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Hello guys. I have a question about nulling a concentric coil. Exactly what wave forms am I looking for on the oscilloscope as far as the receiver goes? I have seen lots of pics of nullling a Double D coil but none of a concentric (at least from googling it and searching the forum)- does it look the same on the scope? Lots of instructions on how to build one though. Any help would be greatly appreciated. Thanks
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Well I'd imagine nulling is nulling. Your looking for a "null" or minimum signal out of the RX. On concentric
you null by adding winds to the bucking coil. That coil sends TX power in a contra direction so reduces
the amount of TX the reciever sees. The big difference is in phase shift. On TGSL the DD gives 20 degrees
and the concentric gives 200 degrees but I don't think you can change that when making a concentric coil... -
Thanks- thats what I figured but before I cut my coil open to re-balance it I wanted to make sure I do it right. I hear its a pain to do. I did think there was a difference between the phase shift of a DD and a CC wasn't sure what it is. Never built my own coil- my hat off to those who have. Sadly I really don't have the time to get that involved. I have the connectors coming in a soon so hopefully I'll have time to get to it soon. I plan on making a nice box up so I can balance the coil without taking the Nautilus apart. I've already modded the rod to something I feel is better than the common Whites set up as its adjustable and balances the weight better. Its a nice detector but it does have its quirks. If I can start re-balancing my own coils it'll be great not having to send it in every 2 years (sometimes even more often).Comment
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I'm not sure how this will work at a CC coil, but at some test I did at DD coils, its possible with a small piece of ferrite to finetune the Null. Front or back end at the DD coil, in the area the RX and TX are crossing eash other, a small ferrite can change the balance.
This isn't my idea, I came in another thread some time ago. If you can accept a glued piece of ferrite outside, you don't need to open up your searchcoil.
Henrik.Comment
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Oh your re-nulling a completed commercial detector? I wouldn't imagine they would open the coil to do that.
A water proof coil has got to be glued and you'd have trouble sealing it again...
Most likely they check the residual RX and just change a tiny tuning cap if it's off.
Oops I checked the schematic and you have a preamp in the coil so they would have to open it to rebalance it.
I just bought a Bandito that was made back in 1990 -91 and it's never had it's coil rebalanced and it work's great...Comment
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You can try a small piece of ferrite on an enclosure surface before opening it. Most probably you'll find a position that nulls a coil just right.Comment
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Do you have a working set up?
If yes, you wont improve it by opening it up and altering it.
If it currently rejects ferrite, iron, and accepts ally, gold, copper alloys, silver with single beep on pass - then the phase alignment of the Rx signal WRT to the Tx signal phase and GEB and Disc CH gating is as good as required - and the null value is just what you get when everything else is good.
If you have exceptional air test and twitchy on your ground, it may benefit from a mild detune of Rx tank, but watch the phase change..
If its a little deaf, moving the Rx tank closer, no more than 80% of Tx freq for most soils will help, again watch the phasing.
The null value does not directly influence performance on a 2nd differential design as the residual signal has no rate of change associated with it, so never gets thru to trip a tone so doesnt cause issue. ( I did months of detecting with a 2v null and found loads of tiny roman and the odd medieval 0.6g hammered etc )
You do have a rate of change of residual signal on power up, none to some, thats why you often can hear a beep on power on.
The phase is the priority, null less so. Maybe not true for all architectures, but true on IDX, ace250, tesoros,classic, etc etc SComment
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Thanks Davor. Yes you are correct it would work but would probably lose sensitivity from it (not to mention I really like to do it right). I once bought a used coil and it had to of had nearly $1 worth of change hot glued inside to rebalance it. I only found out after I used it for some time and a quarter came off and was rolling around inside of the coil. I was not very pleased with that and wish the guy had told me that he had "balanced" it that way. From what I gather it is best to take the bottom cover of the coil off and renull it. I have had it off before- its not hard to do. Just trying to figure out when I do hook it up to the scope how the rx signal should look compared to the TX signal. I gather that the closest to 0v the better. I will not be taking the coil completely apart- there is a small wire that from what I understand is part of the feedback or bucking loop and you unglue it move it around until it looks right on the scope and then reglue it in place. Just trying to figure out how its supposed to look on the scope.
