TGSL troubleshooting questions

[simonbaker]

I have checked the pcd , aparently i don't have any shorts , i also measured the voltage of pin 4 with respect to ground on every operational amplifier and i got 6.25 V. It seams like the discrimination rejects non ferous objects before it rejects ferous objects... i gues the only option remaining is to put the circuit on a scope.

There are many things to check.

Do you have RX coil leads reversed? (makes double-beep with target if you do).

Do you have DISC pot leads reversed? (will work backwards, but OK).

Have you mounted and fixed your coils, or are they still loose for testing?

What are you using for coil cable? Sometimes certain cables very touchy, cause beeps easily, or connectors are noisy.

Which exact circuit did you build (there are several versions)?

Does your sensitivity pot affect the beeping?

Is the beeping the same with headphones and speaker?

Is the speaker or headphones far enough away from circuit board?

Do you have power lines nearby or other strong EMI sources nearby?

Regards,

-SB

[soverey]

7808 shouldnt be problem - being hot is just consequences of another problem. Shorts, wrong components, components wrong way, damaged components(may be another consequence).
Without scope its hard to troubleshoot such circuits like VLF metal detector.

Check PCB very carefully - most problems arise because of builder errors not because parts themselves are bad.

I have checked the pcd , aparently i don't have any shorts , i also measured the voltage of pin 4 with respect to ground on every operational amplifier and i got 6.25 V. It seams like the discrimination rejects non ferous objects before it rejects ferous objects... i gues the only option remaining is to put the circuit on a scope.

[habitbraker]

7808 shouldnt be problem - being hot is just consequences of another problem. Shorts, wrong components, components wrong way, damaged components(may be another consequence).
Without scope its hard to troubleshoot such circuits like VLF metal detector.

Check PCB very carefully - most problems arise because of builder errors not because parts themselves are bad.

[soverey]

I have read them , there is an isue related i think to the lm7808, the beeping sound that i've mentioned in the firts post still continues to happen, related to the lm 7808 overheating i have measured the current used by the circuit , and in standby is 35 mA , and when the beeping starts is oscilating 45 - 60 -70 -89 mA also this oscilation happens when i wave a target over the coil. Could 7808 be damaged and causing this problems?

And by the way it works worse each time , now every time i wave a target over the coil , it beeps and it does not stop

[soverey]

Read those if you havent already: http://www.geotech1.com/forums/showp...postcount=4649

I have read them , there is an isue related i think to the lm7808, the beeping sound that i've mentioned in the firts post still continues to happen, related to the lm 7808 overheating i have measured the current used by the circuit , and in standby is 35 mA , and when the beeping starts is oscilating 45 - 60 -70 -89 mA also this oscilation happens when i wave a target over the coil. Could 7808 be damaged and causing this problems?

[habitbraker]

Read those if you havent already: http://www.geotech1.com/forums/showp...postcount=4649

[habitbraker]

Well, DC voltage on C15 should change when waving different metals. It will only stay const. when waving metal which is just on accepted-rejected border.
Yes - also when turning disc pot.

But maby your multimeter cant register such fast changes(target waving speed). So probobly its OK.
Its hard without scope.

Silver ir rejected also when GND bal. is set incorrecly - see Dfbowers TGSL guide for correct addjutment.

Its good you have found that V-regs getting hot - it shouldnt!
Look for shorts somewhere on pcb. Did you made your pcb yourself?

[soverey]

Yes calculated f of tx is about 14Khz (f=1/(2*pi*sqrt(L*(c1*c2)/(c1+c2))))
Remember that cable, shield and interwinding C adds up - real f will be lower, so your messured f seams to be correct.

I think your coil is ok (at least for first one and for testing)

I would be more woried about disc. Have you acces to scope?

If no - you can try measure DC on C15 (in disc mode) to see if it changes when target is waved accross the coil.

I do not have access to a scope, i have measured the DC on C15 and it is constant when waving a target over the coil , it changes when I turn the discrimination pot. This time i got detection in disc mode but ,when i disc out iron i also disc out silver or other metals. (gues is due also to the ground pot adjustment ) I noticed that after lets say 25 minutes or more of usage LM 7808 becomes hot.

[habitbraker]

Yes calculated f of tx is about 14Khz (f=1/(2*pi*sqrt(L*(c1*c2)/(c1+c2))))
Remember that cable, shield and interwinding C adds up - real f will be lower, so your messured f seams to be correct.

I think your coil is ok (at least for first one and for testing)

I would be more woried about disc. Have you acces to scope?

If no - you can try measure DC on C15 (in disc mode) to see if it changes when target is waved accross the coil.

[soverey]

Just measured the caps ,got for C1 0.218 uf , C2 0.023 uf, and C6 0.015uf ,seems to be ok. I will go with the coils at a smal coil making facility to see of i can measure my coils there and compare whit my results.

[soverey]

Thank you habitbraker, i have measured the frequency and i've got for tx 13.66 khz and for rx 13.47 , i gues either my inductance meter is wrong or the frequancy meter. Another thing that i'm not sure , when measuring the frequancy i've connected the positive lead to the colector of TR1 as you said and i've got a frequancy and then put the negative lead on the ground and the frequency increased whit 0.22khz and I took this last value, hope it is corect. Going to measure now C1 ,C2 and C6 to see if they are close to spec.

[habitbraker]

Hi!

You can measure f on many points, for example on collector of oscillator transistor (TR1).

Looks like your disc channel doesnt work - do you have access to scope?

[soverey]

Hello colleagues,

I recently finished the TGSL EDU circuit, i want to measure the tx frequency with my multimeter and don't know where to connect the leads to the circuit. I have build also the coil, my results being 16,5 cm for 2cm bras coin in all metal , in discrimination i do not have any detection, neither iron or other metals. My tx coil is 6.15 mh and rx is 6.55 mh (105 for tx and 110 for rx whit 25.3 and 26.1 ohms) Another thing that noticed is when the sensibility is more then 50% ,it stars to make "beep beep, beep continously an sometime this happens after a test target is waved over the coil) I hope to find the fault , thanks in advance.

Soverey.