Need help with Scope

[plumabob]

Hi All,

I am very new to this. How would I go about hooking up my Minelab Sovereign to my Tektronics 2225? What settings would I be using and how would I interpret what I'm looking at. I have already built some PI coils and I'm familiar with those and I know what the readings are telling me, but these Sovereigns are completely different. I have built a working Sovereign coil but it needs improvement. Would like to hook-up my scope to it to see what it does as compared to a stock coil. If anyone can make some suggestions I would appreciate it.

Thank you all and have a nice day!

Bob

[ApBerg]

Scope coil

What type scope ? Think it wil not work, Scope uses most Hartley osc.,for the transmitter: 2 coils needed (instead 2 caps for collpits osc., most detec.)
For scope 20 cm coil dd think it was 35+35 turns for transmit, center tap is ground.
70 turns for rec. adjust the coils with cap. for the right freq.(wire .25mm)

[ApBerg]

Oooopps.... it is not for Cscope, need glasses.....

[DarthCarl]

If you look at the transmit signal, it should look like this:



Sorry for the poor image quality. Since the TX signal comes off a driver circuit, I doubt it will vary with different coils.

If you look at the RX signal, I'm not sure what you will see. I suspect that, for balancing, the best way is to use a sine wave, but you will need to check balance at different frequencies as it can vary due to capacitive parasitics.

- Carl

[bbsailor]

Need help with Scope

Bob,

Here are a few ideas.

First make a break-out plug with a male and female connectors matching the coil connector and teh control box connector. Make it so the wires can be attached to the scope by leaving an 1/8 inch area striped of insulation staggered on each wire so it won't short out. If you want to look at TX current, then put a .1 to .5 ohm resistor in one leg of the TX connection and view the voltage drop across it to see the current.

Another more direct way is to use a current probe that clamps over the TX wire. I got mine from e-bay along with a Tektronix 134 current probe amplifier which acts as a current probe terminator and it also extends the low frequency response of the probe.

I make breakout plugs to match the coils that I work with. I standardized on a 4-pin mic connector for both mono and DD PI coils. The breakout on the Minelab Sovereign requires a 6-pin connector and plug set. You just do the same thing. Match up all the pin connections so they go straight through the breakout plug. You only need about 3 to 4 inches of wire to make this breakout plug. Now you can tap into the TX, RX or voltmeter signals while the detector is operating normally and not worry about opening up the control box. If you get or have a current probe, there is just enough wire to allow you to easily slip the wire in the probe.

The frequency range of the Sovereign is 1.5KHz to 25KHz so that should help you set the scope time/division control to see a good, clean waveform of a few cycles.

Use a 10X probe, as it loads the circuit down less. Since the Sovereign coil has a built in preamp in the coil, loading may not be as much an issue compared to coils that do not have this feature.

I hope this helps.

bbsailor

[plumabob]

Hi bb,

Thank you for answering. That's a great idea you have there to get at the coil wiring. I don't have that problem as I can get right at the circuit boards on both my sovereigns easily. They are both in waterproof Lexan cases that pop open quickly for instant access. What I'm trying to do find out is how to use the scope to help me build a homemade coil or to basically see how to improve it or make adjustments. I've made several PI coils that are very easy to set up on my scope to see what is happening. I wanted to do the same with my sovereign coils. If you could give me some explaination of settings or what I'm looking for on my scop, that would be great.

Thanks

Bob

[bbsailor]

Bob,

Sovereign coils are arranged as an overlaping back-to-back double D configuration. The overlap region creates a null and that null affects the sensitivity of the coil. There were even some photos on this forum of an open Sovereign coil showing all the inner details.

One practical way that these type of DD coils are nulled is by using a small coil about 1" in diameter with one or two turns that can be moved and secured more easily than moving the large coils. The small coil is moved until a good null is seen on the receive coil. Since there is a preamp in the Sovereign coil, some way to power that preamp must be found if the coil is being nulled without being connected to the Sovereign control box.

