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Post  Prospector on Sat Jul 06, 2013 6:12 pm

Hi all, I have picked up a GP Extreme later model 70,XXX serial number. Just watched JP's DVD and been reading every bit of help I can. I have just done a few air tests with some lead sinkers 4g,7g,11g & 31g. Using the 11DD stock coil the max I can pick the 4g one is about 8" and the rest about 12". I also tried them with my 14DD coiltek and barely got anymore depth at all. Does this sound normal? I have also tried playing with Deep mode+Sensitive which helps by maybe another inch. I thought the 14DD would have a lot more on the stock 11DD.

I know air testing & lead are different to gold but i'm sure most people would have done similar tests when they pick up a machine for the first time.

The other issue is my 18DD seems to be dead, i.e won't work at all. If I put the Rx into Enhance mode it works. Can anyone tell me what pins are what in the coil plug so I could measure it, hoping its just a wire broken off.

Cheers

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Post  davsgold on Sat Jul 06, 2013 7:34 pm

G'day Prospector

Here is a link to a diagram for the wiring of the coil plugs for both DD & Mono coils. Scroll down to the 4th post for diagram.
https://golddetecting.forumotion.net/t3614-minelab-dd-and-mono-coil-plug-pin-wiring-diagram?highlight=wiring+diagram+for+coil+plug
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Post  Prospector on Sat Jul 06, 2013 9:53 pm

Thanks very much for that link. I'll meter it out tomorrow. I hope i've just lost a connection on the Tx side of things. When the rx is in enhance (psuedo mono) it only uses the rx side yeah?

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Post  davsgold on Sat Jul 06, 2013 11:00 pm

DD coils in pesudo mono transmit on the whole coil and receive on half the coil. This is what makes the coil sensitive on one side of the coil only when hearing the target.

A broken wire somewhere along the cable or just an unsoldered joint in the plug perhaps.

cheers dave
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Post  Prospector on Fri Jul 12, 2013 11:28 am

Hi guys finally doing some checking on the coil between pin 1&2 im getting 520 ohms... seems a bit high for just a straight coil. Around 6 ohms on 5-4 & 5-3. Can anyone help me out here, should it be so high? Cheers.

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Post  CostasDee on Fri Jul 12, 2013 12:25 pm

I just tested 2 of my 11"DDs and got the same results for both, which seem quite different to yours. 1&2 gave me about 15.8ohms and 5&4 (5&3 also) gave me 0.5ohms.
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Post  CostasDee on Fri Jul 12, 2013 12:30 pm

Ok, just tested my Coiltek 18"DD and that gave me between 1&2 6.8ohms and 4&5 0.6ohms. My 11"DDs that I tested before are both GP series Minelabs.
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Post  Prospector on Fri Jul 12, 2013 12:56 pm

Thanks for checking that, something is def up with mine then. Not in the plug Sad just been stripping to coil down. Looks like all the wires are laid inside a carved out channel inside foam & then filled up with a resin. Hmmmmmm. I guess I could start cutting the cord from the plug and hopefully its inside the cable then I could join it.

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Post  Prospector on Fri Jul 12, 2013 1:14 pm

Hi Again have just pulled it all apart and found a 510 ohm resistor across the pin 1&2 winding. Guess it's meant to be there. Doesn't explain why my coil wouldn't work though.

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Post  CostasDee on Fri Jul 12, 2013 1:34 pm

What resistance are your other DD coils giving?

If the 510ohm resistor is in parallel, ie across the 1 & 2 terminal and the resistance of the winding is normally about 10ohm (15.8 + 6.Cool, then by having the resistor in parallel it should infact lower that 10ohm reading to about 8 or 9 ohms. It sounds like it's in series and then the 510ohm would add to the winding resistance of 10ohm making it about 520ohm. I still don't understand why your's has got the resistor there when it adds so much resistance as my coils are far lower so obviously don't need it.
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Post  CostasDee on Fri Jul 12, 2013 1:38 pm

PS.. I wouldn't be cutting the cable just yet. I'd be reading what my other coils give as a result and if they're similar to my results, I'd be unsoldering the resistor and seeing what results you get then. If the results get to be similar to my results, then I'd be retesting the coil on the detector to see if all's well now.
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Post  Prospector on Fri Jul 12, 2013 1:54 pm

Yeah Its definitely in parallel. I have determined that the fine wire must have break in it somewhere in the epoxy. I stripped it back just near where both ends come out of the epoxy and can't get any continuity. Darn.... had a quick look but couldn't find anything obvious. These things weren't meant to be pulled apart.

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Post  CostasDee on Fri Jul 12, 2013 1:58 pm

These things weren't meant to be pulled apart.

lol!  Oh you can pull them apart quite easily, getting them back together is a different kettle of fish.... lol! 
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Post  Mechanic on Fri Jul 12, 2013 11:20 pm

G'day Prospector,

STOP!

The coil windings are made from insulated litz wire, just peeling back the insulation and trying to measure the resistance will not work as each copper strand is enamel coated. The problem will not be in the coil bundle. It will most likely be in the cable where the strain relief clamps it as it leaves the coil housing. To find the exact spot where it is broken, just pull on the wires back into the coax, if the strands are broken they will just pull out. Cut the coax an inch or so past that point and re-connect to the coil inside the coil housing where the cable originally joined and use minimal solder.

Also see this thread for some other hints and tips I have given on coil repair. https://golddetecting.forumotion.net/t15979-11-commander-mono-coil-problem

And as for the resistor inside a DD coil, yes it is in parallel and it is necessary. It is the damping resistor for the DD receive coil. Without it the receive coil would oscillate after the tx pulse and not be usable.

Cheers Mick

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Post  Prospector on Sat Jul 13, 2013 7:26 am

Hi Mechanic,

I understand the wires enamel coated however measuring directly across the resistor still only gives me 510 ohms which means there isn't anything else connected. If the coil was there in parallel it should surely be less. It came second hand to me, I'm guessing it's had hard life.

Thanks

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Post  Mechanic on Sat Jul 13, 2013 12:41 pm

Hi Prospector,

Ahhh, my bad! Very Happy Yes if you are getting 510 ohms across the damping resistor with it still attached to the coil windings, then the receive winding must have a break in it. Very odd as they are potted.

Cheers Mick

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Post  Prospector on Sat Jul 13, 2013 3:37 pm

Yeaha its knackered. I have a teracotta coiltek 14"DD which should be a worthy replacement. I've noticed a couple of small splits along the edge where its coming apart, what is a suitable glue to seal it back up & tape to retape the scuff plate back on with?

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Post  Mechanic on Sat Jul 13, 2013 4:20 pm

Hi Prospector,

I assume you have the skidplate off the coil? To repair the join between the top and bottom housing use good quality superglue.

Cheers Mick

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Post  Prospector on Thu Jul 18, 2013 7:26 pm

Hi guys, can any experienced extreme operators share some gold finds they've had in the field. Eg. Gold size, depth and coil used? I'm interested in knowing what my machine should be capable of. Cheers.

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