Besides obvious abuse what causes a White's coil to go bad?

[ArizFlash]

A few months ago I sent my XLT and IDX in to Whites for a refurb along with the coils. Both of the 950 coils were bad as was the 5.3 Black Max and the service tech said the "null" reading was off. I can understand the 5.3 coil being bad because I used the heck out of it and probably banged it around somewhat. But I very seldom used the 950 coils on either detector. I know the coils were stored in my garage in AZ where I'm pretty sure the temps got up to 120º at times.

[Dave J.]

It's not just White's. The problem of how to maintain induction balance over product lifetime is basic to the design and manufacture of induction balance searchcoils no matter who's making them. It's learned the hard way, there's no book on how to do it. That's why PI's are such popular home-brew projects here-- the simpler designs don't care much if anything at all about induction balance. And the seemingly simple searchcoil is the Achilles' Heel of commercial counterfeiters: it passes some sort of test before it gets loaded onto a ship in Shanghai, and doesn't work right when it finally arrives in the hands of the customer half a world away.

A key to understanding the "big picture" is what you said about sending the machines including searchcoils back to Whites, and a tech checked the searchcoil nulls. I don't work for White's, but I'm pretty safe in saying that when they get bad coils back, someone is asking why the coil went bad, and eventually the lessons that can be gleaned from customer returns can be turned into engineering experiments that lead to better searchcoil designs and manufacturing processes.

If you want to know what could cause an induction balance coil to go bad besides "obvious abuse", that list is a long one and nobody in the industry knows everything that's on that list. Established manufacturers selling trademarked products try to stay on the learning curve.

Counterfeiters have no trademark of their own, they stole someone else's! Counterfeiters don't give a damn what happens to that trademark they stole, they don't even understand how the thing they're manufacturing works. And if you ask about things like "warranty" or "service", that's not part of the counterfeiting business model.

--Dave J.

[Skippy]

Quite small mechanical changes can upset the null. Plastic will absorb water, changing shape as a result - not necessarily evenly. The moulding process introduces stresses in the plastic, the outer skin may be different to the inner regions. Excessive temperatures can relax these stresses (by softening the plastic) causing twisting, etc. Foam-like filler materials inside the coil can dry out over time, shrinking and cracking as they do. This movement can move the wires that are in the foam. Screening and shielding of the coils often involves mechanical connections,(rather than welds, solder-joints etc) and these can fail do to mechanical stresses. These shape-changes don't have to occur once, or permanently, either. Repeated heating/cooling, dry/wet, humid/arrid cycles eventually cause cracks and other permanent failures.

[Dave J.]

Yep, Skippy, all that stuff, just for starters.

The manufacturers of the materials used to make searchcoils don't provide the information needed to know how those materials will perform in searchcoil manufacturing processes, much less in assembled and delivered manufactured searchcoil service. Worse yet is when you rely on manufacturer's data sheet information for design purposes, and discover too late that the information on the manufacturer's data sheet is just plain wrong, their engineers didn't know how to measure the parameter in question.

--Dave J.

[Dave J.]

In my situation I occasionally see posts inviting a response or I even get PM asking, about how to build a searchcoil.

Because I know from more than 30 years' experience what's involved in that, my advice is almost always one of the following:

1. Screw around with pulse induction and a mono coil, if you can't get that to work you need to find some other hobby.

2. If you insist on building a VLF induction balance project, begin by saying to heck with building your own searchcoil, buy an off-the-shelf searchcoil built by a company that actually knows how to build 'em, and make sure you understand what that searchcoil you're buying consists of. Design around that searchcoil, do not get creative with it. This is really good advice and failure to take it is why VLF projects have such a high dropout rate.

*******************

A lot experimenters are working on very low budgets, to where the cost of a searchcoil is an important element in affordability. My advice is this: buy a BH 4 inch searchcoil and develop with that. The transmitter is parallel resonant at about 6.7 kHz and the receiver is non-resonant, both the transmitter circuit and receiver windings being approximately 2.4 mH and the transmitter parallel capacitance about .22 mF.

I won't entertain questions asking for more accurate informaiton about our searchcoils. If you can't make do with what I just provided, you're a hopeless basket case anyhow.

Anyone who doesn't believe this is good advice for them personally, I beg you please ignore this advice.

--Servidor de Vdes.

Dave J.

[ArizFlash]

RE: Besides obvious abuse what causes a IB coil to go bad?

Skippy:

Thanks for your response. I move a lot in my job. In the last 10 years I've lived in Dallas, TX - Hot and Humid in the summer and can get cold in the winter, Phoenix AZ - Hot and Dry from May thru Oct and bearable other times, Houston, TX - Hot and Very Humid from May to Sept and Mild and Humid the rest of the time. Currently I'm in the San Francisco area in the East Bay and the weather is just about perfect most the time and no AC required. Sounds like storing the coils in my garage in Dallas, Phoenix and Houston most likely stressed and twisted the foam and/or shielding. I'll be sure to keep the coils out of the hot garages from now on.

Thanks again,
Alan F.

Quite small mechanical changes can upset the null. Plastic will absorb water, changing shape as a result - not necessarily evenly. The moulding process introduces stresses in the plastic, the outer skin may be different to the inner regions. Excessive temperatures can relax these stresses (by softening the plastic) causing twisting, etc. Foam-like filler materials inside the coil can dry out over time, shrinking and cracking as they do. This movement can move the wires that are in the foam. Screening and shielding of the coils often involves mechanical connections,(rather than welds, solder-joints etc) and these can fail do to mechanical stresses. These shape-changes don't have to occur once, or permanently, either. Repeated heating/cooling, dry/wet, humid/arrid cycles eventually cause cracks and other permanent failures.

