TGSL Tuning & troubleshooting

[simonbaker]

update

Just an update on what I'm looking into regarding TGSL tuning....

The main issue I'm interested in right now is why some of us have working MDs but don't get more than 15 - 20 cm depth.

My theory is that the reason is a combination of:

1. noise received by the coil is breaking up the target signal and the pieces can't go through the final low-pass filter.

2. The sensitivity threshold depends on the LM308 "output bias voltage" (DC level when no input) and the "input offset threshold" of the LM393 comparator. These two voltage levels may differ between chips and cause some of us to have poor max sensitivity.

---------
To test (1), dfbowers sent me one of his TGSL models that gets 30 cm air depth. I am able to confirm that in my "workshop", I do not get 30 cm depth, more like 20 cm (maybe one day I got near 30). I believe the reason is the noise in my area. However, it is possible something happened to the MD during shipping. To try to confirm which is true, I plan to take the MD as far from noise sources as possible, into the mountains. However, I have no real guarantee that there isn't noise there (when will it end?). So that is my next test -- hard to believe, but I haven't had time to go there yet (actually I went once but by mistake left my Euro target behind).

If I can find a place where dfbowers MD gets 30 cm air test, then I can try (A) attaching my coil to his PCB, and (B) my PCB to his coil.

If case (A) does not get 30 cm, I know my coil is somehow not compatible with his PCB or my coil is just plain bad. I will concentrate on building coils to see what I am doing wrong.

If case(B) does not get 30 cm, then I know my PCB is incompatible with his coil or my PCB is not good. I will first try tuning my oscillator to same frequency as his, then try to make sure my capacitor C6 is same as his to get same RX resonant frequency.

If I still don't get good air depth, then I suspect the level problems at output of LM308 and input to LM393. And proceed to test (2).

--------------
To test (2), I test by connecting dfbowers coil to my PCB, changing LM308 chips and LM393 chip, and also implementing the "dfbowers" mod which makes the sensitivity threshold go negative. If I still can't get 30 cm depth, then there is something more subtle with my PCB that requires careful analysis, such as audio circuit, bad phases at front end, etc. I hope not!

-------------

If I'm able to get around 30 cm depth with my PCB and dfbowers coil, then we have better understanding what to do to fix PCB.

If I can get 30 cm depth with my PCB and dfbowers coil, but not with my own coil, then I need to concentrate on building coil and learn what I'm doing wrong and what is critical. Then can recommend what I found.

---------------

I hope lunamay, Stefano, Dennis the Mennis, Mullihaka, Ultimate Haze, Aiko, Darko, and others can report current status of their TGSL, air depth, any other problems or successes.

-SB

[dfbowers]

Just an update on what I'm looking into regarding TGSL tuning....

The main issue I'm interested in right now is why some of us have working MDs but don't get more than 15 - 20 cm depth.

My theory is that the reason is a combination of:

1. noise received by the coil is breaking up the target signal and the pieces can't go through the final low-pass filter.

2. The sensitivity threshold depends on the LM308 "output bias voltage" (DC level when no input) and the "input offset threshold" of the LM393 comparator. These two voltage levels may differ between chips and cause some of us to have poor max sensitivity.

---------
To test (1), dfbowers sent me one of his TGSL models that gets 30 cm air depth. I am able to confirm that in my "workshop", I do not get 30 cm depth, more like 20 cm (maybe one day I got near 30). I believe the reason is the noise in my area. However, it is possible something happened to the MD during shipping. To try to confirm which is true, I plan to take the MD as far from noise sources as possible, into the mountains. However, I have no real guarantee that there isn't noise there (when will it end?). So that is my next test -- hard to believe, but I haven't had time to go there yet (actually I went once but by mistake left my Euro target behind).

If I can find a place where dfbowers MD gets 30 cm air test, then I can try (A) attaching my coil to his PCB, and (B) my PCB to his coil.

If case (A) does not get 30 cm, I know my coil is somehow not compatible with his PCB or my coil is just plain bad. I will concentrate on building coils to see what I am doing wrong.

If case(B) does not get 30 cm, then I know my PCB is incompatible with his coil or my PCB is not good. I will first try tuning my oscillator to same frequency as his, then try to make sure my capacitor C6 is same as his to get same RX resonant frequency.

