ML EQUINOX coil discussion thread

[dbanner]

Upon further examination of the schem, it appears as though the rx emf is intended to swing between?????

[dbanner]

There are some obvious problems with the schematic.

A cursory look at the schem points out some obvious curiousities, for example, why is the positive rail connected to the non inverting input.

[6666]

Hi Skippy, did that broken blue coil wire get repaired by you, did it come to you broken ?

[Skippy]

I wondered if it may be this part:
https://eu.mouser.com/Search/Downloa...7A-1220769.pdf
But there's a fair number of these devices, from several manufacturers, and it's not important, really, it's the analog stuff we need to reverse-engineer properly.

I'm wondering about the SOT23-6 device attached to the differential output. It could be a protection device, to limit spikes on a long cable. Something with diode clamps, transzorbs/zeners that limit voltage pulses to the 0V and 3.3V supply rails of the device. I've noticed them quite a lot in commercial equipment.
These type of things:
https://www.littelfuse.com/products/...rotection.aspx

[Altra]

Something like this might be the security device? Data sheet states that package markings are intentionally non-part related.

http://ww1.microchip.com/downloads/e...-40002025A.pdf

[Skippy]

Yes, Carl, plenty of errors, we can work on them. The two diodes on the front-end are probably between the bases of the npn transistors.
I think there's a virtual ground in there, probably what's labelled on the PCB as 'REF', though not shown on the schematic.

[Carl-NC]

There are some obvious problems with the schematic.

[Skippy]

There's been some previous chat about the workings of the Equinox, including the coil, in this thread:
https://www.geotech1.com/forums/show...ineLab-Machine

Of particular interest are the X-ray images of the coil, that have been posted on the MD-Hunter and MD-Arena blogs:
https://md-hunter.com/minelab-equino...lf-of-machine/
and
https://translate.google.com/transla...o-polovina-md/

Points worth noting from these images include: The TX winding is multi-stranded, probably Litz wire. The RX appears conventional, a single strand of wire.
The electronics is all contained inside the coil, there are no extra parts, like 'security/anti-piracy' devices in the coil free-socket ('plug'). It would be sensible to assume the circuit board has anti-copying/identification device(s) , and a signal pre-amplifier.

Thanks to the great work of Foma, it seems the circuit is just a pre-amplifier, and a security device. They are distinct functional blocks, so could be seperated. The security parts could be fitted in the coil socket, or in a short extension cable. The pre-amp alone fitted in the coil. Multiple coils could then be fitted onto one 'security dongle'.
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The connector and cable:

These are a common connector, usually known as the " M12 " series, the Equinox using the 8-pin variant. The control box has a chassis-mounted male plug, the coil has a 'free socket' with female contacts. Pins are numbered 1 to 8, but don't seem to be commonly marked on the actual connector. The pinout diagrams are attached below.

--
The cable ML use has a single-core screened cable, with a grey outer sheath, and 5 single insulated wires: brown, red, blue, yellow, green , for a total of 7 connections. There are no additional drain wires, foil shield, or braided shield. Photo below of mouse-chewed cable:
------

Pin 1 : TX hot (Shielded wire inner conductor)
Pin 2 : TX cold (Shielded wire outer)
Pin 3 : Brown
Pin 4 : Red
Pin 5 : Blue
Pin 6 : NC ?
Pin 7 : Green
Pin 8 : Yellow
Only the shielded cable has thick wires, the other 5 are low-power, and small. Inside the control-box, Pins 1 and 2 have much thicker tracks on the flexi-rigid cable that solders onto the coil plug. Resistance measured between Pin1 and Pin2 is approximately 0.5 Ohms ( TX coil + cable ). Inside the coil, the shielded cable joins directly to the Litz wire of the TX coil. Unlike some earlier ML machines, power is not 'robbed' from the TX drive to operate the pre-amp etc. There is a seperate DC power wire, of 3.3 Volts.

Foma has given us a photo of the PCB. By comparing it with the X-ray images, it can be seen that it is fitted 'upside down' in the coil housing. The component side of the board is next to the underside of the coil. The wire connection pads are on the component side, too.

Foma has provided a partial reverse-engineered schematic of the PCB. It is a low-noise design, with a discrete transistor differential pair at the front end, followed by a single op-amp. The opamp output feeds back into the front end, to control the gain, at what looks like about G = 16. This opamp output then goes to a unity-gain inverter opamp circuit. This makes the circuit have a differential output, feeding back to the control box.

The security device is in a single 3-pin package, with just 0V, +3.3 V power, and a data signal. This device could easily be relocated to the coil socket, perhaps with a PSU decoupling capacitor for company. The PCB can be hand cut, 4mm square is enough space. If this is done, only 6 wires need to go down to the coil: TX +/- ; Ground; +3.3V supply; RX +/- signals.