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GPZ 7000 Tear-down


phrunt

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Got my carbon fiber today, it's a roll of fabric itself. Not sure what the actual resistance is because I'm stuck using a really cheap multimeter right now, it's saying 5 ohms/inch but I don't put a lot of trust in it. The 6000 coil screams on it when I drape it directly over, then rapidly dies down and stabilizes.

Anyways a few interesting things - It doesn't seem to degrade the sensitivity, at least on a US quarter. Or if it does, it's minor. This was just a real rough, brief test though just to see what happens, nothing more. 

But, it seems to me that you might actually be able to build coils out of carbon fiber where the case itself is the shield, and potentially save some weight by eliminating the coil cover if the resin was stout enough. If it was just 1 layer of carbon fiber, the coil might be lighter in general than plastic coils too. Might not be worth the potential increase in knock sensitivity though.

But interestingly - carbon fiber didn't appear to redcuce the EMI as I hoped. Which means it's either not acting as any better shielding than the graphite paint already is, or that the EMI is mostly still self generated.

That, combined with the fact I was testing inside a large metal building that itself is shielding a lot of external EMI like a Faraday cage (my cell phone almost doesn't work inside), makes me wonder if some or even most of the random chattering I'm hearing on the 6000 post-fix might still be internal/self generated EMI too (and presumably this EMI could be improved with a mod?). I'd have to experiment a bit more to see where it's coming from though.

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1 hour ago, jasong said:

The 6000 coil screams on it when I drape it directly over, then rapidly dies down and stabilizes.

    The automatic ground balance is probably adjusting to the material similar to mineralized ground. It would probably bias or reduce the ground balance circuitry.
    I just now tried some indoor of experiments. I sanded the clear finish off of a lower carbon fiber shaft and measured the resistance to be 25 to 35 ohms. It is not detected when waving it over the GPX 11” coil. I put my Galaxy A10e phone against the GPX 6000 housing and did not hear any additional noise. I removed the GPX 6000 arm rest and wrapped the entire housing with heavy aluminum foil, leaving a small opening for the speaker sounds; it did not reduce the EMI noise. I will try these tests again when I get back to the desert.
     I played with a large sheet of aluminum foil placed on top of cardboard boxes. I got a similar effect that you got with your quarter test. But as I reduced the foil distance the detector quieted down similar to what Woody was accomplishing with the copper mesh. The distance was very critical; just a slight change would cause it to go completely silent or go full EMI mode. The copper mesh may have insulated strands that would have less effect than the solid foil.
     In the desert if the EMI is low I try to run at Manual 10 when EMI gets annoying I go to Auto 2 which is much of the time. The last time I was out it was running so quiet that I checked it on my test nugget to make sure it was still working. About an hour later it went crazy. Auto tuning did not help. I looked around and spotted some distant aircraft. As they got closer I could make out that they were three twin engine Osprey helicopters. It quieted down again after they disappeared behind some mountains. 
      Other than the known speaker EMI problem I think we are just dealing with a very sensitive detector that was designed to detect very small nuggets. I am content to accept that and enjoy the light weight and ease of operation. I look forward to the seeing what is in the next model.

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Try facing your phone screen towards the control box, on the right hand side of the box. My phone sets the detector off every time if I bring it close to the box, but only within about 1/4" or less. I have an S22 I believe it's screen is different than the A10e though. If I do it with the screen facing away from the box it's much harder to hear. 

The carbon fiber doesn't help reduce that effect though - so I don't think it's EMI related. I think something in the phone screen is electromagnetically coupling to something inside the control box. Oddly though, placing a large hammer or wrench to the side of the box produces no such effect, so I'm not sure what's happening really. I'd film it but my phone is my only camera right now, so that doesn't work. 

Also, the effect seems to be decreased now after the fix. But that could just be my imagination. I do seem to remember it being much bigger over more distance pre-fix though since I discovered it by accident just by my phone being near the box and not because I was pressing my phone onto the box intentionally like I am now to reproduce the effect.

Anyways, it's a minor effect, not really a hinderance to detecting. But it did make me curious to see if it was EMI related or not because if so, it might have been an easy fix to quiet mine down more if the same mechanism was also potentially allowed additional EMI from other sources in. Even after the fix, mine can be quite noisy in some locations which makes me think (like your helicoptors) there may be localized EMI sources that are present 24/7 for me, so I was hoping to see if there was some easy way to increase the shielding. I'd really have to try out in the field though to see if it makes any difference, and there is so much snow left here that it'll be a while till I can get out. 

In my head I wanted something simple like making some carbon fiber stickers to place on the side of the box or on top of the coil as basic shielding enhancements, but it doesn't appear so far that this will be much help. It was worth a shot though, and I'm definitely going to test again when I get into the field into a much quieter environment where everything isn't being blown out by 60hz. 

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     I passed a rare earth magnet along the right side of the housing. Just below the GPX lettering it sounds off like a target passing under the coil. There is probably and inductor or transformer at that location. The speaker magnet inside the phone is probably affecting it a similar way. Nonferrous materials will not shield magnetic fields. So it would require tin or powered iron epoxy for shielding if it was a serious problem.
      Much of the EMI we are experiencing is coming from many sources that are hundreds and thousands of miles away. It is bouncing off the ionosphere and varies in intensity with changes in the ionosphere.
       In my days of working with cryogenically cooled radar receivers we had a constant noise/EMI problem that we could not fix. It was the distant noise/static from the big bang sill travelling through space.   
 

