TESORO GOLDEN SABRE

[ivconic]

What I remembered when making the coil for the TGSL (and other Tesoro copies) is that you should stick to the ratio of the difference in inductance between the TX and RX coils around 0.5mH.
RX is always for 0.5mH more inductance than TX.
Second, with the Tesoro models, the two capacitors in the oscillator always define the frequency factor.
They are mostly rounded values: 223+224, 333+334 or 473+474.
And because of that, it was shown and proven to me that it is best to wind the TX between 5.6-5.8mH and the RX between 6.2-6.3mH.
Of course, different inductances will work, but then fine-tuning the capacitance is an additional effort.
For the sake of easier balancing of the coils, it is preferable to always keep a difference of 0.5mH inductance between RX and TX.
Another thing that makes it easier to balance the coils correctly is the geometry of the TX and RX coils.
I think I strictly mentioned this once upon a time because it makes life a lot easier for hobbyists.
The geometry published by Artyom is "crueler" and it will be easier to balance if both "D" halves have a "belly" towards the middle.
In other words; if instead of flat sides towards the middle both "D" halves are elliptical.
How will this affect the performance of such a coil?
Balancing will be 50% easier to do, and the coil will lose about 10-30% in the width of the "focus" of detection.
That is, the "focus" will be narrower and shorter.
If the hobbyist has the conditions for machine winding coils; then a better choice is the geometry that Artyom posted, of course.

[kt315]

thank you my friend. put your pic in a listing GMT King Cobra VLF metal detector, Tesoro Royal Sabre, bare board pcb DIY | eBay
people often ask me how to do coil.

[Artyom]

After having low perfomance on my previous TGSL build (homemade PCB), I decided to build a new one, now from a purchased PCB of TGSL-EDU from PCBway. Although my performance issues are still there. All the components in the new detector are new (resistors, capacitors and transistors), except the ICs.

I think now I have another problem - I can't see any RX waveform changes when passing a metal over the coil, anyone have and idea of what could be the cause?

Here is a small video of it:

https://www.youtube.com/shorts/t4rpszgiYY4

A 2 euro coil is detected at about 5 centimeters with sensitivity at maximum. GB seems to reject ferrite at the almost the end of the turn.

Thanks.

[ivconic]

Ugh, it was a long time ago when I dealt with TGSL.
Maybe some of this will help you... You can't easily replace LM308 with TL071.
TX&RX generally range from 5.5-5.7mH for TX and 6.2-6.3mH for RX. Resistances are in the range of 18-22 ohms.
If I remember correctly; The RX coil resonant frequency is 2kHz higher than the TX coil resonant frequency... or is it the other way around!? I can't remember.
The difference in phase with balanced coils is about 20-22 degrees.
Someone correct me if I'm wrong, I'm really not sure.

[cristiano.sar]

After having low perfomance on my previous TGSL build (homemade PCB), I decided to build a new one, now from a purchased PCB of TGSL-EDU from PCBway. Although my performance issues are still there. All the components in the new detector are new (resistors, capacitors and transistors), except the ICs.

I think now I have another problem - I can't see any RX waveform changes when passing a metal over the coil, anyone have and idea of what could be the cause?

Here is a small video of it:

https://www.youtube.com/shorts/t4rpszgiYY4

A 2 euro coil is detected at about 5 centimeters with sensitivity at maximum. GB seems to reject ferrite at the almost the end of the turn.

Thanks.

[Adam24]

I was not able yet to solve the flat bottom of RX coil waveform at higher amplitudes, but at low amplitude it seems ok.
I did some changes in the board like removing the 100pf cap of the TL071 (I forgot that it was only needed for LM308 and also replaced some resistors around RX preamp with 1% precision ones.
I was able to null at aprox. 20 degree phase shift and the RX signal is at a very low level as you can see in the video.
I can reject ferrite and a mineral (tiger eye rock that contains iron in the end of the video) and detect a silver ring and a 1 euro coin. The GB trimmer is about 40% clockwise.
The problem is performance, I just get the tested targets at about 15cm.
Another problem is that coils have 6 and 6.5mH, TX freq. measured at 14.53khz, but I am only able to properly null at 20 degrees when my resonance capacitors have 12.96nf, what gives a resonance freq. of 17.3khz. If I try to use the expected capacitance (15nf), the phase shift increases and I can't properly null.
The coils are unshielded for now and I'm using a USB cable (shield is floating) to connect them to the board.
The coil shape was copied from an original tesoro 11x8 coil.

