First Texas Products & Fisher Labs August 2009

Dave Johnson, Chief Designer @ FTP & Fisher

Because of the high sensitivity of modern metal detectors coupled with the proliferation of sources of electromagnetic interference, you are likely to encounter electrical interference at times during the use of your metal detector. It is important that you recognize electrical interference when present, and take appropriate measures to deal with it. This will prevent you from giving up on a worthwhile site unnecessarily, or from sending in for a repair a machine which is working properly.

Symptoms of electrical interference

Electrical interference can cause a metal detector to “chatter” spontaneously, to lose sensitivity for no apparent reason, or to cause periodic audio “wobble” or slow waves of spontaneous sound. What you’ll hear will depend on what model of metal detector you’re using, what operating mode you’re using it in, how you have the adjustments set, and what the source of the electrical interference is. The most common manifestation is spontaneous chatter.

All metal detectors are susceptible to electrical interference, but they vary in what kinds of electrical interference affect them. In a given environment some metal detectors may be affected by electrical interference whereas others may not.

Two metal detectors of the same model in the same environment may be affected differently, because of minor differences in operating frequency or because the controls have been adjusted differently.

Common sources of electrical interference

Common sources of electrical interference include: overhead electric power lines, underground power lines, other metal detectors, telephone lines carrying electronic data, computer systems, electric fences, old CRT-based televisions, cell phones, thunderstorms, fluorescent lights, metal vapor lamps, military aircraft with electronic warfare countermeasures turned on, electric motors, VLF military communications systems, and automobile ignition systems. It will sometimes be the case at home, in the showroom, or in an urban environment that there are several different sources of electrical interference present simultaneously.

All metal detectors generate a certain amount of electronic noise internally. On most metal detectors, especially the higher performance models, the sensitivity can be adjusted high enough to “work into the noise”. This is not a defect, but an intentional design feature. Experienced users striving for maximum depth often adjust the machine to “work into the noise”, and then they “listen through the noise” for the sound of real targets.

Is electrical interference a bigger problem than it used to be?

Stricter regulations have cut down on interference from electric light dimmers and auto ignition systems. However there has recently been a proliferation of VLF-UHF wireless communication systems (cell phones, Bluetooth, Wi-Fi, etc.) which often affect metal detectors. Overall, the potential for electrical interference is greater than it was a few years ago.

Also, modern high-end metal detectors are a lot more sensitive than older units, which increases their vulnerability to electrical interference. Engineers are working on ways to reduce that vulnerability, but the battle will never be won because metal detectors are by their nature designed to detect magnetic fields, and electric current always produces magnetic fields.

Metal detectors which operate in the time domain (pulse induction, “BBS”) have gained some popularity in the last few years, and these types tend to be more vulnerable to electrical interference than the more traditional machines which operate in the frequency domain. Pulse induction machines are mostly used for gold prospecting outside of urban areas so their vulnerability to electrical interference is not usually regarded as a serious liability.

What can a user do about electrical interference?

All metal detectors are equipped with a sensitivity control, or with other controls (for instance gain or threshold) which have the effect of controlling sensitivity. The primary reason metal detectors provide sensitivity control, is so the user can reduce sensitivity in order to eliminate response to electrical interference. Some users are reluctant to reduce sensitivity out of fear of “losing depth”. Well, you’re going to lose some depth, but you can still search. If you give up and walk away, you lost 100% of your “depth”. The sensitivity control (or its equivalent) is your first line of defense against electrical interference.

Many midrange and high end metal detector models have a feature called “frequency shifting”. This can be used to reduce or eliminate certain kinds of electrical interference, especially from other metal detectors. It is usually (but not always) effective in dealing with power line interference. It is not effective against electrical interference from thunderstorms, electric fences, or auto ignition systems. For additional information, consult the user’s manual for that particular model.

Many metal detector models have both a discrimination mode, and a motion all metals mode (often called “autotune mode”) which has a slower, smoother response than the discrimination mode. Electrical interference is often more controllable in the all metals mode than in the discrimination mode.

In the discrimination mode, setting the discrimination level into the foil region will usually reduce electrical interference problems: however there are many different discriminator designs out there in beeperland, and this trick doesn’t work for all of them.

In the discrimination mode, you’ll often find that although the machine may be chattery if the searchcoil is not in motion, once you start sweeping over the ground, the signal from the ground will suppress the electrical interference chatter except for an occasional pop or click.

When you’re doing “air testing” (demo’ing the unit indoors), you may find that changing the orientation of the searchcoil makes a big difference in electrical interference pickup.

If you carry a cellphone or other “high-tech’ electronic equipment with you while metal detecting and run into problems with electrical interference, try turning off the electronic equipment (all the way off, not just standby) and see if that solves the problem.

