Whites 6 1/2" VLF concentric coil

[Skippy]

Here's some simple maths: Assume the ECW is square, and you make a nice square cross-section coil with it. It will be square-root of 146 =12 x 12 wire-diameters in size. 12 x 0.25mm (approx 32 SWG) means it will be 3mm x 3mm in size. Ok, the wire is round, so you have hexagonal close packing, and the enamelling thickness is not insignificant, but it would still be 3.5mm x 3.5mm, I bet.
As we've established it's a 'standard' Whites spec coil, it's probably worth just looking out for a compatible vintage Whites coil on eBay. The 8 inch one (4B?) would probably work with it. Carl-NC would probably have some suggestions. I think some current coils they make would also work, have alook, depends if you're wanting authentic retro look or not...

[nalag46]

Yes, I understand the maths but I cannot see it nor will I be able to compress 66meters of 32swg as neatly and tight as the manufacturer can do so that it can fit in this casing. Yet, there is still the not so quite straight forward issue of working out the two tappings to add to it all. I will have a go in my good time but I was already thinking it might better to start a complete new coil project or indeed, as
you say, try to source a used Whites coil from Ebay. Not so simple as you most probably need to buy a complete new Beachcomber detector which are now very scarce (I have owned 5 different models from the basic BFO to the TR/IB and latest GEB units during the 80s, all using the same blue and gold hardware).
I am very grateful for all your info and time.

[Skippy]

I didn't actually state it, but I made the assumption that the main TX coil is wound with 32 SWG wire too. This might not be the case, it could be slightly smaller gauge. One way to possibly judge this is by examining the coil resistance. A TX to the specs we've estimated has R = 20 Ohms approx, plus the bucking coils 3 Ohms = 23 Ohms. If you can find out the R of an original coil, you can judge how far out you are, and maybe tinker with wire gauges as a result. I'm sure this R measurement is on this forum somewhere.
I'm not convinced the TX is any more than just the two coils, the bucking and the 146T. The 'tapped' coil is probably an error. Could the extra wires be for a screen / shield, for example? This is where someone elses opinion would be handy.

[nalag46]

My reasoning for the "tapped" TX coil is derived from what I can see remaining of this set up? The start of the bucking winding is connected to the control box cable shield and one side of the three tuning caps, which is the earth on the p.c.b. but it also has a soldered piece of cable (I am sure it is 32swg) left on which I believe should connect to start of TX coil? The green lead is connected to the other side of the caps and has a piece of wire soldered on to connect to "second coil" start, the white cable going to the other end. And so it leaves us with the loose end of the bucking coil or fourth connection.
Have I got this right?

[Skippy]

I honestly don't know, I can't follow the wires in your photo's, I have no experience with these particular coils. It strikes me as odd that the 72nF tuning capacitance only goes across the main TX coil, I would've thought it more likely to go across (TX+bucking coil).

[golfnut]

The Rx is probably tuned below the Tx.

S

[nalag46]

I have attached a closer pic of the wiring left on this coil. You would be able to see it even more clearly if you magnify the percentage to 150% on the browser. In any case, it is quite clear that the start of the bucking coil wire goes to one side of the caps and the shield of cable feed to pcb earth. On the solder joint there is a short length of wire which should go to?? TX coil top end, my guess. The green TX lead is connected to other side of caps and there is also a short wire to connect to?? The white cable is showing to also have a piece of wire to connect to???? These two leads are the TX feed to pcb board so they must be attached to a coil and so where does the bucking coil other loose end go to?
In my first posting I attached a drawing of how I see the layout to be from this information, ignoring the capacitance and resistance quoted then (all wrongly) does it make any sense?

