XP deus coil modification

[dfbowers]

Hmmm, Yea ran into that problem with my first THREE coils.. Mine were not for the XP though, so cannot help there, but maybe your grounding configuration.
Wet grass was driving me nuts until I changed wiring configurations. The cure I came up with was using a Metallized Mylar survival blanket and keeping Tx and Rx shielding separate all the way to the cable connector on the control box. Below are llinks to my "wet grass" testing.

Bad coil : https://www.youtube.com/watch?v=XhRboS639no
Fixed coil: https://www.youtube.com/watch?v=7UBOca3btMw

[Henrikras]

Thanks Don...
Same problem I got and reason is properly the same, I connected the RX and the TX Sheilding at the coil.
Before I start a new coil I'll will try a panic version. See Pictures. (Lucky me that DB Shells fits together, Very nice job you made).
Tomorrow this will Be tested. Thank god that I got my own field with short grass.
Henrik.

[dfbowers]

That's pretty innovative. I would not have thought about doing that. Was the 7" DD shell deep enough? I think I cut them at 5/8" thick.. better aim for 3/4" next time?

[Rivers rat]

Henricas you are doing a really good job cant wait to see the finish product i m desperatly needing a small coil ,when it is too much for the deus i m using a tesoro with a 4" coil and managed to pick up the goodies



RR

[Henrikras]

Thank you RR.. Seem to that I need all the support I can get.
The version with closed Shell DIDN'T work either. It was better but not calm. (Sens. 70 and it could Be used).
NEWS... I need to find out how XP made the shielding... Please look at the pictures. It looks like they made a Shell with shielding !!!
I had connected the instrument to the RX and no matter where I measure its around 500 ohm - 1,5 kilo ohm...
Can this been explained. ?
THIS is only the RX there is connected to this shell Sheilding...
Henrik.

[Skippy]

Using conductive plastic for the case is common on better machines. I don't know whether it's carbon powder, or carbon fibres that they use. I've seen internal pics of a Minelab coil, where a copper wire has been melted into the case at several points to make a connection. AND, I can tell you that XP like conductive plastic....inside the GMP / GMAXX2 control box, the front-end analogue circuitry, amplifiers etc, are partially shielded by a plastic cover. This is conductive plastic - the idea I think was that when the cover was melted to the PCB attachment holes, contact would be made. It seems to have not worked out for them, the example I have seen had a thin tinned copper wire melted into the plastic, and then soldered to the PCB ground.
I guess in your coil, the cold end of the coil is joined to the case - maybe careful resistance measurements may confirm this (70 ohms difference between hot/cold ends).
Another thing they may do, is use conductive resin/filler to seal in the coil. It has been done on other commercial coils.

[Henrikras]

Okay good Friends. First try is now thrown away. Before the next one will Be made, I need a good plan to follow.
Ive just open up the Stock coil. I was Very difficult, but some good information came up.
XP made not only one Sheilding by conductive plastic, but allso some kind of graphite / conductive thin paint inside the Shell, with Very low resistance 100-500 ohm.
Found allso the sonder place where the thin wire to connect this inside shielding was sondered to the RX cold side.
interesting was that XP had used silver-plated copper wire, and it was going from back to the front of the searchcoil in the top and the bottom... How can that Be done ! Easy. After they had filled the Compound mass up to appoxly 75-80%, then they paint this graphite / conductive paint, to close the sheilding 100% arround the coils and then they filled up the last 20-25% Compound mass to close the searchcoil.
One more thing I found - one extra wire was coming out from the RX, which could Be the XP way to finetune the Null and phase. Strange was that they was place in the middle of the searchcoil, but Thats properly the only place they could find space !
This extra wire which was hard to locate and see If it was made like a small loop had no sondered place, it look like they had taken some 10-20 cm wire from one of the first Winding of the RX, so they could finetune in the end... These was in same level with the top end shielding paint.

Any idea why they used conductive plastic and grahite shielding and what the bennefit is, of using both types ???

Pictures not perfect, but hope you can see something usefull.

I would Be great If you had some good ideas, I could use in the work to make no.2.

