Teknetics Coil

[Daves]

Hello, what is the resistance in the picture of the coil of the T2 model? What is it used for and what is the resistance value? Does anyone have the details to build a coil for the T2 model?

[Skippy]

I've measured a T2 coil ( years ago ) and there was no resistor between these points, open circuit.
The First Texas bi-axial coils all look alike. Are you sure it's a T2 coil shown in the diagram?

If there is any such resistor, it will be very high in value, to couple a tiny portion of the TX voltage into the RX circuit, to achieve a better null. I recall seeing PCB photos of Tek G2 or Fisher F19 boards with a high value ( 2 Megohm? ) resistor added between TX and RX. I believe this was to help obtain a better null, as the G2 coils are really best at 7.8kHz, and not quite so optimised for 19 kHz machines.

[Daves]

I've measured a T2 coil ( years ago ) and there was no resistor between these points, open circuit.
The First Texas bi-axial coils all look alike. Are you sure it's a T2 coil shown in the diagram?

If there is any such resistor, it will be very high in value, to couple a tiny portion of the TX voltage into the RX circuit, to achieve a better null. I recall seeing PCB photos of Tek G2 or Fisher F19 boards with a high value ( 2 Megohm? ) resistor added between TX and RX. I believe this was to help obtain a better null, as the G2 coils are really best at 7.8kHz, and not quite so optimised for 19 kHz machines.

Thanks for your reply Skippy, yes I assume it is used for this purpose. This picture is taken from a Russian site. The exact topic is about T2 but the picture may be of a clone coil. Do you have a coil connection diagram for the original T2,F75,G2 coil that you can share with me?

Note: Yes, I have seen resistance at the megaohm level. how you can know the megaohm level? How to determine the resistance to be used here?

[Skippy]

"Do you have a coil connection diagram for the original T2,F75,G2 coil"

Those 3 coils are all completely different. And if you search the forum, you will find the details for them.

How to determine the resistance to be used here?"

Experimentally, that's the best way. The T2 coil makes this easier, as you have the choice of two different TX signals, that are complementary to each other.

[Daves]

"Do you have a coil connection diagram for the original T2,F75,G2 coil"

Those 3 coils are all completely different. And if you search the forum, you will find the details for them.

How to determine the resistance to be used here?"

Experimentally, that's the best way. The T2 coil makes this easier, as you have the choice of two different TX signals, that are complementary to each other.

Normally I am looking for a T2 coil. I searched the forum but couldn't find anything.

[Skippy]

Yeah, it's hard looking on here for some things.
Here is a start for ya:
t2 teknetics coil - Geotech Forums (geotech1.com)

SEF Coil for Teknetics T2 - Geotech Forums (geotech1.com)

[Daves]

Yeah, it's hard looking on here for some things.
Here is a start for ya:
t2 teknetics coil - Geotech Forums (geotech1.com)

SEF Coil for Teknetics T2 - Geotech Forums (geotech1.com)

Yes, i saw and read this contents but only pinout. Im searching full details and maybe diy video

[Carl-NC]

How to determine the resistance to be used here?"

Experimentally, that's the best way. The T2 coil makes this easier, as you have the choice of two different TX signals, that are complementary to each other.

The resistor is likely used for an R-null tweak which will depend on the exact way the coil is built. That is, what they did and what you do will probably be different, and you may not need the resistor at all.

[Daves]

The resistor is likely used for an R-null tweak which will depend on the exact way the coil is built. That is, what they did and what you do will probably be different, and you may not need the resistor at all.

Thanks for the info Carl,
Additionally: is there a formula to calculate if we want to use a resistor?

[Carl-NC]

No, it's all try-and-see.

[Skippy]

"Im searching full details and maybe diy video"
What did your searches come up with?

The end-to-end inductance of the TX coil is the same as the Fisher F75 coil. For a T2 version, wind the coil bi-filar, with half the number of turns , don't wind half the turns, bring out a tap, then wind the other half. Then join the ends together so it's a proper centre-tapped arrangement. The RX is identical to the F75 coil.

[Skippy]

To save you the effort:
Fisher F75 TX coil : 690 microhenry ; 2.0 Ohms
and RX coil : 6.95 millihenry; 37 Ohms.

So if your calculations say you need 40 Turns for an F75 coil TX , you need to wind 20 turns bi-filar for a T2 coil. Then when you wire up the windings to the cable, do so the TX is a centre-tapped 40T winding.

[pito]

so the TX is a centre-tapped = ???

[Carl-NC]

so the TX is a centre-tapped = ???

For the T2, yes. Not for the F75. But the inductances are the same.

[Skippy]

Quote: "So the TX is a centre-tapped = ?"

Why the surprise? Your first post has a diagram attached showing this, even if it is a poor diagram.

That's one of the electronics differences between the F75 and T2. The F75 is a modified T2. The coil was changed to single-ended, to make it easier to manufacture concentric coils for it. You will notice there are no commercial concentric coils for the T2 |: it's technically possible, but they would be expensive to manufacture, I think.