TGSL Tuning & troubleshooting

[Turjake]

Hello everyone!

Just finished my TGSL circuit and have been playing with it for couple of weeks now. I'm trying to balance the coils by ear and would like to know how critical it is to have the GEB trimmer stay in middle position. Will it work on real ground if I dial it for example to 3/4 parts from other end? I'm balancing by Ivconic's instructions: adjust until rejects ferrite, accepts silver coin. I'm planning to use just one set of coils.

Things I have found so far:
- CRT televisions can be a humongous noise source
- Badly mixed epoxy touching metals can become conductive
- Rx tuning capacitor value very critical. Some will give 40cm detection for 1 euro coin but with serious microphonics. In my case lots of capacitor changing to find best depth/stability/discrimination combination.
- Calculator + magnets + superglue will make simple but reliable turn counter

Finally, Very big thanks to posters in this thread for your helpful insights.

[lunamay]

i will post some photos when i'll make a chasis for pcb..its hard to test when all wires hang in a different way:P

in the meen time

regards to all

[simonbaker]

Hello everyone!

Just finished my TGSL circuit and have been playing with it for couple of weeks now. I'm trying to balance the coils by ear and would like to know how critical it is to have the GEB trimmer stay in middle position. Will it work on real ground if I dial it for example to 3/4 parts from other end? I'm balancing by Ivconic's instructions: adjust until rejects ferrite, accepts silver coin. I'm planning to use just one set of coils.

Things I have found so far:
- CRT televisions can be a humongous noise source
- Badly mixed epoxy touching metals can become conductive
- Rx tuning capacitor value very critical. Some will give 40cm detection for 1 euro coin but with serious microphonics. In my case lots of capacitor changing to find best depth/stability/discrimination combination.
- Calculator + magnets + superglue will make simple but reliable turn counter

Finally, Very big thanks to posters in this thread for your helpful insights.

I have not finished all my experiments and analyzing on how I feel is best way to set the null, so I'll leave it to others to describe. However, you will probably get many different answers!

I guess I could give this advice:

1. Make sure you have the polarity of your RX coil leads correct first. To do that, I would use AC voltmeter on LF353 pin 7, and adjust coil overlap to make voltage close to minimum.

Then test detection with coin at about 12 - 15 cm distance. If you get two beeps, wrong polarity -- switch the RX leads. You should get one smooth beep. But don't put coin too close to coil -- you get multiple beeps there no matter which way leads are.

2. Then continue nulling coil using any one of the Voodoo methods you find described in these TGSL threads.

Qiaozhi recommends choosing null slightly away from minimum, on side with more coil overlap. You also null by "ear" for best detection distance.

3. When you think you have found the best "null" position for your coils, I would check to make sure JFets operate correctly. Put DC voltmeter on capacitor C15 (same as JFet TR4 source lead, DISC channel), and move the DISC pot from min to max. If the DC voltage falls below -.5 volts at some setting (maybe Ok if at max pot setting), then you may have some trouble and need to choose another null.

4. Now adjust the GB pot using ferrite for ground balance. If you cannot find a pot position that rejects the ferrite, your null signal phase may be a problem and you need to adjust the null. If you are able to reject the ferrite but the GB pot is very near maximum (depending how you wired the pot), you may find it reduces your air depth. Changing the null so the GB pot is near middle or more toward minimum setting may improve your air depth.

If you built your MD with the GB pot external so you can adjust it for different ground (many people don't), then it would be good to have the ferrite rejected with the GB pot near the middle. You might be able to adjust the null signal slightly to achieve that.

It really helps to have an oscilloscope of course to make fine adjustments in the null signal phase. However, it may be difficult to keep the null signal phase exact when you "glue" your coil into its final form -- so maybe all waste of time! This is where building technique is important.

Those are just some tips. If your MD works well, great! That's all that matters.

-------------
By the way, feel free to tell us more about your experiments with RX capacitor, epoxy, etc. in detail.

Regards,

-SB

[simonbaker]

i will post some photos when i'll make a chasis for pcb..its hard to test when all wires hang in a different way:P

in the meen time

regards to all

Ok keep us posted.