[ArizFlash]

Dave:

Thanks for sharing your knowledge. I've been contemplating building a P.I. detector along with a mono coil. Carl's GeoTech site is a great resource and you're one of the many gurus on this site that I glean knowledge from.

In my work I manage designs of industrial electrical and control systems but specialize more in traction power substation design for rail transit systems. Even with my basic knowledge in electrical engineering the electronics side still baffles me. At 60 years of age i continue to be a student of electronics and am looking forward to have time to build a working P.I. detector. I'll leave the IB design the pro's and just enjoy the fruits of their knowledge and resulting products.

Alan F.

In my situation I occasionally see posts inviting a response or I even get PM asking, about how to build a searchcoil.

Because I know from more than 30 years' experience what's involved in that, my advice is almost always one of the following:

1. Screw around with pulse induction and a mono coil, if you can't get that to work you need to find some other hobby.

2. If you insist on building a VLF induction balance project, begin by saying to heck with building your own searchcoil, buy an off-the-shelf searchcoil built by a company that actually knows how to build 'em, and make sure you understand what that searchcoil you're buying consists of. Design around that searchcoil, do not get creative with it. This is really good advice and failure to take it is why VLF projects have such a high dropout rate.

*******************

A lot experimenters are working on very low budgets, to where the cost of a searchcoil is an important element in affordability. My advice is this: buy a BH 4 inch searchcoil and develop with that. The transmitter is parallel resonant at about 6.7 kHz and the receiver is non-resonant, both the transmitter circuit and receiver windings being approximately 2.4 mH and the transmitter parallel capacitance about .22 mF.

I won't entertain questions asking for more accurate informaiton about our searchcoils. If you can't make do with what I just provided, you're a hopeless basket case anyhow.

Anyone who doesn't believe this is good advice for them personally, I beg you please ignore this advice.

--Servidor de Vdes.

Dave J.

[Skippy]

Quote:"I won't entertain questions asking for more accurate information about our searchcoils". There's actually a couple of threads on here about the BountyHunter / F2 coils detailing more about the innards of them, for those wishing to know.

[Carl-NC]

A few months ago I sent my XLT and IDX in to Whites for a refurb along with the coils. Both of the 950 coils were bad as was the 5.3 Black Max and the service tech said the "null" reading was off. I can understand the 5.3 coil being bad because I used the heck out of it and probably banged it around somewhat. But I very seldom used the 950 coils on either detector. I know the coils were stored in my garage in AZ where I'm pretty sure the temps got up to 120º at times.

One thing you didn't mention was whether or not the units seemed to be having any problems. Lots of people send perfectly working detectors in for a "tune-up" which is generally not needed at all. And when the Service Dept tests a loop sometimes it doesn't quite pass the test, but it is otherwise a perfectly serviceable loop. If fact, almost all the loops I use when I hunt are Service Dept rejects that they replaced; but they work just fine. I have to admit that our tests are probably set too tight.

[ArizFlash]

Carl:

The screen on the XLT stayed blank when powered on. The tech replaced the LCD screen, screen cover, pod housing, membrane panels and contact switches. Also replaced some gaskets and installed a new coil connector along with the replacing the 950 coil. I had sent the XLT in previously in 1999 because the screen would constantly keep scrolling on start-up. At that time it was still under warranty. The tech told then the coil was bad and needed to be replaced. I don't ever remember using the 950 since then.

The IDX ran okay. But the black sand circuit didn't shift the discrimination level. It operated the same as in the normal switch position. Also the speaker wasn't working. I used it successfully with headphones. The only reason I sent it in was to get the black sand switch/circuit checked out. If that hadn't been a problem I could have replace the speaker myself.

I usually run the 5.3, 800 or the BigFoot coil on both the XLT and IDX. I sure wish Jim Karbowski with Applied Creativity had kept better notes. I would like a BigFoot for my MXT Pro but aftermarket ones sell for $400 - $500 now when you can find one. This seems like a market that needs a new supplier and not just for the White's lineup. The BigFoot coil isn't deep but for recent drops and ring hunting on large fields there is nothing better that I've used. I know Tesoro has a BigFoot type coil but I haven't heard to many positive remarks about it.

I'm not sure what type of electronics or loop configuration (other than what I've read in your book and on Geotech) were used on the BigFoot to stop the EMI effects. But I wish First Texas (Dave J. and Co.) would come up with something similar to partially solve the T2 / F75 EMI issues. Turing down the sensitivity and threshold does help in some areas. However when your in a urban area surrounded by overhead lines on two or more sides within a one block radius the F75 is useless. I really like the ergonomics and lighter weight of my Fisher F75E but the EMI issues are a handicap and frustrating to deal with in the field. I usually take both the MXT and the F75 with me and sometimes the IDX cause if it is a location with EMI issues because the IDX is rarely affected by EMI.

Alan F.

One thing you didn't mention was whether or not the units seemed to be having any problems. Lots of people send perfectly working detectors in for a "tune-up" which is generally not needed at all. And when the Service Dept tests a loop sometimes it doesn't quite pass the test, but it is otherwise a perfectly serviceable loop. If fact, almost all the loops I use when I hunt are Service Dept rejects that they replaced; but they work just fine. I have to admit that our tests are probably set too tight.