Measurements for GP Extreme 1800 coil?

[bklein]

Hi,
My Gp Extreme did not come with an 18" coil so I bought one off Craigslist.
I am underwhelmed with it in comparison to the 11" stock coil. Perhaps it is bad?
Measurements:
shields pins 1/5 to pin 2: 27 ohms, 381uH
shields pins 1/5 to pins 3/4: .39 ohms, 287uH
Pin 2 to Pins 3/4: 27.5 ohms 682uH

I was expecting a huge improvement in sensitivity to say a quarter but it is not the case.
A small nugget (<1/4" dia.) is not detected.
Is this normal for this coil? Is it designed then to just find deeper larger things?

[bklein]

To add to this, given that older SD coils may not be appropriate for the GP and above detectors, why is this? Why would a coil be better on one PI detector and not another? How do we know what the best inductance or resistance will be for a given detector design?

[Mechanic]

Hi Barry,

How does the signal received on the receive coil compare to the 11" coil that you repaired? I saw in the other thread that there is a decaying sine wave signal on the rx coil. This does not sound right. The rx coil should follow what the tx coil is doing, but the flyback spike should be much smaller, due to it being an induction balanced coil. As far as capacitance goes, the coil needs to be settled(front end fets on and preamp settled) before the first sample is taken. Check the test points for the front end and find where the earliest sample is being taken by testing the control signals for the demodulators.

How does the coil go when you set the coil switch to "E"? This should be mono mode. It should be more sensitive than in "N" You would be better off with a dedicated mono coil IMHO.

Cheers Mick

[bklein]

Hi Mick,
My testing with the scope was on the Surf PI board but too crude - I used it to drive the TX coil but RX signal was floating when I looked at it and saw the decaying sine. Does the RX coil circuit have a damping resistor at the coil? I'll go back to the 11" as you suggest and get comparative scope shots. But reflecting back on the experience with the second 11" coil I had that may not tell me enough. In fact, this 18" is a lot like that 11". I don't get how it supposedly work better on another detector than this one. If capacitance is the issue it shouldn't.
Are guys actually taking these DD coils and using them with the Surf PI? How is that done?

[bklein]

Attached are two files. First is a shot of the TX with RX of both 11" and 18" coils but with 750 ohm damping resistor on RX at the connector. The next one is without the RX damping resistor. Here you can see that the 18" has a lower frequency "ping".

[bklein]

Ok I don't get this at all. I'm probably making a stupid mistake.
I ran my Wavetek frequency generator through a 1M series resistor across the each coil and I really don't see anything with a scope. The signal is too small across the coil.
I replaced the 1M resistor with a 10K and I get something to work with.
Then:
11" good DD coil, RX(pins 1/5 to 3/4): 439.9khz resonance peak frequency
11" "bad" SD coil, RX: 439.9khz
18" new coil, RX 473.5khz

So if anything the 18" should be better.
Have you guys actually tested Minelab SD/GP coils this way??
I did not use a 1pf cap in series with the scope probe but I doubt that's a factor. My scope probe capacitance is 12pf.

[IBGold]

Hi I have a match box made up that I plug the sig gen, cro and coil into to sweep the coils for resonance same as bbsailor describes in his coil building article but I have mine switched so as I can sweep both TX and RX coils on induction balanced coils as well as mono coils it is a must have when making coils.

Regards, Ian.

[bklein]

Ok, so what am I doing wrong? Looking at BBSailor's article, his 1pf cap is an open-ended twisted pair wire set. Is that really so critical? Is the 12pf of my probe killing what little signal there is? So you are getting a signal even on the TX coil? Huh.
What is the signal amplitude of your signal generator?
I think I read an older thread where you were experimenting with coils between the SD and GP series and seeing vast differences in performance. Was this mainly due to inductance? Why would the poorer coil on a GP work better on a SD?

[Mechanic]

Hi Barry,

Can you take some scope shots from the testpoints at the end of the front end stage from the gp? Set the coil switch to "N" this will show the amplified output of the rx coil. If there are any problems with the coil it will be evident here. Also do a shot with the coil switch in "E" and cancel.