If I still don't get good air depth, then I suspect the level problems at output of LM308 and input to LM393. And proceed to test (2).

--------------
To test (2), I test by connecting dfbowers coil to my PCB, changing LM308 chips and LM393 chip, and also implementing the "dfbowers" mod which makes the sensitivity threshold go negative. If I still can't get 30 cm depth, then there is something more subtle with my PCB that requires careful analysis, such as audio circuit, bad phases at front end, etc. I hope not!

-------------

If I'm able to get around 30 cm depth with my PCB and dfbowers coil, then we have better understanding what to do to fix PCB.

If I can get 30 cm depth with my PCB and dfbowers coil, but not with my own coil, then I need to concentrate on building coil and learn what I'm doing wrong and what is critical. Then can recommend what I found.

---------------

I hope lunamay, Stefano, Dennis the Mennis, Mullihaka, Ultimate Haze, Aiko, Darko, and others can report current status of their TGSL, air depth, any other problems or successes.

-SB

That is so strange.. In my basement I can get a 30cm+ bench test EASY.. More likely 35cm. with just about any coil that I build...at just about any time. assuming that my coil has a half decent null, the connections to the coil are are all short and tight. And the coil and detector that I sent you performed almost exactly like the one I still have. Even if the coil I sent you became out of wack somehow, you should be able to quickly attach another on the bench.

Simon, can you tell me a little about the area that you live in?
High density of TV sets or something? Cell towers next door? I have been trying to see if I can find a nationwide EMI map or something.. someone has to have done a study.

Maybe it's just the nature of how things work. I have read that some detectors like the Minelab Relic Hawk are useless in a house. Anyway, I think that the next step would be a trip to the beach.. or like you said, to the mountains. If we truely thought that it were components I could swap with you again just to verify.. I have another circuit... but at this point, I don't think it would matter.

What kind of air test do you get with the Compadre?

Don

[simonbaker]

That is so strange.. In my basement I can get a 30cm+ bench test EASY.. More likely 35cm. with just about any coil that I build...at just about any time. assuming that my coil has a half decent null, the connections to the coil are are all short and tight. And the coil and detector that I sent you performed almost exactly like the one I still have. Even if the coil I sent you became out of wack somehow, you should be able to quickly attach another on the bench.

Simon, can you tell me a little about the area that you live in?
High density of TV sets or something? Cell towers next door? I have been trying to see if I can find a nationwide EMI map or something.. someone has to have done a study.

Maybe it's just the nature of how things work. I have read that some detectors like the Minelab Relic Hawk are useless in a house. Anyway, I think that the next step would be a trip to the beach.. or like you said, to the mountains. If we truely thought that it were components I could swap with you again just to verify.. I have another circuit... but at this point, I don't think it would matter.

What kind of air test do you get with the Compadre?

Don

I sense the depth is there on your TGSL and that it is the noise pickup that is thwarting the last 5 to 10 cm, and yes, I need to get it to a low noise environment. The problem is, how do I prove I'm in a low noise environment??? There really could be some extremely low frequency (ELF) transmissions out here, I just don't know. Or your MD could have changed, but I've handled it fairly carefully. The null is around 1V pp, but I think that is OK - everything seems solid about it. Eventually, I'll send it back to you and you can do a final check and we'll know.

I think the sensitivity knob has to be set just right to get the full depth, and with noise around, I probably can't do that. As I mentioned once, I took your (dfbowers) MD to a park and had a terrible time with "motorboating" beeps after about 5 minutes where I wandered. I didn't have time to fully investigate, but I'm wondering what's up?

I live in a condo in an urban sprawl area. I'm sure there are cell towers not too far. I'm kind of in a valley and there are big microwave repeaters and stuff on the peaks, but I doubt they are a problem. NASA facility is about 5 miles away, I don't think they make that much noise, but who knows. There are other condo buildings beside and across street from me. I suspect mainly urban cables all over, pool pumps, air conditioners. The condo on other side of wall is running a business in his unit and has a bunch of computer stuff, wireless router, etc.

