Gpx 6000 Circuit Board

[PimentoUK]

The '14 chips' are analogue switch circuits, general-purpose cheap CD4053 types, widely used for sampling signals, and frequently used in the demodulator stage of many VLF machines.
3 switches in a package, each one single-pole changeover:

https://www.ti.com/product/CD4053B

the nearby IC's look like op-amps, two amps per package.

[jasong]

Also, after thinking a few minutes on the logic of it, I'm prone to believe the X Coils pinout because the 2 TX wires are not shielded. If they were RX wires, they would both have heavy shielding since any tiny amount of noise on them will mask signals plus affect how much RX gain you can effectively use. Plus, the GPZ 2 wires are heavy gauge wire, which you'd need for TX but not RX since there is almost no current going on the RX side of things.

It seems counterintuitive given the DOD design, but I really think the GPZ has 2 TX wires, not 2 RX wires. I'm unsure how they control which lobe is active. (or I probably misunderstand how the DOD works and maybe both are active?)

So, I think the red wire in the 6000 is also an RX. And the 3 black wires are something else, including at least one TX. One wire has a red band around it. I'd say that one wire might be a lobe control or something but why run it through a choke? All 5 of those wires going through the choke are like to be either shielding, TX, or RX, unless they have something weird going on like a wire carrying a constant reference voltage to some part of the coil for engineering reasons beyond my understanding.

Edit: one last logical progression thought - a constant voltage to some part of a coil (through an additional mystery wire) could I guess in theory be closely monitored to determine how saturable (hot) ground is affecting the inductance of the coil independently of the RX signal. Get a little parallel processing going on there since the FPGA can handle all that and more (basically, creating a magnetometer of sorts in the coil). But I'm way off in left field designing my own detector in my head here now, trying to guess what an extra wire might be. 

[egixe4]

The choke is purely for reducing transmitted and received EMI, it is needed for FCC compliance.

[jrbeatty]

3 hours ago, jasong said:

Not according to the X Coil pinout for the patch lead. It says 2 TX in 2 different diagrams. Do they have it wrong twice?

I'd post it here but I'm uncertain if those were private or allowed to be shared.

Jasong: To clarify, I'm referring to the number of TX windings in a DOD (1 only) so both heavy TX wires are marked correctly and connect at each end of said winding. The shielded black RX wire must service both receive windings which are possibly connected in series (according to my ohmmeter)

I recently made an adaptor for my Z and closely followed the X Coils wiring diagram (which, incidentally, Simon drew up for them)  

The colour band on one of the 6000 black wires could merely be an identification code for assembly purposes. Perhaps all three are shielded and therefore RX wires for multiple windings. That's my pet theory for now!