When working near overhead power lines, you’ll often find that you get the best results right under the power line, and the worst results at about a 30 to 45 degree angle away from the power lines. When you read reports of people bragging that their metal detector worked great right under power lines, this is often the situation. The person doing the bragging often couldn’t get out from under the power lines because of weeds, right-of-way fences, etc. and so may have been unaware they didn’t have a magic metal detector in their hands.

Many sources of electrical interference are intermittent. You may find that an area which is difficult to search at one time of day may be easier after 5 PM or on weekends. Power lines are usually quietest late at night, and early weekend mornings.

Small searchcoils usually pick up less electrical interference than larger searchcoils. For a given size searchcoil, concentrics usually pick up less electrical interference than does a DD. However, the differences are not great. …….On a site with really bad electrical interference, a small concentric searchcoil may be the best choice.

Distinctive characteristics of certain Bounty Hunter, Teknetics, and Fisher models

In general, it can be said that the “hottest” models experience the most frequent problems with electrical interference, and that the least sensitive low end models have the fewest problems.

The T2 and the F75, when used in discrimination mode, usually provide better electrical interference rejection at low discrimination settings, than at high discrimination settings. This contrary to the pattern of most discriminators.

The JE discrimination process (often called “jewelry mode”) in the F75 and F70 is more vulnerable to electrical interference than the other discrimination processes. However if you are searching for small jewelry on a site where you’re experiencing electrical interference, you may choose to search using the JE process anyway, and to control electrical interference by other means (or just listen through the noise).

Among our medium to high performance metal detectors (as of August 09), the ones which are overall probably the easiest to use in environments with electrical interference, are the F5 and the Tek Omega.

The Gold Bug II discrimination system was designed specifically for gold prospecting away from sources of electrical interference, and to be used for checking a target, not for searching. It is not like a conventional discriminator. By design, it tends to run noisy, and the user interface provides no convenient means for cutting chatter from electrical interference. If you are experiencing electrical interference with it, just sweep it over the ground, and the signal from the ground will usually suppress most of the chatter unless the source of electrical interference is unusually strong.

A few years ago before Bounty Hunter came under present ownership, one Time Ranger version had some sort of antenna mounted on the control box which was supposed to suppress electrical interference. We don’t believe it was ever useful, and it was eliminated from all later revisions of the Time Ranger.

Distinguishing electrical interference from other problems

The loudness of electrical interference will usually vary as you walk around holding the machine, and also it will usually vary with changes in the orientation of the searchcoil. This is almost never the case if the problem is in the metal detector itself.

The most common cause of a “noisy metal detector” where the problem is not electrical interference, is a defective searchcoil (including cable and connector). Searchcoil manufacturing is as much art as science, and although we wish we could say that searchcoils never go bad, sometimes they do. In many cases a defective searchcoil is intermittent which can be determined by giving the coil a whack with your hand (not with a hammer!). If whacking the searchcoil with your hand fixes the problem at least briefly, the problem isn’t electrical interference. Also, it is often the case with a noisy searchcoil that the noise has a sporadic character, and is almost as bad with sensitivity adjusted to a low level as it is with sensitivity adjusted high. Electrical interference sometimes exhibits this characteristic, but is usually more continuous and exhibits a greater sense of proportionality with respect to control settings.

The second most common cause of a “noisy metal detector” is an internal calibration which has drifted over time. This is rarely seen with the most modern designs which have few calibration adjustments because nowadays so much of the function is in software. However some of the most popular machines are ones which have been around a long time with a proven track record, and some of these older mostly circuit-based designs have a number of internal calibration adjustments to maximize performance.

The third most common cause of a “noisy metal detector” is dirt or water in the searchcoil cover (“scuff plate”). If you use a searchcoil cover to protect the searchcoil from abrasion, it should be periodically removed and cleaned. Dirt or water in the searchcoil cover can move around while you’re sweeping, causing false signals.

If your complaint is “noisy metal detector”, before you decide it needs to be repaired, please first make sure that the problem is not electrical interference, and verify that you are using the machine properly. The factory can fix the machine only if there’s something actually wrong with it.

* * * * * * * * *
File: Electrical Interference last update 4 Aug 09 DEJ copyright First Texas Products – Fisher Labs

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This time I take apart the Display/control pod! In the first part of this series, I “deconstructed” the under-arm housing. LINK HERE. Upon removing the screws on the control pod, I am very impressed by the use of a nice heavy rubber bushing/seal. So far, all parts appear to be very high quality on the Spectra V3. Each half of the display seat into a recessed groove…nice and water tight! There’s also a beefy rubber grommet/seal where the pod back connects to the back of the handle…once again helping keep the elements at bay….

READ MORE BELOW!  More Pics…

Partially Apart Vision Pod

*Use CAUTION when reassembling! Put the whole thing back together with pressure from your fingers…making sure all the parts are seated properly. DO NOT use the screws to “pull” it back together or you may strip them out!

