coil terminal not used for the negative connect a 1N4007 diode oriented so current flows when the pushbutton is depressed.

So I decided to use some brute force: I put a 2-turn coil of heavy wire on the poleface of the pickup, and touched this on the terminals of a 7 amp bench power supply set to 12V. (Which I happened to know had a 1000uF capacitor on the terminals internally, to give it even more wallop.)

That made the neon flash all right! Unfortunately, when I introduced a simulated "shorted turn", the bulb would still flash anyway.

Now we are driving the two-turn coil, not the pickup coil. I missed that the first time I read this. If you vary the drive voltage, you may be able to find a value that causes the neon lamp to flash only for well-shorted pickups.

I guess this makes sense: look at pickups with nickel silver covers. Isn't that cover just one big shorted turn? And yet they still work with just some damping of the resonant peak.

A cover is a shorted turn for sure, but nickel silver makes for a high-resistance turn and reduced effect. So, we again see the need to in some sense measure the amount of shorting.

I wouldn't recommend trying this experiment with any pickups that have value: it might screw up the magnets. (I could feel them vibrating when I did it.)

The magnet wire insulation is rated for a few hundred volts, and so one could puncture the insulation if one does this test without the neon tube, which breaks over at ~80 volts, protecting the insulation.