I'm still working on my PCBs, testing and comparing with dfbowers MD. Very valuable to have his good MD for comparison -- I must thank him again! And even then, difficult to track down small things that affect maximum air depth. I'm pretty sure my PCB will detect in the 20 to 25 cm range -- it is just that elusive 30 cm range that I want to continue analyzing.

Like you, I have also decided to continue building coils and make a more finished detector to help with testing... got to do something.

And I'm working on a PCB with same circuit but different parts values to see how a slower response TGSL behaves.

Cheers,

-SB

[simonbaker]

Thanks Aziz

On thinking, I realize I don't know a simple electronic equivalent of nudging a pendulum, so maybe there are practical limitations on my quest for a super underdamped coil... I'll keep thinking...

-SB

Aziz's latest circuits for his laptop VLF showed me the simple circuit that is like nudging a pendulum that I was looking for.

Well, close -- it is not part of an oscillator, but is driven by any oscillator output (with low impedance I assume).

What's important, if I'm not mistaken, is that it takes advantage of high-Q TX coils, where the TX voltage rises way above the driving voltage, and I assume that provides a higher coil current accordingly.

Of course the issue of whether such a high-Q TX coil makes more difficulties than advantages remains (ref: mikebg), but it is at least a simple circuit to play with if you can find a nice way to drive it (relaxation oscillator?) -- or build it into an oscillator.

On the other hand, maybe the easiest way for us to utilize high-Q coils with the TGSL is a center-tapped coil.

-SB

[Aziz]

Hi Simon,

quite impressive this simple LC pendulum. I took a higher Q TX coil (R=0.9 Ohm, L unknown) and the TX coil voltage went up to 45 Vpp with only 1 Vrms input voltage (@29.812 kHz). The source impedance is around 30 Ohms I think (from the sound card). But such a configuration is sensible to reactive and resistive responses. So the source impedance should be quite low and the coil could be damped a bit to make it less sensible to such responses.

I can measure the losses of the TX coil and its frequency shift. So the ground delivers a lot of useful information here at the TX coil.

Well, the RX coil isn't processed yet.

Aziz

[lunamay]

hi Simon .
i would ask you about pc oscyloscope. there is a way to do a mesurements using a sound card and soft on pc but in ttl standard. that means i can't achive higher voltage on IN than 5V another thing is taht it can only mesure a freaquency up to 20 kHz. so my question is that is it posible to mesure parameters of our project using this pc oscyloscope and some voltage or freaquency divisors? i don't know where to get traditional osc and i don't have enought money to buy my own

what do you thing??

best regards

[simonbaker]

hi Simon .
i would ask you about pc oscyloscope. there is a way to do a mesurements using a sound card and soft on pc but in ttl standard. that means i can't achive higher voltage on IN than 5V another thing is taht it can only mesure a freaquency up to 20 kHz. so my question is that is it posible to mesure parameters of our project using this pc oscyloscope and some voltage or freaquency divisors? i don't know where to get traditional osc and i don't have enought money to buy my own

what do you thing??

best regards

I think it is not quite good enough unfortunately. I think we really need at least 200 kHz bandwidth bare, bare minimum, including DC. Probably 1 mHz is really a minimum. Also dual trace is quite important for seeing phase relationships.

This is not cheap but looks very good for the price.

http://www.amazon.com/USB-2-Channel-.../dp/B000LR4ZJY

This might barely do it:

http://www.parallax.com/Store/Microc...me,ProductName

You could try the sound card software and tell us what you think, but it may mislead you with some measurements. It may be OK for looking at output of LM308 (important signal) if it can measure low enough frequencies.

Regards,

-SB

[Mullihaka]

Hi dfbowers,

What is the phase shift in your best TGSL?

I can get 20 degrees, but then I get also double beeps. Whne I swap the RX wires around, there's only one beep but I can't get the phase shift even close to 20 degrees. It's like 70 or so.

[dfbowers]

Hi dfbowers,

What is the phase shift in your best TGSL?

I can get 20 degrees, but then I get also double beeps. Whne I swap the RX wires around, there's only one beep but I can't get the phase shift even close to 20 degrees. It's like 70 or so.

I have so many coils.. At least 5! The last coil I made seems to be as good as any and it's "around" 20 degrees. I was trying to come up with a 0 degree null phase as well and it's slightly over-coupled.

