ML EQUINOX coil discussion thread

[EL NINO]

My experience with the multi-frequency Spectra V3 and the 13 "ultimate coil balance - with the help of the Signal Probe function - implemented in Spectra - showed that the coil, which is heavily balanced with a few 50 eurocent coins, has not lost sensitivity or reach in field tests. range was quite comparable to another 13 "coil, and better than a 12x10 sef coil ..



The initial value of the coil balance in the signal probe was above 65% of the signal at RX gain 9 ...
After adjustment, the coil balance has improved by more than 2X. The coil could then work on the maximum Rx Gain 15 ..




Field tests have shown the correct sensitivity even the depth of the 13 "coil ultimate ..






I would like to say that the work of balancing the coil at the multi-frequency has its meaning .. ,, and I think it will mainly manifest in more mineralized terrain, or in the separation of rough iron ..- where it is more resistant to overload ,, ...So each coil when working above its signal-before overload .. will identify non-ferrous objects as iron...

[nordic]

Thanks Skippy, much appreciated input. I don't call it an improvement, as I don't know how it will perform, but will give it a go. Kinda like this empirical stuff. I have no doubt ML squeezed most if not all from the setup.

I haven't noticed any drop in distance during air tests, sure that is not on real ground, so time will tell. What I have noticed, that the ID would come sooner as soon as I hear iffy sound, it gets the TID rather quickly and close to real value. Without nulling, TID would jump around until object gets close enough. Gold modes on 800 were quite unusable to me due to background noise, but it is now very faint, that is actually nice. It looks like Gold modes feed audio directly to ears.
I've tried all frequencies, all of them seem to have dropped significantly, only 40k has the largest Vpp out of all, but it is multiple times smaller than before. As you have suggested, I have tried smaller ferrite pieces I had, but they could not bring Vpp low enough, perhaps it's just the distance from the coil.

[Skippy]

I think it's correct to state that ML have nulled the coil correctly/adequately, from the factory. Any "Improvements" you make will have no performance benefits even if they appear to give a lower signal from the coil
Adding ferrite ( and non-iron objects ) can affect the null, but you have to consider the wide freq range. It's no use improving the 8.2kHz null if the 39 kHz null gets worse, etc. If you have the 800 model, you can choose all the freq range, from 5k to 40k, to try out various nulling techniques. Owners of the 600 model are restricted in this sense.
And I have to say - that effing big lump of ferrite is not appropriate. The pieces I've used on other coils have been 6 x 6 x 2 mm, it matters where you place it, of course, but in general, the less you use the better. And I should remind you that the coil needs to be kept away from all metal objects when trying this kind of nulling. No point nulling out the legs on your workbench.

When we eventually do try and make our own coils, this 'bits of metal' nulling technique has some use.

[nordic]

Need someone's educated opinion. Is there any drawback nulling the RX coil using a small ferrite attached to the RX coil? Here are few screenshots of the scope before and after finding a certain place for the ferrite.

What I see with air tests, that control box is a lot more stable with TID even significantly away from the coil. Haven't done any real ground tests though. Or is this all just worthless time waste...

The other thing noticed, gold modes have extremely low background noise.

Many thanks!

Default, multi freq


Moving ferrite around to get the lowest Vpp


End result

[dbanner]

I played a little bit with the 6 inch coil yesterday. I've noticed that the VDC cable has a lot of ripple in it, around 60-70mV, not sure whether it is due to crosstalk or sagging, but it was contaminated with the TX coil signal. I could bring the noise down to about 10-15mV with some filtering, but oddly, all targets on control box started coming in the 11-13 range! The other thing, I tried change coil voltage to 3-4V, but on extremes it subtly reduced the air test depth, the best distance I was getting at around 3.6-3.7V, which is a battery voltage...

Perhaps there already exists some filtering in the control box that takes care of that tx noise. But also consider mismatch of impedance.

[nordic]

I played a little bit with the 6 inch coil yesterday. I've noticed that the VDC cable has a lot of ripple in it, around 60-70mV, not sure whether it is due to crosstalk or sagging, but it was contaminated with the TX coil signal. I could bring the noise down to about 10-15mV with some filtering, but oddly, all targets on control box started coming in the 11-13 range! The other thing, I tried change coil voltage to 3-4V, but on extremes it subtly reduced the air test depth, the best distance I was getting at around 3.6-3.7V, which is a battery voltage...

[nordic]

Thanks Skippy. The sound from the control box is way before TID is displayed. Changes are tiny, it was even reacting to the scope probe somewhat.
It is interesting to see that when TX circuit transconducts, coil oscillates and there is a small spike on the RX coil.

