step by step building

[windaube]

I tried cable cat5 but the frequency decreased

I tried the capacitor in parallel with c6, at 19nf it seems to work better than with 15nf, it is the best result with 19nf
I also have better results with the GB Pot at MIN

despite the low sensitivity, the discrimination works well
I removed the shielding from the coils but no improvement

I don't know where to look for the fault

[waltr]

Measure the DC Voltage on output of U103 & U104. These should be very near zero Volts.
Which op-amps did you use here?

Did you adjust the Sens. control (R36) all the way up and did the audio 'chatter' then.
If no chatter with Sens all the up then the outputs of U103,104 have too much offset.

You could try the Sens adjust mode.. Take bottom of R36 off ground and connect to -5V.

[windaube]

Measure the DC Voltage on output of U103 & U104. These should be very near zero Volts.
Which op-amps did you use here?

U103 - pin 7 = +0.5v
U103 - pin 1 = -10mV
U104 - pin 1 = -15mV
I use those of the documentation, U103 LM358N and U104 LM308N

Did you adjust the Sens. control (R36) all the way up and did the audio 'chatter' then.
If no chatter with Sens all the up then the outputs of U103,104 have too much offset.

no effect at MAX,sensitivity doesn't seem to work
I see no difference between MIN and MAX, i use 10K pot

You could try the Sens adjust mode.. Take bottom of R36 off ground and connect to -5V.

I'm going to try

here are two videos :
discri test :
https://www.youtube.com/watch?v=UoVk...ature=youtu.be

TGSL faill:
https://www.youtube.com/watch?v=iwme...ature=youtu.be

thank you very much for the help

[waltr]

Opoops...I meant U105 and U104 pin 6. I didn't zoom in enough on the schematic and mis-read the refdes.

Recheck the Sens pot- Measure with you DVM the Pot wiper, It should change from 0V to +8V when turning the pot.

The LM308 is an old part and VERY Hard to obtain. Many, many Fakes are sold as LM308.
I and many others use TL081 for U104, 105.

When I built the TGSL increasing the Sens pot did not cause audio chatter so did the Mod of applying -5V to the pot instead of ground. Now audio chatter just past half turn and sensitivity increased.

[windaube]

U104 Pin 6 -0.10 -0.15mV
U105 Pin 6 -0.10 -0.15mV

the sens pot work, 8.2V at MAX and -6.06V at MIN, the mod with the -5v does not change anything

i buy the lm308 in china, i think i lost my money

i will look to buy tl081...

[windaube]

I do not know if this is important but I replaced C24 and c25 by two 0.01 capacitors in parallel (I had more than 0.022)

[waltr]

-0.15mV is ok on U104,105 outputs. So the LM308's should be fine but if you can get TL081's these are worth trying.

20nF for C24 & C25 should be fine. These are simply add some filtering to the comparator output.

Odd that with -5V on sens pot and adjusted all the way up the audio doesn't chatter.

Possibly issue in the Audio circuit. I have read some having issues with the type of speaker used.
Check at TR2's base with your scope. With Sens pot down it should be DC and then have audio low duty square wave with sens pot up.

[windaube]

Odd that with -5V on sens pot and adjusted all the way up the audio doesn't chatter.

unfortunately the audio doesn't chat

Possibly issue in the Audio circuit. I have read some having issues with the type of speaker used.
Check at TR2's base with your scope. With Sens pot down it should be DC and then have audio low duty square wave with sens pot up.

I look at tr2, the pin is still at -5.80V, when there is metal the wave goes back to 0v and peaks at 2v,
if I move the pot sensitivity tr2 remains stable at -5.80v

I have a doubt about the assembly of the j107 (TR4 TR5), how to check that it works?

[windaube]

the u107 (lm35 was out of order !, I replaced the u107 now the pin 7 is at -5v and switches to + 11v when the metal rings.

I look at pin 5 of the u107, the pin is at -5v and goes up to 0V when there is metal.

when I put the sensitivity to MIN, I have -5v (stable)
when I put the sensitivity to MAX, I have -2 -3V and the wave oscillates
the sensitivity seems to work but not enough to trigger the threshold

maybe the u106 is the problem ?

[waltr]

Not sure what may be wrong now.
Since you did use sockets for the ICs do try swapping in new ones.

Check pins 2,6 of U106. When lower then both U104 & U105 outputs Audio should sound.

What are you using for a speaker?
Maybe try something else or disconnect speaker and connect to O'scope.

[windaube]

hey
I opened a dedicated topic for the problem
https://www.geotech1.com/forums/show...tivity-problem

Check pins 2,6 of U106. When lower then both U104 & U105 outputs Audio should sound.

the voltage goes from -25mV to +40 mV when I touch the discri pot

What are you using for a speaker?

I use an 8ohms 40W, I tried with 4Ohms but it does not change anything

Maybe try something else or disconnect speaker and connect to O'scope.

I looked and I have no false signals, when I have a signal there is necessarily noise, so it's not a problem on the audio side

I think more and more that the U106 does the job badly

[gost373]

Hi, I also tried to make this detector! I spent hours breaking my teeth on it! do and redo the map several times!
I never succeeded !! good luck ! yes look at the side of the transistors! are they well positioned? I will follow you on our project! courage!


Salut , j'ai essayer moi aussi de r?aliser ce d?tecteur ! j'ai pass? des heures a me casser les dents dessus ! fait et refait plusieurs fois la carte !
j'ai jamais r?ussi !! bon courage ! ? oui regarde du cot? des transistors ! son t' ils bien positionn?es ? je vais suivre t'on projet ! courage !