Whites 6 1/2" VLF concentric coil

[golfnut]

for reference posting pics works well on here if the pic is 2Mega Byte - Jpgs

[nalag46]

6 1/2" VLF concentric coil

Ok. I have now finished winding the new TX coil and I am still none the wiser after having experimented with it and finding that it does not work. Following the instructions to connect it according to Don's
IDX concentric schematic does not work with this circuit and I pointed that out at the beginning of the thread when I mentioned and showed pictures of the components left on the coil casing and how
they are connected (they are the originals and have not been disturbed), meaning the three caps. across the Shield and the Green leads and the complete windings of the RX and bucking coil.
To make this clearer, I have attached a diagram of what I have and how I have connected the new one TX coil, which does not seem right and it does not work and by that I mean there is
no resonance, nothing at all.

[Qiaozhi]

To make this clearer, I have attached a diagram of what I have and how I have connected the new one TX coil, which does not seem right and it does not work and by that I mean there is
no resonance, nothing at all.

That doesn't look correct with the shield connected between the TX and bucking coil.
What TX circuit are you using?

[Skippy]

It would be easier if you simply attached a .jpg image, so I don't have to save zipfiles, extract them, open Acrobat....
I suggest wiring the TX coil up as in my previous post. Not saying it will work like that, but it seems to be more correct, with the 72n across TX and buck in series, so it's a step in the right direction. You can also try the TX oscillator with the parallel combination of your new coil and a 68nF capacitor. The bucking coils inductance is negligable compared to the main TX coil, so the effect on frequency is not important. If you can persuade it to oscillate, say, by connecting this combination between any two of the following 3 wires : white / ground / green, then we've made progress. I'm a little surprised by your TX coil resistance of approx 10 Ohms, I estimated it as 19 Ohms last week, though it's not the main issue at the moment.

[nalag46]

Whites 6 1/2" VLF concentric coil

I have attached a diagram of the TX circuit used in this simple but yet intriguing set up

[Qiaozhi]

I have attached a diagram of the TX circuit used in this simple but yet intriguing set up

Have you back-engineered this circuit from the PCB?
If not, then where did it come from?

[Skippy]

I'm more puzzled than intrigued. This circuit doesn't seem to match the PCB photo's at all. The oscillator looks to be based around a single TO92 transistor, there's only 2 diodes, one (the small one) is labelled as a zener on the PCB copper. The fat blue one, 1Nxxxx, has a normal diode symbol on the copper layer.

[Qiaozhi]

I'm more puzzled than intrigued. This circuit doesn't seem to match the PCB photo's at all. The oscillator looks to be based around a single TO92 transistor, there's only 2 diodes, one (the small one) is labelled as a zener on the PCB copper. The fat blue one, 1Nxxxx, has a normal diode symbol on the copper layer.

I agree. Something horribly wrong here.

[kt315]

I'm more puzzled than intrigued. This circuit doesn't seem to match the PCB photo's at all. The oscillator looks to be based around a single TO92 transistor, there's only 2 diodes, one (the small one) is labelled as a zener on the PCB copper. The fat blue one, 1Nxxxx, has a normal diode symbol on the copper layer.

this is a schematic from another whites. tx is similar?

[nalag46]

Of course I have made the diagram by looking at the pcb. You cannot get data anywhere for this daft piece of electronics. The drawing matches exactly the components seen on the board (have you not looked at my previously posted pics? attached again below). All the components on bottom right hanside, i.e. 3X IN4148 diodes + 1 other diode (fourth from right) not sure, looks like a zener but can't read markings, the resistors, caps and IC, that is where the TX coil cables go to. Anyway, I will continue experimenting with the coil cables but the whole thing is too time consuming and I have decided to look out for a used Beachcomber 4b coil and that will sort this matter out once and for all. That coil is a multi-tapped coil and will work here. I know it because I opened one up in the 80s to repair a dry jointed connection and it's just a pity I cannot find any notes or remember much else about it.

[nalag46]

Skippy. Just reading through your message again, I can see why you have got rather confused here. You are looking at the wires mounted on the pcb and have assumed that the white and green leads going to the part of the circuit with one TR and a large IN blue diode are the wires from the coil? You would be wrong then. Those four wires are the connections to the signal meter and the push button
auto reset. The leads from the coil are hardwired to the copper tracks side of the pcb so my diagram posted before is the diagram for the TX circuit.

[Skippy]

Ah, I see. I think I was remembering the line "the cable is hardwired to the PCB and it hasn't been disturbed", so the idea that it had been desoldered from that row of pads on the left-hand end of the PCB didn't occur to me. And it makes sense about the push-button, that circuit based around the CA3140 mosfet opamp is a sample-and-hold circuit, using the big non-electrolytic cap, with a guard ring on the PCB to reduce leakage. So it looks like the inverter gate drives the transmit coil? It's possible there is a second 'feedback' winding on the TX coil. If so, it would most likely connect between the cable shield and the white wire, but it would be in anti-phase (ie. not a tapping). I will give it more thought, maybe a drawing will help me explain better...

[Skippy]

I've just scrutinised the last good pic you took of the innards of the coil ....and can see TWO short bits of copper wire joined to the cable shield. That's an important clue, I think this is the 'tapping' on your TX coil, one of these wires is the main TX (that you've made), the other is the end of the feedback winding, the other end of which joins the white wire. So green wire is TX drive, white is feedback, in opposite polarity to the drive. If this is the case (George, KT et al, feel free to comment) then we have the problem of finding out the no. of turns on that winding also. Though a bit of experimentation wouldn't be hard, you have spare wire, I guess, try 30 turns for starters, just lash it together on its own, place it very close to the main TX.

[Skippy]

I've attached a plausible idea for the TX and feedback coil. Worth a try, I've really no idea at the moment about how many turns the feedback coil might have, could be 5, could be 50.

[nalag46]

Thanks for all that. I have been away for a few days and have not had the opportunity to look into this any further. However, the above idea is what I had plotted myself except that it does not work.
The feedback (or loose wire) is rather short so it is no more than 3 may be 4 turns in length, one end goes to the shield cable and the other to the white. I do not believe 146 turns for the TX coil to be
accurate (I am only going by the size of what the original one "looked like to be" by the width of the groove left on the foam). This coil is far too thick. I have been careful with the winding and got it really tight but still is more than twice the size of the groove, both in depth and width and would have to be cut in firmly in a wider channel if it turned out to be right.
Anyway, I have managed to source an used Beachcomber 4B coil. It is the 8" one but it will be the right coil if it still works? It comes with another non working detector and I am just keeping my fingers crossed (talk about looking for punishment??) but I will not be defeated. It is not as though I would be wanting this detector for my own use (I only use top of the range Minelabs and others like Deus etc) it is getting to know how they got this to work on such a simple circuit that bugs me, and you too I am sure