There were some posts a while ago on this topic. You might want to so a search to look for what the experts say about this.

The interesting thing is that DD coils on VLF machines depend on the null to be sensitive, while a DD coil on a PI machine is not dependent on obtaining a null.

bbsailor

[plumabob]

Hi bb,

I have all that information already and 2 "dead" coinsearch coils I have been using for measurements. I have made a working 14x10 Sovereign coil but need to change the tx windings as my 13/30 litz turned out to be too heavy, unless it was to be used for a water coil. I just was asking if there is any way that changes could be made by interpreting what was seen on my scope. There is very little info out there on Sovereign coils. If it's just a matter of trying different windings and turns I will just keep at it till I have something that I'm happy with.

Thanks again

Bob

[Charles (Upstate NY)]

I think what you may be looking for is an LCR meter. I build coils for my Minelab Explorers and use my Tektronix scope for little more than balancing the windings.

Scope - mine is a two channel scope, I connect channel 1 to TX, channel 2 to RX, trigger on channel 1/TX. I adjust channel 1/TX to 5v div, I start channel 2/RX at 1v div and adjust to 5mv div as I bring the windings into balance. I have my probes set to 1x as 10x is too noisy.

NOTE: I connect my scopes ground lead to the Explorers ground, this eliminates a ton of the normal EMI/RFI in my house, I can actually run the machine at 33% sensitivity with no interference e.g. stable. Remove this household ground source and the machine goes nuts. Do this at your own risk on a Sov though since I know nothing about them.

I highly recommend you make a safe test fixture as bbsailor suggested, I have the same breakout plug he describes. In your case you may want to fashion something you can tape down to ensure that you don't have an accident. One short is enough to zap it dead.

LCR Meter - if you want to experiment with difference coil designs, sizes, shapes, etc. an LCR meter is the way to go. I sampled several different sized factory coils for my Explorers to establish a range to work within. You want to measure...

Inductance (L) (better meters measure Q which is useful)
Resistance (R)
Capacitance (C)

Also capacitance of the coil cable. Once you know these measurements you can vary coil size and shape so long as you stay within these parameters. Coils will of course work (sort of) when wound outside of these parameters but the phaze shift may be off and so the machine may not ID targets accurately. On the Explorers balance is also critical, balance the windings just so and the ID will be accurate, just a tad out of balance the wrong direction and the ID will be off.

So lets say you establish a range for the measurements above on a factory 10 inch coil, now you want to build a 10x14. The guage of the wire probably isn't going to vary from the 10, what will vary is the number of turns, fewer turns on the bigger coil. L and R fight each other, for a given guage wire if you increase L, R will also increase.

Lets say you need to hit 500uh for L and 6ohms resistance. Lets say you are short on L only 420uh and you need to add a couple of turns but R is already too high at 6.4ohms and if you add any more turns this will only go further out of spec. The solution is to drop to a heavier guage wire, that will drop R (and L somewhat). So you experiment with different guages until you find the one that brings it all together within spec.

The difference between 25 turns of 19 verses 20 guage wire is not so great for either L or R, however when you get to smaller guages like 29 and 30 L and R vary more from guage to guage. RX on the Explorers uses a fine guage wire and is the most troublesome. I can use either 19 or 20 on TX and be within specs.

Capacitance...thats a lengthy difficult subject best left to the Eric Foster and Dave Emery types to discuss.

Shielding, very likely your only choice will be carbon black/graphite type anti-static paint. I know the Explorers will pick up the nickel flake paint e.g. its too conductive.

Hope this helps.

[plumabob]

All you guys have been great! Carl, Bb and Charles. Thanks for all the input. I will try your settings Charles on my scope and see what happens. I do have an LCR meter and have been taking readings off of several coils I have available to me. Like I said I do have a working 14x10 coil I have already made. I just really wanted to know is there any way to tune it in perfectly or I just have to be lucky to hit on the right combination, working within the parameters of the stock coils? I'm guessing this is probably the answer.

Thanks again

Bob