Whites 6 1/2" VLF concentric coil

[Skippy]

I wonder what that coil is for? The cable shield of nalag's coil is clearly the TX ground. Interesting, though, especially the resistors.

[nalag46]

Thanks for that drawing. Quite puzzled though and as Skippy says, my shield cable is the earth cable and there aren't any resistors in my set up. I will be winding my coil up soon on the data already
provided and will see what happens. Still, I will keep that layout at hand just in case I need to make any alterations.

[kt315]

I did take my pic of XLT 950 coil for some base, with diagram I did. yes there is some evoliution if in nalag early coil the shield wire used like a signal one. I wrote about
King Cobra coil and there too shield wire is signal wire. but in modern coils shield wire is splitted out and uses not like a signal wire, uses only as electrostatic shield connection wire.
take attention on whites schematic of coil at corner of the pic that it is absoliutely unformative and has not needed details.

[nalag46]

This 950 coil, although similar in design, it still is very different to the one used on my Treasuremaster VLF. I have the Whites Classic schematic (from somewhere in this forum) using the same coil layout and from the pics. attached, it can be clearly seen that the TX coil windings are of a much heavier gauge than the 32swg used in the TM VLF. Also the capacitance of my small coil is just 0.072uf with no coupling resistance on any of the windings while the shield lead is not used as a signal coupling, it is connected to earth/negative side of the 9v.power supply.
I cannot see how this set up would work here and I more or less can prove it. A friend of mine has just brought me his "antique" Coinmaster 6000D Series 2 to be "serviced" (it works fine but the push button auto reset does not function). This machine uses the classic Whites 8" coil with a 5 pin plug connector, which I have removed, hardwired it my TM and IT DOES NOT WORK. Enough said......

[kt315]

I see you mistaken in values of caps. NOT 0,1 NOT 0,01. I see 332 on one, it is 3300pF. another values are not clear seen at your photo.

[kt315]

???

[Skippy]

I can read the cap values as 102K, 332K, 683K on the TX, plus the total capacitance made sense when put together with the estimated TX inductance.The only thing that bothers me is why the coil was mutilated in the first place. Was the detector faulty, leading to the previous owner stripping it for a source of ECW?

[kt315]

Skippy, having the data L and caps value I would like to calculate the TX resonance frequency. yes may be it is early coil with 5,xxkHz resonance, not 6,59-6,6kHz that would be later time, for Classics, IDX Eargle, XLT etc. then it is due what we see in the patent described above you gave. I still do not see at pics where WHITE wire is going...
it is dilemma... starttopic can do more modern coil design and do nor repeat this oldie...IMO.

[kt315]

Nalag, can you give more clear pics of yout pcb at daylight, at open air, more resolution, both sides? can you give part list?
your photo is very blur and bad, I see nothing at it.

[Skippy]

I agree, KT, there are a lot of unknown bits of information about this coil. I am hoping if nalag can get something working, even badly, then we can progress to a good solution. I think it is very likely to be a 6.6KHz operating machine, but obviously not identical coil specifications to more modern Whites machines. If the TX coil shows a reasonable null, AND the frequency is about 6.6KHz, then I think that would be a successful start. It's a shame that nalag doesn't have an inductance meter, but I think it is possible to manage without. The white wire must go to some shield or screen, I think. I don't think VLF IB machines work well without some shield between the ground and the coils. It's possible to do without shielding the RX from the TX /bucking coil, if you keep both 'cold' (non-oscillating) ends close to each other.
I am hoping that the total transmitter coil inductance is about 8 mH, resonating with the 72 nF capacitors to give 6.6KHz. What we don't know is whether there may be other capacitances in the oscillator circuit that affect the frequency.

[nalag46]

capacitor are 102k, 332k and 683k total 72600 = 0.072uf. It has been mentioned already in the thread below.

[kt315]

ok. this coil is something another than classic ones... have to see tx generator diagram to judge more...

[nalag46]

Ok, I can't give any more information about the missing coil because I haven't got a clue about what might have happened to it or the reason for someone taking it out. My bet is the detector did not work properly, ( there are clear signs that the chips have been either replaced or resoldered, but I have checked the complete assembly and everything seems to be working fine including the oscillator and audio circuits) then they decided to open the search head (has a coil cover only that serves as the coil bottom's cover, so easy to open) messed about with the wiring and removed the coil. I bought it at a car's boot sale knowing it would power on but there was no metal "pick up" reaction, the coil "cover" was glued on so I had no reason to suspect the missing coil.
I have already mentioned I have had a lot of experience with these machines as I have owned most of the first Beachcomber series made in UK by Savo Electronics of Inverness, now Whites.
This coil layout must be one of their first designs as it clearly is different to all the later, similar types even if the operating frequency is the same and I believe that the first Beachcomber 4 fitted with a variable ten-turn pot for discrimination might well work here. That 8" coil had a hole under the top coil blue sticker to get access to a variable cap/trimmer to tune the null. This VLF 6 1/2" coil with a standard linear discriminating pot and auto tune reset push button was an update from the latter.
Anyway, I have ordered a reel of 32swg wire but will not arrived before Thursday 28th. so I cannot do much more before then but I will definitely come back here to post the result.
I am also attaching some more shots of the pcb as requested by KT. I always thought this would be a good little winter project and indeed is turning out to be rather a challenging one, at least on the exchange of information so far.

[kt315]

sorry Nalag. I am not able to work with low resolution photos. must refuse to help you ((

[nalag46]

That's ok mate, I do not need any further help. I have got all the information I need. Thanks anyway