[Skippy]

The divider is two 1k8 resistors, with a (probably quite big) cap to GND for decoupling, this is brought out to PCB pad 'REF' , and connects to pin 7 of the dual opamp. Pin4 of this opamp is the -ve supply, connected to GND, pin10 is the +ve rail, to 3V3. The 'power-down' pins, 5 & 6 are both tied to +3V3.
Here's the datasheet for the opamp ( if Foma is correct ):
Texas OPA 2835:
http://www.ti.com/product/OPA2835

The VSSOP package is the one used.

@Nordic: You won't see the smallest detectable signals on a scope, that's for sure. On a typical single-freq machine with plain coil, changes of 5 microvolts will be detectable. If this coil has a pre-amp gain of about 15, then maybe 50 microvolts out of the amp may be detectable, just a guess.

[nordic]

I just hooked up the coil and control box to the oscilloscope and can see it registers items and displays TID when RX amplitude changes within 1-2mV at sensitivity 20. Can barely see the change as RX idles at 0.3-0.5V Pk-Pk and is biased.

Nothing too new really, just showing. I was interested if ML have exhausted room for sensitivity in analog domain.

Square is TX signal, triangle is RX.

[dbanner]

In the schematic posted, if the 2835(2) is supposed to be the split rail supply op-amp, then it looks strange to me. Shouldn't both 1.8K's be connected to pin 7 to form the voltage divider? I think the schematic should look more like this.

[Skippy]

Some analysis of the TX coil, based on measurement, not by dismantling the winding:

Measured L = 119 microHenry ( 0.119 mH )
Measured R across free socket pins = 448 mOhm
Resistance of screened cable outer = 50 mOhm
Estimated resistance of screened cable inner = 50 mOhm
So ... resistance of coil winding = 350 mOhm approximately.

For the stock 11" coil, the TX winding measures up with a circumference of 640mm, equivalent to a circular coil of R = 102mm.

A bit of work with the coil calculator suggests the number of turns is N = 15T , total wire length = 9.6 m.
To get the correct resistance, a wire equivalent to 0.8mm diameter is needed. R = 36 mOhms per metre.
Common Litz wire is based on 0.04mm, 0.05mm or 0.071mm diameter ECW.
0.8mm diam solid is equivalent to (400 x 0.04mm ), or ( 256 x 0.05mm )

See here for some possible choices, such as ( 243 x 0.05mm ):
https://www.wires.co.uk/cgi-bin/sh00...00710090_2d100

For a theoretical 17" round coil, no. of turns = 11T , needing about 11 metres of 0.85mm diameter equiv. wire.
And a 5" round would need 25 T , length 7.5 metres, of 0.67 mm diameter equiv.

So that gives a ballpark figure of a typical TX winding - 10 metres of 0.8mm equiv.

[nordic]

Legend!

[6666]

Pinout update:

Pin 1 : TX hot (Shielded wire inner conductor)
Pin 2 : TX cold (Shielded wire outer)
Pin 3 : 3.3 V DC power (Brown)
Pin 4 : RX + or - (Red)
Pin 5 : RX - or + (Blue)
Pin 6 : NC ?
Pin 7 : 0 V ground (Green)
Pin 8 : Data (Yellow)

The 3V3 supply is split, probably with just a resistive divider, to give a virtual ground of about 1.6 Volts.
The RX outputs are biased at this 1.6 V level, the AC signal added to this. So one increases in voltage while the other decreases, etc.

Notice the revised TX hot/cold wiring, I got them the wrong way round in my original post.

I made a little breakout cable for experiments. Pic attached.

neat

[Skippy]

Pinout update:

Pin 1 : TX hot (Shielded wire inner conductor)
Pin 2 : TX cold (Shielded wire outer)
Pin 3 : 3.3 V DC power (Brown)
Pin 4 : RX + or - (Red)
Pin 5 : RX - or + (Blue)
Pin 6 : NC ?
Pin 7 : 0 V ground (Green)
Pin 8 : Data (Yellow)

The 3V3 supply is split, probably with just a resistive divider, to give a virtual ground of about 1.6 Volts.
The RX outputs are biased at this 1.6 V level, the AC signal added to this. So one increases in voltage while the other decreases, etc.

Notice the revised TX hot/cold wiring, I got them the wrong way round in my original post.

I made a little breakout cable for experiments. Pic attached.

[nordic]

Hey guys,

Upon looking at the schematic and the x-ray photos, aren't amplifier outputs connected to the Tx side of the coil? It says Rx? Judging from the x-ray photos, the thick shielded wire is connected directly to the Rx coil and is fed right into ADC though low pass filter in the control unit? Looks like the amplification stage for the Tx is moved into the coil itself and is fed with low level signal, but it is also unshielded. There is probably no need to shield low level Tx signal because it get smoothed out by the coil and the ground...

I don't think any other coil will work, even if one defeats the coil ID. It is an active coil.

It is probably too much to ask. Is there a good picture of the coil board and/or which contacts correspond to which pins in the connector?

Tremendous work trying to reverse engineer it.