There is a damping resistor for the rx coil in the coil housing where the main coil lead is soldered to the rx coil windings. Make sure when you are testing the coil that it is up off the floor(on a box, or the missus' plastic washing basket, psssst it also makes a nice coil winding aparatus!!!!)

Cheers Mick

[bklein]

Hi Mick,
To be honest, I'm shudder when thinking of opening the Extreme back up now that it's working ok - plus I'd have to look back at all your writings to see where those test points are... I'd rather make up or buy some apparatus that does a good job testing the coils I come up with. Something like this?: http://www.geotech1.com/forums/showt...nant+frequency
I also put up a separate thread asking about using a Dip Meter with home made low freq. coils.
I wonder if the internal damping resistors are screwing up the commonly accepted 1M series resistor to signal generator approach. Perhaps it is just that I need to incorporate the 1pf twisted pair wire. I'll try that next.
On top of all this, there is something about my housing tract AC wiring that really messes with detector signals. The AC system is all underground in the front yard somewhere. To get my detectors quiet I have to walk up the street to a park. The system is over 27 years old - I wonder if something's out of spec and we are getting blasted.

[bklein]

Hi Mick,
Got some time to try the 18" coil again during my lunch hour.
Hmmmm... Wow, you're right! The E switch setting is waaayyyy better than the N normal or Cancel. Same goes for the SD 11" DD coil. Weird - that is supposed to be the mono switch position. I didn't bring my nugget to test.

I get some variation if I move the coil cable's connector at the box. Is this just ground variation or indications of a bad receptacle? It happens with any coil (and its cable).
Does this normally occur on the Minelabs?

Barry

[Mechanic]

Hi Barry,

Yes E is the mono position. For an 18"DD it would convert it to a 17"X9" elliptical as far as sensitivity goes. It will be alot more sensitive in mono. If you go detecting try running in E and see how you go. You will have to ground balance more often most likely. Do some comparisons with a range of target between DD and mono, you will be surprised!


As far as the coil plug wriggle noise goes, yes thats the norm!!! Thats why I suggested bridging pins 1&5 as well as 3&4 in another thread. The noise will be reduced considerably.

Cheers Mick

[bklein]

Ok, I guess I just had an incorrect assumption that a DD coil would have better sensitivity at N because that is how the manual suggests to use the coil. There is such a great difference between the original 11" and the SD DD 11" and GP DD 18" that I just thought they were bad. The original 11" is just a bit hotter for some reason.
Now if I am to go about making a 18" mono I need to make a separate TX coil. On the original 11" I think that was made of litz wire. I have some litz, but a many strand version. Are guys always using litz for the TX, or other wire?
Then RX can be made of the (spiral) twinlead speaker wire I'm thinking.

[mario]

Hi Barry,

without getting too much into details, no, the old stock SD coils are not the best performers.

I'm not sure what makes them bad, but do know that there would not be a thriving aftermarket for Minelab coils had Minelab included decent coils with their equipment.

I have managed to improve performance by halving the timings and increasing the flyback voltage, but even then those coils remain very heavy and cumbersome to carry.

Unless they're Commander coils, you may want to stay away from them.

[Mechanic]

Ok, I guess I just had an incorrect assumption that a DD coil would have better sensitivity at N because that is how the manual suggests to use the coil. There is such a great difference between the original 11" and the SD DD 11" and GP DD 18" that I just thought they were bad. The original 11" is just a bit hotter for some reason.
Now if I am to go about making a 18" mono I need to make a separate TX coil. On the original 11" I think that was made of litz wire. I have some litz, but a many strand version. Are guys always using litz for the TX, or other wire?
Then RX can be made of the (spiral) twinlead speaker wire I'm thinking.

Hi Barry,

If your making a mono coil then there is only one coil, the tx coil. The detector receives and transmits on the same coil, so the tx is also the rx.

As for the requirement for the litz I think it was 32 strands of .2mm(diameter not awg)enamel coated copper, wrapped with teflon tape or something else with a low dielectric constant. For the strand count look back to where I first mentioned it, I think it was in another thread, as I had just counted it before posting the number of strands. This time it is from memory and I think it is wrong.

It is important to get the wire size and strands correct otherwise the coil you build will not work correctly.


Cheers Mick