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T2 & F75 Electrical Interference


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11 hours ago, phrunt said:

My T2 hates powerlines and urban areas, It's not uncommon I can't run it at all or have to run it wound right back to 50% gain to get it stable, it runs better with the Mars Tiger coil than any other I have for it but still has the same problems.. 

It seems to me 13khz is the worst for EMI.

Hi Simon… I realize that your T2 is somewhat different from the F75. But let me describe how I operate the original F75 to deal with issues related to EMI. 

The original F75 version is very much subject to erratic behavior in areas where EMI is present. Hunting urban areas in zero discrimination with the stock 11” DD coil is frequently impossible. I avoid using this coil in urban environs because it is quite vulnerable to EMI issues (extra windings / antennae effect) compared to the 10” elliptical concentric coil, and this is especially true when compared to any of the smaller coils. So don’t hesitate to switch to a smaller coil, preferably a concentric coil if the ground conditions permit.

An added benefit if hunting micro jewelry, is that the smaller coils, particularly the 6” elliptical concentric coil, is quite sensitive to small stuff compared to the stock 11” DD coil. In fact I put it to good use for hunting naturally occurring native silver in rocky environs. It’s not unusual to find sub-grain material with this coil. 

If using a discriminate mode, avoid JE mode in EMI areas. It is extremely high gain, and therefore much more sensitive to EMI than are the DE or PF search modes. 

Another very useful technique, if necessary, is to increase the iron discrimination level until erratic behavior settles down to an acceptable level. Do this while moving / holding the coil on the ground, and not while waving it around in the air. Keeping the coil to the ground much reduces EMI instability because it reduces the coil’s antennae effect.

Adjusting the sensitivity control is not necessarily the final step in stabilizing how the machine behaves in EMI environments. Do as you please with it. Keep in mind that you may wish to limit iron discrimination to about 6 or 7 and no more as Steve describes, depending on your preference. At that point, if necessary, you may wish to experiment with decreasing the sensitivity control to achieve stability.

Of course for prospecting applications, and low trash urban areas, the first choice is to search in the motion all-metal mode. It is much less vulnerable to EMI than are any of the discriminate modes. It makes a huge difference.

In closing, I should add that in remote prospecting areas, my original F75 is normally as quiet as a churchmouse regardless which search mode, settings employed, or the type / size of coil that is used. On rare occasion we do experience an intermittent EMI induced instability from what I suspect is the local microwave tower. These occasions are always temporary, rarely lasting more than a half-hour…………….. Jim.
 

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My original T2 and F75 were the same as yours.   Then, what, 10 years went by and out comes DST,  which made the F75 a dream machine.

 Also I find that sometimes the coils need to stabilize a little while, temperature wise.   I get different stability from the start of the hunt as compared to about 15 minutes in.   So let the coil temp adjust, then go back and freq shift.

HH
Mike

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On 9/28/2018 at 2:55 AM, phrunt said:

I followed your advice and went outside with the 5" coil on which I've rarely used on it as I normally just leave the Mars Tiger on, turned the sensitivity down to 80 and it was almost stable so I set my usual settings, and it went nuts with falsing, Turned it to all metal and it had the odd false signal but nothing major and ran at about 80 sensitivity with no falsing at all, cranked up the sensitivity again and found it would run at about 88 with no issues

I know a lot of people like to run the gain really high, particularly when hunting native gold, but at least for my F75 (black model with DST) I did quite well coin hunting with gain of 60, and even ran it down to 50 at the suggestion of an experienced/accomplished F75 user.

I'm not saying don't run it high, but definitely don't feel like you've severely limited your search zone by turning it down.

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Just note that the F75 only has two(2) gain settings, Low and High.  The Low Gain settings are 0 to 29.    The High Gain settings are 30 to 99.   The numbers are actually threshold settings.  You pick your Gain setting (low or high) then adjust the threshold setting for that Gain selection.   I run at 0 gain quite often in order to have the smallest coil footprint the F75 can give me and still get excellent depth.

I don't remember if any of this transitions over to the T2 or not.   Its been a long time since I've owned a T2.  

HH
Mike

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  • 4 weeks later...
On 9/27/2018 at 11:21 PM, Jim Hemmingway said:

Hi Simon… I realize that your T2 is somewhat different from the F75. But let me describe how I operate the original F75 to deal with issues related to EMI. 

The original F75 version is very much subject to erratic behavior in areas where EMI is present. Hunting urban areas in zero discrimination with the stock 11” DD coil is frequently impossible. I avoid using this coil in urban environs because it is quite vulnerable to EMI issues (extra windings / antennae effect) compared to the 10” elliptical concentric coil, and this is especially true when compared to any of the smaller coils. So don’t hesitate to switch to a smaller coil, preferably a concentric coil if the ground conditions permit.

