Intermittent humbucker problem - one coil w/ correct DCR but no inductance reading?

[jamesmafyew]

I've got an older Gibson humbucker (from a 1985 Explorer, a black/black Dirty Fingers w/ the 12 filister screws) that has been doing a weird intermittent thing recently. It worked fine for many years before now; I even toured with it a bunch. One of the coils will randomly make no sound. I have a coil tap switch on the guitar it's installed in, which selects either coil or both in series (convenient for testing). I figured the switch was bad, so I replaced it, but the problem continued. The pots are 100% good, wiring is all correct and relatively new. Pulled the pickup's tape, leads coming off the coils are fine and not pinched or distressed.

When it acts up (which is most of the time now), the neck-side coil works, but there is no sound from the bridge side coil. The weird thing is, the DC resistance of each coil is ALWAYS correct (8K each, 16k in series together) whether it's working or not. No shorts to ground, and neither coil is open. But if I measure inductance when it's acting up, I get ~3.2H on the meter (DE-5000) for the neck-side coil, but 24mH for the dead coil. If I measure inductance with the two coils in series, it's still 3.2H, compared to ~6.5H like it should be.

Any ideas what's happening here? This problem comes and goes suddenly, doesn't seem to be related to physical abuse (I can't get it to come and go by banging on the guitar or pickup). Shorted turn(s)? Moisture? It has been really swampy around here this summer.

[Jim Darr]

Check the wiring to the switch...if that checks out okay and the problem still exists, then try jumping the switch.

If still no resolution, I'd "re-flow " the connections on the bad coil and/or replace the leads on the bad coil.

If all else fails it could the be coil itself, but doubtful based on your description.

My take, its the lead wire or connection to the coil wire.

Let us know.

[jamesmafyew]

That's the weird thing, I've already bypassed the switch to make sure it isn't that. If there were a wiring/solder issue, I would expect continuity/DCR to change, which it hasn't (even when there's no sound from the 'bad' coil, it still measures 8K). Only the measured inductance and output change. That would normally point to a degaussed magnet or similar (this pickup has three) but the problem comes and goes suddenly.

[big_teee]

Sounds like a coil tap switch issue, or the neck coil connection.
Here's some rewind data on a dirty fingers, you may want to review.
Posts 123, 124.
http://music-electronics-forum.com/s...ad.php?t=39746
GL,
T

[Helmholtz]

To me it sounds like an intermittent short of several layers in the bridge side coil. This won't change the DCR much but strongly reduce the inductance due to coupling.

[jamesmafyew]

Terry, you actually re-wound one of my other DFs, which I believe is what that post refers to. That one is doing fine, by the way. This one might be headed your way soon...

I should clarify that this behavior is happening with the pickup OUT of the guitar. So it's not the guitar wiring or the switch. I just originally noticed it because of the coil tap switch.

[Jim Darr]

...That would normally point to a degaussed magnet or similar (this pickup has three) but the problem comes and goes suddenly.

If it were a degaussed magnet(s) the symptoms would be consistent, not "coming and going" as you describe. Likely the coil based on your latest response.

[the great waldo]

Hi James
It could be a break in the coil and temperature expansion makes or breaks the coil. Try warming or cooling the pickup to see if its a thermal problem.

Cheers

Andrew

[Lollar Jason]

Having made 10's of thousands of pickups and testing each one may times during the manufacture on rare occasion youll find one where the resistance is what you expect but the inductance is WAY off, invariably its a bad solder joint from the coil to a pigtail or eyelet. Depending on your soldering you may find more or less defects