[Skippy]

Good pic, thanks for making the effort. I'm tempted to say that the two left-most ecw's, the one going to the green wire, and the one that becomes the nulling loop, will connect to the main TX coil. The short length of ECW that goes to the cable screen is probably connected to some internal screen / shield - does it have any carbon-painted surfaces etc inside the shell, or carbon-coated paper? But that leaves the white wire, it could be a seperate shield, really don't know. I notice what looks like a very small groove in the foam, inside that for the main TX. Is that just a hoptical hillusion?

[nalag46]

There is no other internal screen or carbon painted magnetic shield (they brought the latter much later on than this primitive design I am sure). Equally, there is no other groove on the foam either, it is just a wide band foam pattern raised 3mm above the TX coil groove that goes all the way round and might look different in the pic. It would be great if that TX coil were just a straight forward winding. I would start winding it right away and experiment with length etc but I can't see it clear and I still believe that White used various multi-tapped coils in their discriminating coil designs. Wouldn't life be so much simpler for a bit of collaboration from the powers that be? Just you try. Technical data of detecting equipment is the most secretly guarded business on Earth, why? As a radio ham myself, I can obtain (paying of course) a complete repair manual of the most sophisticated transmitting equipment costing thousands $ or £, the complexity of the circuitry leaving the most advanced detector design in the shade. Need I say any more....
Anyway, back to this, there is a guy called Don who has posted a PDF of his coil/s design for the Whites IDX on this forum. I have contacted him and asked his advise and also to join this thread as I am sure he might have some good input to put forward? Can't try any harder, can I ?

[golfnut]

"It would be great if that TX coil were just a straight forward winding."

It is.



" I still believe that White used various multi-tapped coils "

Even if your right you dont need multitapped coils here.



"there is a guy called Don who has posted a PDF of his coil/s design for the Whites IDX"

If you digest that pdf in conjunction with my drawing and what others have given you it should be possible now.



If its still con fusing you, strip it out and measure the coils separately -

If you do actually have 2 coils in the middle the highest resistance one is the Rx and the other will be the buck.

S

[Skippy]

Another approach to working out what wire is what on the cable is to see what they are connected to on the main PCB. In particular the white wire. See which ones connect to ground. Is the coil hard-wired to the control box or plug & socketed?
Also, take a read of this patent:
http://www.geotech1.com/pages/metdet.../US4293816.pdf

[nalag46]

Thanks for that. I have had a good look at Don's diagram and t does look quite straight forward and although I still have my own doubts, I will have a go and wind a TX coil of the length is has been
suggested by Skippy earlier on and will see where it gets us. The bucking coil on this set up has been wound on top of the RX coil and as this is supposed to be wound in the opposite way to the TX coil,
looking at the picture it can be seen that the end of that coil (or loose length for nulling) is on the right hand side, would that means that the coil turns are clockwise and the TX need to go the other way round?

[Skippy]

You can only WIND a coil in one direction. You can place it in the housing either way round, as it's not physically near any other coil. Just sort out the phasing by switching over the start and finish wires as appropriate. Based on my previous 50T coil example, I suggest winding your 146T coil as 146+1+2+4 turns (all isolated). This will give you 139 to 153 turns, giving +/-10% inductance change, and 5% frequency change. When you have the thing oscillating, measuring the frequency should in theory be a good guide as to how far out the main TX coil is. Whites carefully fine-tuned the original with 3 capacitors to get it correct.

[nalag46]

The cable is hardwired to the pcb and it has not been disturbed. The white lead goes to an LM358 op. amp. and the ground lead is the shield in the cable so I don't think there will be any problems with these. Like I said previously, with all the data now thankfully received, I will wind a coil of the length given and will measure the frequency up if I get it to oscillate and will report back here. It might take a few days though as I haven't got enough 32swg cable to wind 150+ turns. I am sticking to 32swg hoping is the right gauge (going by the bits soldered and left on the green and white cables).
With batteries connected the circuit does power up and I get full audio. It has an auto tuner (by reset push button) and that part of the circuit also works. All it needs is a tuned coupling coil and hopefully the variable discrimination might be ok too.

[kt315]

check