Opening a DeArmond = Courting Disaster?

[Joe Gwinn]

In “playing around” with different bread-boarded circuit configurations, I managed to hook up a signal generator directly across the pickup- with no current-limiting resistor. The DCR now reads infinity.

I doubt that the signal generator had anything to do with it, unless it's far stronger than common. With a Rdc of 11 Kohms, if the generator was supplying 100 volts rms, the current would be only 9 milliamps, well within the ability of #45 copper wire. MWS Wire Industries - Frequently Asked Questions - FAQs

I would look for a mechanical problem, probably involving the leads.

[StarryNight]

good timing. I'm expecting a defunct model 210 in the mail soon. It's nice to see some photos of other models.
cheers!

[David Schwab]

I courted Disaster once, but I had a date with Destiny.

(sorry, couldn't resist)

[rjb]

Hmm.
I hadn't done the math.
Looks like AWG 45 *should* be able to handle 15mA. The audio generator (a battery powered BK 3001) puts out about 1.6V full scale.
1.6V divided by 11Kohm equals... a lot less than 15mA.

But here's the thing:
Before posting, I did check for obvious mechanical problems.
I have continuity between the "distal" coax shield and the backplate ground pin.
As well as I can see through the wax, the coil's start appears to still be connected to the ground pin.
Continuity between the distal coax center conductor and the copper tape inside the pickup- check.
Continuity between ends of the copper tape-check.
Carefully lift the copper tape- see the coil's finish still connected, with no obvious break in the wire.

I see an open between the coax conductor and shield whether the pickup cover is on or off- so its not that cramming the cover on is forcing something apart....

Its a mystery to me.
I guess the finish wire could be cracked, but still held together by the insulation. So, how do I check for a break in the wire, without breaking the wire?
I can look again, but I'm not sure what else to look for.

Going back to the "oh no, the sky is falling and the coil is vaporized" scenario:
What about L di/dt? A voltage spike caused by abruptly cutting the signal, or a clip lead falling off?

I'd like this problem to be one that's easily found and fixed.
But just in case it isn't, let me repeat my original question:
If I rewind the pickup with AWG 42, will it sound like sh*t?

Thanks,
-rb

[rjb]

...It's nice to see some photos of other models.

Now I can pretend I know something, and say something like "Oh yea, the 210- isn't it cool the way they did those adjustable pole pieces?"

[rjb]

I courted Disaster once, but I had a date with Destiny.

(sorry, couldn't resist)

Always with the wisecracks. Couldn't you say something constructive every once in a while?

(sorry, couldn't resist)

[jonson]

Dearmonds caried different wire and the k was all over the shop. running from low 3s up to 14.5k and to me they all sound good. There is enough room on that bobbin to get 3.5 to4k with 42 and allthough lower output will still sound good.

[rjb]

What about L di/dt? A voltage spike caused by abruptly cutting the signal, or a clip lead falling off?

This may be water under the bridge- but it could be something for me to be careful about in the future. A clip lead did, in fact, fall off the pickup while signal was applied. Could the inductive "kick" from suddenly "opening the switch" have cooked the coil? I bet that bbsailor guy would know....

Thanks,
-rb

[marku52]

"Could the inductive "kick" from suddenly "opening the switch" have cooked the coil?"

Strikes me as unlikely. I did do this once with a hi power race car ignition that I ran without a spark plug load. without the spark plug arc voltage (about 12kv) to limit the output V, the coil output went up to it's full value determined by the input voltage (12V) and the turns ratio, about 60kV. that was enough to cause a fatal insulation breakdown in the ignition coil.

In this case, the input voltage is much lower, and the breaking of the circuit was probably not very clean (think switch debounce), I just don't think this would open the coil. We are dealing with V= Ldi/dt, so I suppose some high voltages could be created, but at very low currents. I just think that a short is more likely than an open.

[Mike Sulzer]

"Could the inductive "kick" from suddenly "opening the switch" have cooked the coil?"

Strikes me as unlikely. I did do this once with a hi power race car ignition that I ran without a spark plug load. without the spark plug arc voltage (about 12kv) to limit the output V, the coil output went up to it's full value determined by the input voltage (12V) and the turns ratio, about 60kV. that was enough to cause a fatal insulation breakdown in the ignition coil.

In this case, the input voltage is much lower, and the breaking of the circuit was probably not very clean (think switch debounce), I just don't think this would open the coil. We are dealing with V= Ldi/dt, so I suppose some high voltages could be created, but at very low currents. I just think that a short is more likely than an open.

Certainly you cannot blow a pickup with the inductive kick from breaking the connection to a signal generator; there is not nearly enough energy stored in the coil. But you are underestimating the voltage across the primary of an ignition coil when you break the connection to the battery. It can be much higher than the battery voltage because the inductance wants to keep the current flowing. Remember, the voltage across an inductor depends on the rate of change of the current, as you wrote above. Thus, if the current is suddenly interrupted, the voltage becomes very high, and if there is sufficient energy in the field you can destroy the coil if that energy does not go into the spark plug gap.

[bbsailor]

This may be water under the bridge- but it could be something for me to be careful about in the future. A clip lead did, in fact, fall off the pickup while signal was applied. Could the inductive "kick" from suddenly "opening the switch" have cooked the coil? I bet that bbsailor guy would know....

Thanks,
-rb

rb,

In order to blow the coil you would have to exceed the breakdown voltage of the wire insulation. That is not going to happen with just an alligator clip unless that alligator clip was attached to a high voltage that was used to put a high DC voltage across the guitar pickup coil.

In pulse induction metal detectors, a coil of 10.5" in diameter with 20 turns of AWG 18 wire being about 300 microhenries with a 12V voltage being switched across a 0.5 ohm coil can generate a brief (few microsecond) pulse of up to 1000V depending on how fast the turn off current can be reduced to zero. All of this depends on how much capacitance is in the coil, shield, coax cable and the MOSFET driving the coil. Less capacitance and the damping resistor can be a higher value and the flyback voltage can be damped quicker, all based on pulse time constants.

I doubt that any guitar pickup is going to generate this kind of voltage unless intentionally fed a high DC voltage to generate an inductive kick.


Joseph Rogowski

[rjb]

Thanks

Thanks, everyone (except David ), for your support and technical expertise.

Despite all the evidence and logical reasoning that the coil could not possibly be open, the pickup was unconvinced. I peeled the coil, and found a break about one layer down. Continuity is restored.

Still not sure how it happened- maybe got crushed by the fatter-than original coax jammed into the cover? EMP from a nuclear explosion? I'm trying to figure a way to blame the cats, but haven't come up with one yet.

Thanks again,
-rb