An added benefit if hunting micro jewelry, is that the smaller coils, particularly the 6” elliptical concentric coil, is quite sensitive to small stuff compared to the stock 11” DD coil. In fact I put it to good use for hunting naturally occurring native silver in rocky environs. It’s not unusual to find sub-grain material with this coil. 

If using a discriminate mode, avoid JE mode in EMI areas. It is extremely high gain, and therefore much more sensitive to EMI than are the DE or PF search modes. 

Another very useful technique, if necessary, is to increase the iron discrimination level until erratic behavior settles down to an acceptable level. Do this while moving / holding the coil on the ground, and not while waving it around in the air. Keeping the coil to the ground much reduces EMI instability because it reduces the coil’s antennae effect.

Adjusting the sensitivity control is not necessarily the final step in stabilizing how the machine behaves in EMI environments. Do as you please with it. Keep in mind that you may wish to limit iron discrimination to about 6 or 7 and no more as Steve describes, depending on your preference. At that point, if necessary, you may wish to experiment with decreasing the sensitivity control to achieve stability.

Of course for prospecting applications, and low trash urban areas, the first choice is to search in the motion all-metal mode. It is much less vulnerable to EMI than are any of the discriminate modes. It makes a huge difference.

In closing, I should add that in remote prospecting areas, my original F75 is normally as quiet as a churchmouse regardless which search mode, settings employed, or the type / size of coil that is used. On rare occasion we do experience an intermittent EMI induced instability from what I suspect is the local microwave tower. These occasions are always temporary, rarely lasting more than a half-hour…………….. Jim.
 

New member here trying learn @ 78 ,just bought an F 75 whats a good setting for nuggets?----william

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Use the all metal mode, run the sensitivity as high as you can without the machine becoming unstable, get a proper ground balance, and dig everything that signals. If you want to risk missing nuggets but are digging too much trash and don't care, you can pass on targets that id 6 or lower. An alternative would be to use single tone disc mode, and dial in a disc setting of 6.

https://fisherlab.com/hobby/finds-Steve-Herschbach.htm

https://www.detectorprospector.com/forums/topic/357-fisher-f75-strikes-gold-twice-in-a-row/

https://www.detectorprospector.com/forums/topic/7607-fisher-f75-ferrous-tone-quirk/

 

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On 11/3/2018 at 6:13 PM, w.j.mccrary said:

New member here trying learn @ 78 ,just bought an F 75 whats a good setting for nuggets?----william

Hi William… here in northeastern Ontario, we do most of our prospecting for native silver in the motion all-metal mode. There are occasional exceptions where excessive trash is encountered as described further below. Let’s take a look at how we operate this unit up here.

The motion all-metal mode, that conveniently features a target ID, is deeperseeking and more sensitive to ground conditions and to small nuggets than are the discrimination modes. This mode is particularly effective for searching over rough, variable terrain where ‘low and slow’ scanning is necessarily dictated by such conditions, for example, poking the coil between and around boulders. We also prefer its comparatively unimpeded target-sizing ability to better assess what is happening beneath the searchcoil. 

When searching in the motion all-metal mode, use as much sensitivity as the ground conditions will permit while maintaining stable detector operation with a reasonably smooth threshold hum. Adjust the threshold as low as possible but still able to hear a faint mosquito-like hum. Many senior hobbyists have experienced hearing loss, therefore it is even more important to adjust the audio pitch to a level where you can most easily hear faint target signals. Set it appropriately and leave it alone. I use NuggetBuster headphones to help me hear weak target signals. 

The original F75 provides two features to ground balance the detector. The manual GB effectively does it all, including slight GB adjustments to offset neutral GB settings to perhaps enhance depth / sens to small nuggets or possibly to subdue various ground / hotrock effects. The ground balance procedures are described in the manual so we won’t go over it here. 

(a) The important thing to remember at all times, particularly when searching over tough ferromagnetic ground, is to ground balance to the same elevation above the ground that you will be scanning the coil. Otherwise over tough ground you will undoubtedly experience false signals.
(b) If the ground mineralization prevents you from obtaining a ground balance, you must either switch to a smaller coil (DD coils are more effective at reducing the effect of tough ground) and / or reduce sensitivity. For nugget hunting you will likely be using a small sniper coil anyway, so reducing the sensitivity is the only remaining alternative to achieve a good ground balance.

The alternative ground balance procedure is to use the autograb feature by pressing and holding the “trigger” forwards while bobbing the coil a few inches as described in the manual. It’s convenient, fast, and accurate. The autograb GB feature can also be used to supply additional information about a suspect target, but the technique described below benefits from using a concentric coil rather than a DD coil. 

Prior to disturbing the soil, pinpoint the target and do an autograb GB over it. Pump the coil four or five times but no more, and allow the software to function as designed. Note any movement on the ground balance scale readout. We find that positive signals from rusty iron, cobaltite, niccolite, and pyrrhotite generate significant GB reductions frequently shifting from typical search mode GB86 readouts right down into the GB40s. Positive diabase “hotrock” signals produce a relatively small GB reduction. But fortunately native silver samples free from either cobalt or nickeline contaminants will generate little or no downward movement to more conductive GB values.