Henrik.

[Skippy]

The ideal Faraday shield would completely surround the coils, no gaps. Just doing it on the bottom surface is not really good enough, though may work. So it looks like you have conductive paint above and below the coils, I guess there is some around the edges somewhere.

Why conductive plastic for the case? Well, regular plastic is a VERY good insulator, and I imagine static electricity effects ( eg. wet grass) can cause localised strong electric fields. If the case plastic conducts, the static effect will be less, and it will be spread out, too (just my opinion). This makes life easier for the internal shields.

The bare silvered wire probably helps the graphite do it's job. If you make a wire connection at one point only, the furthest bits of graphite will be more ohms away. running a near-zero ohms wire around means that no point is 'bad'. The detector will not see a 0.2mm diameter wire at all, you could have 1 metre of it running back and forwards around your case, no problem - PROVIDED you don't make a loop.

And, as I suggested earlier, there is likely to be some nulling loops, and such-like, I think you can make a coil without slavishly copying their methods.

Hopefully others with more coil-building experience will have ideas.

[Henrikras]

Thanks Skippy... One reason that my first attempts didn't work better, could Be 1-3 kilo ohm shielding Ive made ! Knowing now that XP used so low shielding resistance at the Stock coil.
Another thing, could Be the connection shielding wire and cold RX side, it should maybee had been in the coil, and not at the board.
One thing I am wondering ! How can the shielding Be closed, and not making a loop ? I must investegate the Stock coil more at the middle of outer.. It could Be the case that the painted graphite isn't there or high resistance. ! What I can see, the resistance go higher but not totally off.
A lot to think about.
Henrik.

[Skippy]

You have to be careful when you talk about resistance of graphite shields. You can't really say much about the resistance when you're just prodding it with a meter in two points. The only way to scientifically measure surface resistivity is with fancy probes that have a point in the middle, and a complete ring around the outer as the second electrode - and they only work on flat surfaces and need fancy calibration charts. I tend to favour George Paynes figure of 1 to 5 K Ohms per square. Maybe if you do some Googling about surface resistivity, you may be able to estimate what resistances you would see on a complex surface.
Q:"How can the shielding Be closed, and not making a loop ?"
It does make a loop. Any continuous sheet of something conductive makes a load of loops. That's why you don't want your shield resistance to be too low, it can then be detected, and affects performance. But if the surface R value is high enough, the currents are low, and the drop in performance is minimal.

[golfnut]

In line with what Don mentioned about having the Tx gnd and Rx gnd effectively separate until they get to the box - this is true of my observations of NEL, ML, sunray as these use 2 coax's. one for tx and one for Rx -- the carbon/graphite screen or metallised screen drain wire goes to the Rx coax braid.

The Tx cold is on the Tx coax braid. At the box depending on the Tx design, the coax braids are either coupled together on local pcb gnd or ac coupled together using a capacitor.

On the xp coil , I guess the resistance is so low, as the shell is carbon, and the potting is graphited too, one inside the other plus decent drain wire.

[Skippy]

Golfnut: Do you think it would be a good idea if Henrik used two-core screened cable for his TX coil connection, joining the screen to the TX cold wire at the PCB (or connector).
Regarding conductive plastic: just for fun, I measured the surface resistivity of a sample of black conductive ESD plastic bag. It was 300 kOhm per square, about 30 times too high to be much use, I suspect.

[Henrikras]

Cable ! Remember that the Stock coil with cable was working perfect..!
I just made some measurement at the stock coil shielding.. - 1.5 Kohm from front to back (longest way) but allso 1.5 Kohm from right to left... It seem that it is the same all over. Only If I go Very close together with the pins close to the wire I can measure low resistance.
picture of the cable

[Skippy]

Ok it was just a suggestion about the cable, based on what my Fisher has - a single-core screened for the RX, and a two-core screened for the TX. Then the two are sheathed in plain silicone rubber / similar.

[Henrikras]

yours properly a better cable Skippy.. If you get time some day, a link to buy the cable would Very help full. Uk link or Ebay or the type so I can search it.