The first coil that I made is the hottest and the phasing is really off (probably 90 degrees)! Another thing that is wrong with it is the grounding (I accidently grounded it to the non inverting input (J2, pin1 instead of J2 pin2 inside the coil). But that coil will bench test at over 40cm for a 1e! I will not work in wet grass though.

IMHO, I would worry too much about the phasing if you can GB and DISC ok, but if you can make a good set of coils, use a shielding material that is really thin (like metallized mylar). It seems to make a noticable difference. Get a survival blanket from an outdoors shop and cut it into long, thins strip.


Hope this helps.. I can do some screen shots if you want details.
Don

[Mullihaka]

That's good news, but also bad news.

It's good that the phase shift does not need to be 20 degrees sharp.

The bad thing is, that now it's difficult to null the coils, as there's nothing to look at: no 20deg shift, no absolute minimum.

So, I guess I just need to null and try until I get it up to 25 cm. With the previous coil set I got 25cm, but it was ruined when I poured the epoy into the styrofoam housing.

The newest coil set is worse, about 11 cm.

Boy, the TGSL is a tricky one to get working.

Back to the nulling bench, then.

By the way, what's your tuning target in nulling: deepest minimum or what?

Thansks again!

[dfbowers]

That's good news, but also bad news.

It's good that the phase shift does not need to be 20 degrees sharp.

The bad thing is, that now it's difficult to null the coils, as there's nothing to look at: no 20deg shift, no absolute minimum.

So, I guess I just need to null and try until I get it up to 25 cm. With the previous coil set I got 25cm, but it was ruined when I poured the epoy into the styrofoam housing.

The newest coil set is worse, about 11 cm.

Boy, the TGSL is a tricky one to get working.

Back to the nulling bench, then.

By the way, what's your tuning target in nulling: deepest minimum or what?

Thansks again!

Yea, the coils are undoubtably the toughest thing. I didn't use any targets while tuning at all.. just look for null and phasing... Then check with a few targets. I don't know if you have been following the threads about null phasing or not (zero crossing of sync pulse & Rx signal (Pin 7 U101), but that's quite an interesting discussion in itself. Most of us were just measuring the Rx phase against the Tx signal only. Anyway.. for bench testing neither one seems to make a huge difference.

What kind of Epoxy did you use?

11cm for your new coils doesn't seem very good at all. Where do you live?

Don

[Mullihaka]

I've read all TGSL threads thru and tried all nulling methods: nanovolts at the RX coil wires, 20 deg phase shift, GB pulse at the middle of descending slope of the RX signal and so on.

I'll null my latest coil set for the TGSL to 1 volt / 70 degree phase shift.That seems te be kind of trade-off between the correct phase shift and minimal residual voltage.

When ready, I'll make your sens pot mod and see, if I get the missing 9 cm. I'm happy if I get 20 cm on the field.

Hot glue together with the wooden housing seems to be make a solid coil set. If I stay in when raining, I dont even need to varnish it!

I live in Finland.

Andy

[Jerry]

Hi Andy:

What do your tx and rx coils measure for inductance and resistance? Can you post some pictures of what the scope is showing when you set your null? So far I have made two coils. One very bad and the other very good. The first one (25 cm) was made last winter and was not real close on inductance or resistance. I had a lot of problems with GB and finally gave up on it but not until I saw some real potential. I was seeing responses to coins at the outputs of U104 & 5 well past 30 cm. It just was chattering all the time and I could not get it ground balanced.

This summer I made a smaller coil. (20 cm) This one measures 5.79 Mh Tx and 6.23 Mh Rx with the resistance about 24 ohms. This coil has worked well from the start. I get about 35 cm on a 2 Euro coin. At 30 Cm it is really solid.

My circuit is from the information that Ivconic provided and I have made several substitutions for parts. So it isnt a real critical circuit to get working. This the background on what I have build with good results, so I know it can be done.

Good luck with your project. Keep us posted on you progress.

Jerry

[Mullihaka]

Found the depth problem!

When making the dwbowers mod to the sensitivity pot, I found out that the pot was broken. The voltage at pot viper never went below 6 volts. There was a crack in the carbon arc.

Without the mod I got 23 cm, with mod 26 cm. That's in the middle of the noise from a dozen of instruments.

I'll pack it and get some fresh air, finally!