You will experience some questionable responses at times that will require some interpretation based on your knowledge about local minerals and field conditions. The GB information provided by this technique can be ignored if in doubt, or it can be used in conjunction with target ID and target-sizing to evaluate suspect signals. We practice this procedure to eliminate unnecessary digging in difficult rocky substrates. 

In a prospecting context, we utilize iron discrimination in areas where hotrocks or ferrous targets are excessive to the point where effective detecting is either inefficient or no longer enjoyable in the motion all-metal mode. 

We all encounter areas that are so inundated with blaring signals from small bits of iron trash such as tiny nails and tacks, non-descript shards, and interminable wire scraps that effective detecting in the motion all-metal mode is frustrating or next to impossible. In such conditions we use smaller coils to help with separating target signals, and we make good use of small iron discrimination or iron tones.

Try adjusting to small iron discrimination levels initially as suggested above by Steve, give it a go and make any further adjustments according to what you can tolerate. This will depend on the amount of trash you’re willing to dig. Using iron tones or iron discrimination will result in overlooking deep non-ferrous targets that occasionally do target ID as iron. But it is a necessary compromise to at least enable us to enjoyably detect excessively trashy sites. 

Hotrock discrimination is somewhat different from small iron discrimination in northeastern Ontario. By and large we deal with non-conductive mafic hotrocks, diabase is usually the culprit that can produce either positive or negative signals just depending on circumstances, while conductive pyrrhotite hotrocks are mostly responsible for producing positive signals. There are other minor conductive nuisances, for example infrequent bornite and even more rare graphitic rocks.

Pyrrhotite signal elimination is straightforward. Large samples may target ID into the low foil range, but most of it is eliminated with mid-to-upper iron range discrimination. This material in abundance renders entire sites unsuitable for detecting with either VLF or PI units. 

Discrimination settings do vary according to how each discrimination mode processes our non-conductive diabase “hotrock” signals. For example, all discrimination modes eliminate signals from negative diabase hotrocks at a zero discrimination setting. However while both the DE and PF modes require a discrimination setting of “1” to eliminate positive diabase signals, the higher gain JE discrimination mode requires a full iron discrimination setting of “15” to achieve the same result here. Hotrocks in your area may vary considerably from our experience.

The only other suggestion that occurs to me at the moment is to equip your F75 with a small coil for gold nugget hunting. Small coils are far more sensitive to small nuggets than is the stock DD coil. There really is no comparison. The 6½-inch elliptical sniper concentric coil is superior in my estimate, although the 5” round DD is a very close performer over my ground. A further advantage to these small coils is that they permit using higher sensitivity settings in prospecting country because they obviously see much less ferromagnetic ground mineral than do the larger coils. 

I hope the foregoing will be of some use to you. There undoubtedly will be differences in the ground conditions, the hotrocks, and the trash levels in your areas. Good luck with everything William, and welcome to the forum!!! :smile:

Jim.

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548486696_PYRRHOTITE(BRO)SF.JPG.3e5e1f1525de2b984b01be06895f3756.JPG
 

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Very interesting subject,i use a original T2 that i have had for many years and never really had trouble as such with EMI as i mainly detect farmland well away from EMI sources,but we do have problems occasionally with over head power cables.

So although i dont have trouble with EMI i do prefer my T2 to run smoothly rather than on the 'edge' as they say,you may get some slight depth advantage but at the expense of giving your ears a real hard time with all the falsing and other non desirable audio signals,for a very long time i always used discrimination mode only and 80 sensitivity was the baseline figure and of course adjust up or down accordingly on the ground condition,but this last year or so i have been mainly using AM mode and with smaller coils,i do have a slection of large coils for deep clean pasture sites and for the most part that has worked well,i do occasionally reduce sens down too around 75.

But i mainly use smaller than stock size coils these days mainly due on a wrist injury i received back in the 70s when i came off a bike,so these days i usually run any detecting unit with smaller coils on,also i have found that i am happier reducing the sensitivity down to 70 even in AM mode,always loved the 5'' factory coil as it gives great depth for its size and deadly on trashy sites,but always wanted a slightly larger coil but still gives alot of the characteristics of the 5'' coil for more ground coverage,i finally found the ideal coil recently when i located and bought a 6'' DD coil made by Coiltek,it was a prototype coil from them but it finally has given me everything that i want from the T2,no more EMI issues and runs as smooth as silk and just with 70 on the sensitivity in AM mode.

Always fancied trying a F75 but while my T2 still finds me the holy grail of finds the celtic gold staters then i wont change this winning combination,1/4 staters are are not the biggest of finds,and even cut 1/4 and halves are found with ease.

A great topic on EMI and some great information reply wise.

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