Thanks Golfnut but the Nautilus IIB coils usually go out of balance every couple years- pretty standard with them. I hear its because they run at around 44volts to the coil and the slightest change in the coil from aging or being bumped around can affect them. Due to that they made to take apart (cut through the silicone caulking on the seams and carefully pry apart). There is really no option to do it right but cut the coil open and renull/ balance it. This is not an improvement but a repair. The coils are unuseable until they are rebalanced. It is very different from a "standard coil." I hear that Whites V3 coils are being returned al ot lately - and they only run at 30 volts max on the coil. There is a little wire (probably part of the buck coil) that while looking at the oscilloscope you unglue and move around then reglue in place. You do not disassemble the whole coil- just the bottom cover to get access to that wire. The factory did it all the time when they were still open and it is a very common procedure. I realize you know a lot more than I do on designing and building detectors but this is apparently not a common procedure with most detectors that you are familiar with. I do not intend on modifying the circuitry of the Nautilus as there is nothing wrong with it. According to the manual when the R/C knobs go beyond 9 and 3 oclock positions the coil needs to be rebalanced/ nulled- as it is now out of balance.
Its a great detector but its like driving an old car- it requires a lot of tweaking and fiddling sometimes and even still on the rare occasion is tempermental and refuses to work correctly (its the coils- usually the 15in one which is the largest they made and with the high voltage that seem to have issues).
Even on a bad day it cleans up behind my Etrac (so long as the coils can be balanced at some voltage). It hits deeper and Ids correctly. I dig primarily relics on old sites so anything large or nonferrous I dig. I just dug something that my Etrac said was junk- and old lead bottle top from the early/ mid 1800s in the shape of a ring. On the Nautilus it rang out loud and clear as something worth digging and that runnning at 20v on a 15in coil in a trashy area (not the worst but a lot of old nails from the same period- its a building site).
Sadly right now one 15in coil will not balance above 25volts or so and the other is out of balance period (I have 2 of them). I'd like to get them working correctly again soon. The Nautilus really shines with those 15in coils- without them its not much better than the T2 (great detector just not as deep). My only other coils are an 8in and a 6in. I suppose a 10in would still be competitive but I don't have one so its a moot point.Comment
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Yes I don't think the coils properly speaking are "water proof." Thanks for the suggestion. I have one coil I can't balance above 25v (supposed to go up to 44v). Maybe a cap is dying a slow death on the preamp. I will have to look into that. That coil is pretty old.Originally posted by Silver Dollar View Post
Heres a post from another forum that shows what the coils look like on the inside and gives a basic desciption of what needs to be done to balance it. Hope its okay to post it- please remove if not.
http://www.findmall.com/read.php?39,2019879
Yeah Tesoros are a great detector I hear and thankfully most detectors don't need the coils rebalanced. Can you imagine the industry if everyone had to send their Aee-250 in every 2 years to have their coils rebalanced? lol. In my opinion the Nautilus is a great detector but most definitely not a turn on and go. But if you like having options (unlike dodads) then it can still hold its own with the best of them. One guy claimed he even found a 30lb shell 4 1/2ft deep. How many short of a PI and a 2box can do that?Comment
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hi nm, and all, yes these detectors do need the coils retuning if they go out of whack, though they are no longer produced the official uk supplier still has new stock and rebalances the coils.
there are ways round this though, you could build a dd coil to the right spec, and i have heard of modifying coils off of another machine but cant remember what, a little digging around might help there, the thing i always found strange about these machines was that the simplified machine actually cost more than the full bells and whistles version.Comment
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reading your other thread rang a bell, on the russian pics, there are two Nautilus coils large and small, at the side is a tiny older garrett coil, this jogged old memories about small coils on the Nautilus, if the small ones work just find the larger garrett counterparts if the small ones work so will the large.
you need to identify the 5" garrett coil in the russian pics, then look for its larger version, they may not be as spot on as the original, but they may work well enough and wont need nulling every 2 years.
hope this helps.Comment
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