My Compadre has no sensitivity knob and got about 4 1/2 to 5 inches on a quarter out of the box. It also was chattering in my livingroom. I took it upstairs and was able to avoid the chattering, but I was really disappointed in the depth. On Geotech website I found some advice for tuning a couple of pots inside, and I got the air depth up to maybe 8 inches for a quarter. I had to adjust again on soil, I think I found the GB pot. The coil is 5.75 inches in diameter, so I guess that is pretty good. I probably have not found any coins deeper than about 4, maybe 5 inches in playground wood chips and 2, maybe 3 inches in the hardpacked clay out here. It's easy to think you found a deeper target by looking at the hole you dig, but I think I'm being conservative.

I will try to get to the bottom of this. Maybe some people have fine TGSLs and just don't realize it because of local noise.

I sure appreciate having your TGSL, it would be impossible to do these tests without.

Regards,

-SB

[Vladimir]

The null is around 1V pp, but I think that is OK -

For this type of detector phase zero of the coil must be 0.00V pp or in the worst case up to 50mV. Any other setting is wrong and you will have very poor sensitivity, no more than 20cm in the air. It also dramatically increases the noise from poorly adjusted coil.
This is repeated several times on the forum, but it is consistently ignored and re-creates the confusion, I do not understand why?
Regards

[geminis74]

hi vladimir, to nulled the coil, the sensitivity potentiometer is at minimum, or
maximum impact "on something?, I ask because where I work, I think there is much interference
electric, and I am more comfortable, lower the sensitivity to work, if not the detector becomes
unstable.
sorry for my English

[Stefano]

I hope lunamay, Stefano, Dennis the Mennis, Mullihaka, Ultimate Haze, Aiko, Darko, and others can report current status of their TGSL, air depth, any other problems or successes.
-SB

I had a little of time to play with the circuit yesterday, but things seem to mess up more instead of improve.
I've tuned better the tx freq. wich was 14.08 kHz by changing the .22u to .1u in the oscillator circuit. Now I have 14.8x kHz. Sine wave still good.
Checked the freq. of rx LC tuning circuit with a function generator and scope and tried + and - 2kHz offset.
Not found the combination of null with +20° phase offset and reliable working. I still wonder which is the correct phase with the right parameters... I'm able to make it working good only with negative offsets (some -40 / -50°) but then depth is no more than 20cm.
Sometimes I've been able to get more in all metal mode but then couldn't set GEB to reject ferrite and in disc mode also the quarter coin disapperared.
Then most of time facing also problem with beeping, sometimes increasing speed sometimes slowing down. Seemed like a beat frequency somewhere. Tried to give the -5V from an external battery. No fix.
...so nothing new to report...

Stefano

[geminis74]

I think I do not understand, my question is:
to nulled the coil can be done with sensitivity to a minimum?

[dfbowers]

For this type of detector phase zero of the coil must be 0.00V pp or in the worst case up to 50mV. Any other setting is wrong and you will have very poor sensitivity, no more than 20cm in the air. It also dramatically increases the noise from poorly adjusted coil.
This is repeated several times on the forum, but it is consistently ignored and re-creates the confusion, I do not understand why?
Regards

This has not been my observation. I can misadjust my null to + or - 1v
p-p at pin 7 U101a. It does make a slight difference but I can still air test a 1e over 30cm or more. When Simon mentioned this yesterday, I immediately went and tried it. I do not believe that this is his problem, especially since he can take a set of coils not cast in epoxy and adjust them however he wishes.

Don

[Vladimir]

I think I do not understand, my question is:
to nulled the coil can be done with sensitivity to a minimum?

YES

[Vladimir]

This has not been my observation. I can misadjust my null to + or - 1v
p-p at pin 7 U101a. It does make a slight difference but I can still air test a 1e over 30cm or more. When Simon mentioned this yesterday, I immediately went and tried it. I do not believe that this is his problem, especially since he can take a set of coils not cast in epoxy and adjust them however he wishes.

Don

Perhaps the problem is in the location of the coil, it must be at least 1.5m away from all metal objects.
Regards

[simonbaker]

Perhaps the problem is in the location of the coil, it must be at least 1.5m away from all metal objects.
Regards

As dfbowers noted, I also have not seen evidence or theory that null point is super-critical for this particular detector. It may be more so for other detectors, I don't know. I trust dfbowers experience more than mine because I don't get full depth yet with my experiments.

