TGSL Tuning & troubleshooting

[Stefano]

I open a new thread just for TGSL tuning, once completed, because in the main thread there are so many posts that I think it's difficult for many (at least for me) find the information you need while troubleshooting the circuit.

Well, this is my situation:
The circuit basically works, I mean that it detects metallic object but there are often chatters (cracks, noise) and iron is detected in both discriminating and not-discr. switch position.
The coil being used is a Laser Widescan 8" coil which I bought as faulty on internet but: I checked resistance and impedence and seems to be correct. At the beginning I noted something wrong in the resistence value so I opened it and found that it was intentionally center tapped, I've removed the center tap and got a standard values coil.
The first picture attached is the signal I got from the coil: it is driven by the TX stage but the RX is connected just to a 1K and 15nF in parallel, not to the circuit.
Scope setting is: Ch1 (upper) 5V/div Ch2 (lower) 10mV/div timebase 20us

Second picture is the scope 0V trace position

Third picture is the output of U101a (pin7), setting is: Ch1 (upper) 5V/div Ch2 (lower) 0.5V/div timebase 20us

Now, I got about 13.8 KHz at TX. It's not correct. How change it? Should I decrease C1 or C2 (I think C1 but I noted that changing any of them gives result... so it's just to be sure....)
Then what if, in future, I 'll make my own coil and got a slightly different impedance? I mean: is it so important to get exactly 14.5-14.7 KHz (I can measure it at the moment only by the scope signal shape period, and it seems 72us...).

Then, should C6 be tuned? if yes how to proceed?

...and this just to start.....

[Stefano]

Well, I'll try to post other picture, just hope someone will notice anything strange to be further investigated.

Taking example from what someone else did on TGSL thread, I've taken scope pictures on the labelled points. Reference for all them is tx signal.

[maxbiri]

Hi Stefanno

At 5 point you need a square wave , like forms you have in 3 and 4; it seem that 393 dont work in the right way.

By max


Ciao Stefano al punto 5 devi avere una quadra simile a quella che ottieni in 3 e 4

[Stefano]

Hi max,

it's not clear what you say "at point 5..." do you mean test point #5 as I labelled on the schematic or img #5?

As it appears on the attached legend, image #5 doesn't refer to test point #5 but #14.

I took example in doing that from post 3896 by Molzar on the TGSL thread, in which he posted a useful and clear example of scope pictures of TGS (not L).

In fact my image #5 (tp #14) should be compared to his image F26 (tp#14 as well), but his has RX signal on Ch.1 where my has TX signal.

[maxbiri]

Sorry Stefano

You are right!
i confused image and numbar 5!
On test point 5 you have a correct square wave form.
May be it's a phase problem, tray reverse rx or tx wire.

Bye Max

[Stefano]

I've tried without success.
Pictures posted have been taken with the Rx polarity that seemed me to give the correct Rx-Tx phase difference.

[Stefano]

New pictures this time referring to Rx signal:
there is something strange, expecially when GEB is rotated at 100% I think the signal is not correct, but why?

[Stefano]

.

[maxbiri]

It seem that there is a problem around image 8-9, same spike and a doubled square wave.
When you test a target you have a doubled sound from speaker? try with a iron target and a not iron target e.g. copper, brass, alluminium ecc ecc.
I built golden sabre about 5-6 years ago and now i dont remember very well but i have seen a problem like yours in same tesoro circuit, maybe golden or eldorado or lobo.

sorry for my poor english.

Bye Max

[Stefano]

Yes max, I have no discrimination in any position of potentiometer but, if I remember correctly, I have single sound for coloured metal, and a double sound for iron.
I say "If I remember correctly" because I have an odd logistical situation: my shack is not at my home but at my sister's house

Furthermore I can't completely reject ferrite with geb trimmer.
This makes me think that something is wrong with signal phase (for this reason I gave coil signal scope picture at the beginning and evidenced that I'm using a coil bought as not working...)
I think that there's something wrong also in img#13 when geb trimmer is at 100% (while at 90% seems still good) but I'm not able to understand why.

Your english is good enogh (at least for an italian as I am )

[peih]

and I have the same problem of non-ferrous metals one tonne for iron double tone, the depth is very good

[Stefano]

Happy Christmas to all of you!!!

I'm still far from considering depth (at the moment it isn't very good).
I wanted to be at first the receiver part fixed, get correct phases, to be sure of the coil I'm using for test is good, then I'll go on later with depth improving.

[porkluvr]

When you turn your GEB trimmer to 100% you have effectively shorted your signal to ground. So the waveform display in 13 would be perfectly normal.

When you state that you have a standard values coil, what is your definition of standard? You should be specific, especially as you stated that you bought a known defective coil and then modified it. How about some numbers??

If you have used the correct capacitors in the transmit oscillator then the only other thing that would substantially affect your transmit frequency is your coil.

Until you decide how or if you will raise the transmit frequency you probably should not worry too much about the receiver phase. To raise tx frequency you would probably want to lower the two capacitors in proportion.

For your false triggering problem (fig.9) you could try adding a 5pF resistor across R17 or else a 100k fixed resistor. The amplifier U101B tends to be unstable without notch components that were parallel in the TGS. I'm not sure if the notch pot was a 10k or 100k. 10k works great to tame instability but it noticably affects the curve of the discriminate pot.

[Stefano]

With standard I mean around 20 ohms the rx and tx resistance (I don't remember exactly) but I remember exactly 5.97 mH and 6.57 mH measured with the homebuilt LCF meter project published on Elektor.
I think too that there's something strange around U101b, also because I was reading the "TGSL Experiments" thread which I had strangely not noticed before where this issue and the lack of the oscillator stabilization components is discussed.
One more doubt is about the not perfect linearity of the tx signal waveform.
I'll make more tests in these holiday days.
Certainly a bunch of reliable waveforms of a working tgsl would be useful...

[porkluvr]

On my 8" concentric coil, RX is 6.2uH and TX is 5.6uH. You should try reversing your TX and RX coils. That will bring your frequency up if you have them backwards.