DD-Coil design guide for a VLF detector?

[dingbatca]

It's been a while, but I have managed to scrounge up 4 different coils. All in unknown health. If any one has a guess as to the pin out's of any of the coils, that would be a great starting point, and help me out a bunch. Pics attached.

[Skippy]

That's a random mix.
Your orange Coiltek WOT is most likely for a Minelab BBS machine like a Sovereign, it will have a pre-amp inside it.

The SEF is probably for a First Texas machine, likely candidates being Teknetics greek-series/G2/Fisher F19/F5, or the Teknetics T2, or the Fisher F75. Try and work out the pinout with a resistance test, that will narrow down things.

White's DFX is common enough, the details should be on here somewhere, I believe they are fundamentally the same as those for the Whites V3 machine, but not so tightly specified.

I don't know what the hothead is for, I'm sure someone here will recognise it. Older single freq machines from Whites, Fisher are likely.

[dingbatca]

Ya, a rather random mix, but great for ripping apart, and learning. Here is what I was able to learn with my Volt/inductance meters.

DFX:
Pins 1->2 = 37.2 ohms, 390pF, 414500hz
Pins 4->5 = 3.2 ohms, 344pF, 419850hz

HotHead:
Pins 1->3 = 472.3K ohms
Pins 1->4 = 4.8 ohoms, 785.0pF, 368510hz
Pins 3->4 = 472.1K ohms

S.E.F.:
Pins 1->2 = 1.4 ohms
Pins 1->3 = 1 ohm
Pins 1->4 = 6.4 ohms
Pins 2->3 = 1.6 ohms
Pins 2->4 = 7.1 ohms
Pins 3->4 = 6.2 ohms

CoilTek (suspecting internal pre-amp):
Pins 1->2 = 2.4 ohms
Pins 3->4 = 2.34K ohms

[Skippy]

The SEF beep-out looks a mess. I'd be tempted to remove the connector shell and take a look at the internals. I think if you remove the two cable-clamp screws and the single countersunk one ( which may be threadlocked) the metal body will rotate a small way, then it can be pulled off, kinda like a bayonet light-bulb. Then you can looks for shorts, etc. The cable may also be a guide. If there's one single-core screened cable and one two-core screened cable, the single-core will be the RX, the two-core is the TX in some fashion. These CB-radio microphone connectors are 50 cent junk, with inadequate strain-relief, and wiring problems are common.

The DFX beep-out looks reasonable, the low resistance is the TX winding, the high resistance is the RX. The DFX is a multi-freq machine, forcing a complex square-wave signal onto the TX coil. TX coil inductance is lower on this type of coil, compared to the typical tuned resonant single freq coil.

[dingbatca]

Took a knife to the HotHead. Pics attached.

[dingbatca]

Skippy. Thanks for the notes on the SEF connector. Took it apart and sure enough, it's two-core screened.

S.E.F.:
Pin 5 NA
Pins 3->4 = 6.5 ohms, 130Khz, 584.7uH
pins 1->2 = 1.6 ohms, 120Khz, 704uH

[Skippy]

The SEF beepout looks normal now, but I've no idea what machine it's for, certainly not those Fisher/Tek machines I listed earlier. I would guess the TX winding is the low-resistance one, the 6 ohm winding is the RX.
The Hothead beep-out is suspicious, too. The 400 Kohm is due to the resistor visible in the potting resin, this will be some part of how they get the coil nulled correctly. The two coils will have resistances under 50 ohms, maybe check the connector wiring on that one too.

[Carl-NC]

Took a knife to the HotHead. Pics attached.

Interesting, they made the RX coil quite a bit larger than normal.

[Op04]

I think I am getting this. Well, at least half of it. ;-)

I have designed a TX coil. Still waiting on a Capacitance/Inductance Meter to test, verify, and tune. It maths out correctly, lets see what the real world numbers show. Here are the numbers I have hammered out:
10" coil
26 AWG
80 turns (About 209' 4" of copper needed)
8 ohm
L = 4474.76 uH
f = 7.5kHz (Target freq)
C = 0.1uF

Now the part that I am stuck on. The relationship of the RX coil to the TX coil. If this was a simple air core transformer, I would build the RX coil to match 7.5kHz TX coil. After a lot of research, it does not seem anyone can agree on what the RX coil should look like. Some say +100hz. Others go for -2.5kHz. While other have 3X more turns on the RX then the TX, and some have 1/2 the number of turns. Can any one give me some good pointers? Or should I just build up 5~10 RX coils and just try them in a controlled environment? AKA the 5gl bucket of dirt.

So, have you made this coil by now? I assumed what you trying yo build is the old published e l e k t o r 81's magazine , coz that just exactly what I'm looking for by now. Correct me if I'm wrong.... or is there someone out there successful build the project?

[WM6]

Besides the lower operating frequency (6.5kHz), are the Ace coils just simple TX/RX with no other circuits in the way?

http://metaldetectingforum.com/showt...5429&styleid=4

[dbanner]

Interesting, they made the RX coil quite a bit larger than normal.

Yes, I'm reminded by what Payne said. The ratios need to be kept within certain "reasonable" limits.

[dbanner]

So, have you made this coil by now? I assumed what you trying yo build is the old published e l e k t o r 81's magazine , coz that just exactly what I'm looking for by now. Correct me if I'm wrong.... or is there someone out there successful build the project?

That particular project, is a pll driven TX, bipolar transistor array. But there was an error in the PCB layout.

[dbanner]

The SEF beepout looks normal now, but I've no idea what machine it's for, certainly not those Fisher/Tek machines I listed earlier. I would guess the TX winding is the low-resistance one, the 6 ohm winding is the RX.
The Hothead beep-out is suspicious, too. The 400 Kohm is due to the resistor visible in the potting resin, this will be some part of how they get the coil nulled correctly. The two coils will have resistances under 50 ohms, maybe check the connector wiring on that one too.

That resistor is fundamental to the feedback of the RX coil, and indeed it is meant for nulling or what we refer to as "balancing".

[Op04]

That particular project, is a pll driven TX, bipolar transistor array. But there was an error in the PCB layout.

Thanks for remind this, do you recall which errors on what part of PCB that contains errors??? coz I have build half way for this project, so I get fast way to fix it. I really appreciate you for this