True, I may have metal object nearby. I think though that Ivconic did experiment with metal nearby and it did not affect MD that much as long as metal not moving. But I agree, not smart to null coils with metal nearby! So I'll check some more. Maybe that is why I see larger null from dfbowers coil. All tests subject to further analysis...

I don't know about ability to test loose (not potted in final form) coils because of tiny vibrations causing noise signals. I am interested if others have successfully achieved good air depth with loose coils. Maybe with good solid workbench (and not living in earthquake country) it is possible.

I will do more thorough and careful tests -- so far my tests are just preliminary. I hope to do as follows:

1. Take MD to "noise free" area (preferably more than one).
2. Carefully lay out ruler for measuring.
3. Carefully adjust sensitivity knob and try different settings.
4. Take video to show test, how target is moved, audio beeps, etc.
5. Try pointing the coil in different directions to see if anything changes.
6. Try combinations of my PCB and coil with dfbowers PCB and coil.
7. Maybe play with GB pot if max air depth not achieved.
8. Test discrimination with US nickel.

If I can understand and/or fix my own TGSL, then would like to help work on Stefano, lunamay, etc. Even send through mail if necessary to see what the problem is. It is an interesting puzzle.

Regards,

-SB

[Vladimir]

As dfbowers noted, I also have not seen evidence or theory that null point is super-critical for this particular detector. It may be more so for other detectors, I don't know. I trust dfbowers experience more than mine because I don't get full depth yet with my experiments.

True, I may have metal object nearby. I think though that Ivconic did experiment with metal nearby and it did not affect MD that much as long as metal not moving. But I agree, not smart to null coils with metal nearby! So I'll check some more. Maybe that is why I see larger null from dfbowers coil. All tests subject to further analysis...

I don't know about ability to test loose (not potted in final form) coils because of tiny vibrations causing noise signals. I am interested if others have successfully achieved good air depth with loose coils. Maybe with good solid workbench (and not living in earthquake country) it is possible.

I will do more thorough and careful tests -- so far my tests are just preliminary. I hope to do as follows:

1. Take MD to "noise free" area (preferably more than one).
2. Carefully lay out ruler for measuring.
3. Carefully adjust sensitivity knob and try different settings.
4. Take video to show test, how target is moved, audio beeps, etc.
5. Try pointing the coil in different directions to see if anything changes.
6. Try combinations of my PCB and coil with dfbowers PCB and coil.
7. Maybe play with GB pot if max air depth not achieved.
8. Test discrimination with US nickel.

If I can understand and/or fix my own TGSL, then would like to help work on Stefano, lunamay, etc. Even send through mail if necessary to see what the problem is. It is an interesting puzzle.

Regards,

-SB

Anything that you do, if phase of the coil is below 50mVpp, the results will be poor. I have tried with other values, such as in the SMW, the 250mVpp, but I only got bad results. My TGSL worked well only after additional settings of the coil. The high voltage on the coil result in unstable comparator and can be solved only by reducing sensitivity. In the end, your detector will work with 60% efficiency.
I will try to put a video file on the work of my modifications, and you will be able to see all on what I wrote here.
Regards

[geminis74]

1 V in RX is unacceptable, please check it well, must have at most 45 mV, this afternoon I've tested, and the system which was Vladimir, I could go down more than 13 mV and has remained at 26 mV (I still think it is too, I hope the next go better

[simonbaker]

1 V in RX is unacceptable, please check it well, must have at most 45 mV, this afternoon I've tested, and the system which was Vladimir, I could go down more than 13 mV and has remained at 26 mV (I still think it is too, I hope the next go better

Hi geminis74:

Are you measuring voltage at pin7 of LF353?

Regards,

-SB

[Qiaozhi]

Tesoro coils have a residual voltage of around 15mV (measured at the coil) and sometimes higher. They are not nulled to zero. In fact, if you null the coil to zero you will not be able to ground balance correctly, as the synchronous demodulator will no longer be able to sample at the zero-crossing due to the incorrect phase-shift between TX and TX coils. In other words, the TGSL circuit is not designed to operate with a perfectly nulled coil.