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2 minutes ago, Chet said:


       In my days of working with cryogenically cooled radar receivers we had a constant noise/EMI problem that we could not fix. It was the distant noise/static from the big bang sill travelling through space.   
 

Familiar with that story, it was and I think still is required teaching before starting the chapters on the discovery of the cosmic background microwave radiation that ended up having a significant impact on cosmology and physics. 🙂

A lot of science has been discovered by accident like that. It's why I always try to be hyper in tune with an odd observations, most are nothing. But occassionally....

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      The receiver frontend was operating as close to minus 273 degrees Celsius as could be achieved outside of a laboratory environment. This was to prevent noise from electron flow in the receiver wires and components. On rare occasions the cryogenics unit would fail causing a 3 decibel loss in receiver signal to noise ratio. This is a real loss when you are trying to detect and track incoming Russian submarine launched missiles.

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I have a wacky idea on the submarine topic - ULF frequencies used by subs and also emitted by certain natural phenomenon are known to travel a good distance through the earth itself and detectable inland like in Colorado and whatnot. Would there be a way to use those existing transmission already deep in the earth as "TX", and just use a lightweight "RX" handheld unit, or even a truck mounted loop? Obviously not for small nugget size targets, but maybe for larger conductive ore bodies for geophysical exploration?

Similarly, would it be possible to build a massive "traditional" PI by running 4 stakes over like a 500ft diameter square, then winding a coil over the stakes and using that as a TX mono. Then using a lightweight RX handheld unit that senses the pulse timing for synchronization, use it to scan for really deep targets that are nugget sized? Or potentially a drone or atv mounted RX loop that might not need to be so close to the ground since the TX loop is so large?

I'm curious to experiment with stuff like this that I've thought about for a while, but I just never have the time or resources to try. 

 

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      There has been some work similar to what are describing used to identifying mineralization and discontinuities or possible ore bodies in a localized area. Submarines listen from any ocean for orders from Extremely low frequency (ELF) of 1 Hz to 300 Hz radio signals. I think someone has experimented with the Pulse Induction arrangement. The problem with trying to benefit from large transmit systems is the wide dispersed transmit signal is too weak. The feeble energy induced into and retransmitted by a large nugget would be undetectable. 

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Thanks, meant to say ELF not ULF. Do you suppose a really large ore body might alter the phase...or some other quantity...of an existing ELF signal passing through the Earth already and detectable at surface too, around a large ore body, such that it could be detected with an RX loop? Or something like a gradiometer where one RX loop is separated from a 2nd RX loop by some distance in order to detect changes in the ELF signals passing through the Earth in different spots? Like would it be doable using just existing ELF signals, or would a local TX module be required to have enough control over the TX signal shape/power/etc?

Also, what do you suppose would be the theoretical limit on coil size for a standard PI type detector, before the signal simply gets too weak for nuggets? 5ft? 25ft? 100ft? What about doing something like pulsing a bank of 1000v low internal resistance caps through the coil, using beefy wire like 8 gauge stranded or something that could take the massive current pulse? That should compensate for the large coil size up to some point.

I'm just curious if there is a way to move specifically beyond the coil on a stick model. Or also, if you could remove the TX/power circuitry from the hand held detector itself, you might be able to just get an ultra quiet noise floor and ultra sensitive RX unit, very lightweight lacking all the power circuitry, and use that alone by hand.

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A friend worked at one of the ELF transmitter sites when he was in the Navy in the 1950s. It has miles of antenna wires and miles of cables buried in the ground to essentially make the earth the antenna. It was driven by high transmitter power that charged your car’s surface so there were spring supported grounding rods in each parking space to park your steel bumper against before you stepped out. I wander what they do now with plastic bumpers and plastic splash guards under some cars? 
      The wavelength of ELF signal is many miles long. Submarines receive the signal by deploying a long floating wire antenna. I don’t think small loop antennas would pick up much signal especially for making any kind phase shift or signal strength comparisons between sine waves that have their peaks and valleys many miles apart. It’s similar to the needle in the haystack problem.
      The largest PI coil I recall was one on the Meteorite Men TV program. They had a large PVC drag coil that detected a huge deep meteorite in a Kansas field. It was a known area for large meteorites. 
       You have probably seen some of the power equations before; it is hard to gain much by brute forcing more energy. The concern on detector weight rules out larger coil wire gauges and the extra battery weight to power a heavy coil. It is practical to make gains in receiving and data processing. That is where Minelab has made the improvements. 
        I have built a number of experimental mono coils of varies sizes between 6” to 48” for the GPX 5000 and the GPX 6000. I have built two experimental drag coils for the GPX 5000 and GPZ 7000. I am aiming for a size that has a sweet spot for a 1 oz. nugget. I think my next and hopefully final coils will be a 25” or 30” mono for the GPX 6000. And a similar size Hybrid Concentric coil for the GPZ 7000. The Hybrid Concentric has an outside receive winding with the transmit winding spaced much closer than the normal practice of 50%. The receive null coil is almost against the transmit coil. I have already built this and had it working. But the glue that I used was water based and soaked into the Litz wire windings and never completely dried.
 

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