Board and connectors inside display housing

Thanks again to the great folks at White’s Electronics (Carl and Alan!) A full review of the V3 is coming very soon!

Rubber Bushing near handle

Inside the back of the housing with connectors

Heavy rubber washer/seal between front and back halves of the housing

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V3 board 1

V3 board view 2

Vision Spectra V3 board view 3

Speaker with seal

V3 board ID

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Posted originally on American Relic Hunters Official Teknetics Forum

Hello! and, Omega

Posted By: dave johnson
Date: Friday, 13 February 2009, at 1:10 p.m.

Hello, this is my first forum post on this website. Some of y’all I recognize from other venues.

Since the Teknetics Omega is in the process of being released to production, there will be a lot of curiosity about it. Most of the questions will relate to features and performance in the field. There’s no need for me to go into those matters, since other people will quickly fill in the details. What follows here is a bit of information which is probably better coming direct from engineering department.

Once the Omega gets into people’s hands, it’ll become obvious that there is some sort of relationship between the Tek Omega, and several earlier Fisher and Bounty Hunter products. Here’s what that relationship is.

The Omega circuitry and software are a major revision of the Fisher F5, which although it was a bit slow to “catch on”, is now getting good reviews. The basic software concepts of the F5 in turn, were derived from the F70, but not the code itself because the machines use completely different microprocessors. I was responsible for many of the software concepts in all these products, but John Gardiner wrote the code for the F70 and Jorge Anton Saad wrote the code for the F5 and the Omega. The F70 circuit platform is related to the F75 and Tek T2, which despite their differences all run at 13 kHz. The F5 and the new Tek Titanium series of which the Omega is a member, are a different family of circuits, all running at 7.8 kHz. The Bounty Hunter Platinum and Gold were an earlier version of this basic circuit platform.

The mechanical design of the Omega is a revision of the Bounty Hunter Platinum.

The Omega and F5 have their own unique user interfaces and different features, but both are simple and easy to use. The two machines are fairly similar in performance and in operating characteristics.

Since we came out with the Tek T2 several years ago, we’ve learned a lot about how to do tricky stuff in software but to keep the tricks hidden from the user so as to create less puzzlement on the part of the user. Old-timers will like the predictable analog feel of the Omega, even though what’s actually happening inside the machine is very different stuff from what went on in the older analog metal detector designs.

The basic performance and “feel” of the Omega is rather similar to that of the Delta and Gamma, about which there has been information posted on forums for several weeks. The differences are in mechanical construction and features.

HEADPHONE JACKS AND SEARCHCOIL CONNECTORS

The earlier Delta and Gamma were released with a non-locking DIN connector the same as the F5, and with an 1/8 inch headphone jack. With the release of the Omega, we are improving the Delta and Gamma design by changing it to have the T2-type searchcoil connector and both 1/4″ and 1/8″ headphone jacks which were developed for the Omega.

SEARCHCOILS

At the moment there are two searchcoils available for the Omega: the 10 inch elliptical concentric (standard), and the 8 inch round concentric (standard on the Delta and Gamma). A 5 inch DD coil and an 11 inch DD coil are under development with release expected early spring this year. When these additional coils are released, they will also be offered in a DIN connector version to work with the F5 and the earlier Deltas and Gammas.

PERFORMANCE

The Omega air tests on coins past 10 inches (25 cm)– not as hot as the more expensive Tek T2 and F75/F70 type machines, but close. Because of the difference in operating frequency, those 13 kHz machines will have a distinct edge on lower conductivity targets.

When it comes to target separation, the Omega is in the same league as the Tek T2 and the Fisher F75/F70 and F5. These are machines which have already earned a reputation for superb target separation.

How a machine performs “in the ground” on a particular target entails many variables, not the least of which is the operator knowing how to use the machine. So I will not say that the Omega will do this or that in the ground better or worse than some other specific machine. Suffice it to say that “in the ground” on coin-sized objects, it’s in the same league as the T2 and its successors. The 13 kHz platform machines will tend to have an overall edge on smaller targets.

SO WHAT DO I LIKE ABOUT THE OMEGA?

Straightforward simplicity, excellent performance, sense of direct connection between what’s in the ground and what you hear, and the completeness and refinement of the overall machine.

AND FINALLY……

Although I monitor this forum on a regular basis, I will not frequently post. I know lots of people would like to ask me lots of questions, most of which are ones I either can’t answer or don’t have time to answer. There are some very knowledgeable people posting on this forum who will be able to respond to most questions including some I myself would be unable to answer. Thank you.

–Dave Johnson
Chief Designer, FTP & Fisher

Omega Owner Manual Download HERE courtesy of Jorge Saad

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