Thanks for your efforts,i couldn't play the video is there a link

[cristiano.sar]

Thanks for the help.
I changed the LM358 by 2 others of different brands and the result is more or less the same. Also tested the resistors and capacitors around the power supply and all is within the spec. Anyway the negative voltage is always around -6.4v . Now I'm waiting another LM358 just to do another test. I also noticed that moving the RX coil, in a certain position I have an almost perfect sine wave of the RX. I will try a little more and if I can't get good results I will make another board (purchased some from pcbway) because this one was my first and was home etched.

I was not able yet to solve the flat bottom of RX coil waveform at higher amplitudes, but at low amplitude it seems ok.
I did some changes in the board like removing the 100pf cap of the TL071 (I forgot that it was only needed for LM308 and also replaced some resistors around RX preamp with 1% precision ones.
I was able to null at aprox. 20 degree phase shift and the RX signal is at a very low level as you can see in the video.
I can reject ferrite and a mineral (tiger eye rock that contains iron in the end of the video) and detect a silver ring and a 1 euro coin. The GB trimmer is about 40% clockwise.
The problem is performance, I just get the tested targets at about 15cm.
Another problem is that coils have 6 and 6.5mH, TX freq. measured at 14.53khz, but I am only able to properly null at 20 degrees when my resonance capacitors have 12.96nf, what gives a resonance freq. of 17.3khz. If I try to use the expected capacitance (15nf), the phase shift increases and I can't properly null.
The coils are unshielded for now and I'm using a USB cable (shield is floating) to connect them to the board.
The coil shape was copied from an original tesoro 11x8 coil.

[kt315]

I am sending from Lietuva. I do not offer KITs and components because Lietuva does nothing of electronics now. I use desoldered ones from trash boards.
All my projects are based just on using of desoldered components. You can buy them in India and China for low prices.

pleasure

[Adam24]

i still have a PCBs' batch on GMT KING COBRA (GMT GOLD SCORPION) and can send to you. 18 euro cost with sending.

Where are you sending it for m?
also does it have the components on it?

[kt315]

Hi there this is Adam from Egypt,so have you found a schematic with clear print detailed yet workable.thanks

i still have a PCBs' batch on GMT KING COBRA (GMT GOLD SCORPION) and can send to you. 18 euro cost with sending.

[Adam24]

I am working in Protel DXP,,he is very dificult ,,I recommend you eagle,,but find full version,,,,ask your friend is this board tested ,,does it work,,,I started to losing faith in people and projects on this forum,,i made a board for Coin hawk and he doesnt work,,then I made'd board for white 3900 and he also doesn't work,,I am investing lot of time and effort,,,in last time that was vain..

Hi there this is Adam from Egypt,so have you found a schematic with clear print detailed yet workable.thanks

[pito]

Now I'm waiting another LM358 = wasting the time , it is not going to help.
I also noticed that moving the RX coil, in a certain position I have an almost perfect sine wave of the RX = because input signal is so small that after amplification it is below amplitude limitation.
The simplest solution is reducing amplifier gain by increasing capacitor across pins 6 and 7, 10pF it is wrong.

[cristiano.sar]

Thanks for the help.
I changed the LM358 by 2 others of different brands and the result is more or less the same. Also tested the resistors and capacitors around the power supply and all is within the spec. Anyway the negative voltage is always around -6.4v . Now I'm waiting another LM358 just to do another test. I also noticed that moving the RX coil, in a certain position I have an almost perfect sine wave of the RX. I will try a little more and if I can't get good results I will make another board (purchased some from pcbway) because this one was my first and was home etched.

[pito]

the question was in post #6223

[ArchibaldSTM]

We are not interested in the lower half-wave --- only the upper half-wave is used for subsequent processing