TGSL Tuning & troubleshooting

[lunamay]

the problem is that i do not have a scope..only a good multimeter

when i turn on my md there's a signal only on"one chanel" that means it works good when it's switched to all metal position..when i switch to other position there's no response..firstly i thought that maybe a half of 393 was broken but i don't think that this may happen..so i was looking on schematic and maybe i'm wrong but i thing that one of j107 is responsible for all metal function and secound is responsible for disc function..so there may be my problem..but like i said i can be wrong..i have replace all ic's so its very posible that my transistor can be broken

another question about ballancing coils: to balance a coil i should check the voltage on rx coil on beetwen pin 6 and 5 right

maybe my questions are stupid but i'm trying to lern how much i can to know this project as good as posible

anyway i'm greatfull for any help

when i finish and repair my md i will post mesurements to help other people

thanks a lot and regards to all

[Qiaozhi]

anyone thought about my problem??:P(#451)..i was looking on schematic and i think it may be one of j107 tranzistors..any thoughts???

best regards to all

The problem with your request is that it's too vague. In essence you have said, "My detector is broken. Whats wrong?". There is simply not enough detail. You will need to debug the problem logically and use an oscilloscope, otherwise you are just firing arrows in the air.

[lunamay]

anyone thought about my problem??:P(#451)..i was looking on schematic and i think it may be one of j107 tranzistors..any thoughts???

best regards to all

[Qiaozhi]

Monk,
No, I meant C13. I missed C12 when I looked at your circuit, but I see now that C12 and C13 are in parallel, so both tune the coil. I really wanted to know what the loss in amplitude is when the Rx coil is untuned, and that means leaving both caps out.

They don't seam to shift the phase appreciably, but C12 certainly increases the gain - although the tuned circuit is way off resonance.

The Rx coil of Micronta has a much higher inductance, which I suppose is why they left it untuned, but it would probably benefit from tuning.

Are you sure it's not tuned? Have you checked inside the search coil to see if there's a tuning capacitor there?

[Qiaozhi]

Yes, I guess my question would be: Should the Rx zero crossing (Source pin of TR5) coincide with the 0V crossing at the outputs of the GEB channel?

You will not get the same response at the outputs of the GEB channel, because it reacts only to a change in the signal, and not the DC level.

[dfbowers]

Hi Don! That's great you can get quarter at 10 inches at the beach -- can't wait to get out on some sand. Beach seems like a place where targets would sink easily down, so a really deep detector would be interesting.



Can you elaborate that question? I'm between models here so need to know what assumptions you're referring to. In the past I assumed that ground balancing had nothing to do with the RX zero crossing because I treated ground as a target with a specific phase. This is how my ferrite targets seem to behave. In such a case, you wouldn't care about the RX zero crossing (which essentially is the null signal zero crossing). And that was my advice.

But I'm going with Qiaozhi's model of how ground influences the RX signal, which is a pure amplitude modulation, so the ground has no distinct phase of its own. In that case, we just want to zero out the null signal (with the addition of any steady signal from the real ground) at the output of the Synchronous Detector (capacitor C12). To do that we have to make sure the null zero crossing is in the range of the GB control (by shifting our coils until the zero crossing is under the GB sync pulse center), and we might as well choose the middle position so if the null drifts either way, we can readjust the GB pot.

However, I'm still musing if there is a possible issue with ferrite targets that do act like a real target and create a signal with a distinct phase. If so, it can still be dealt with I think, but it puts a tighter constraint on your coil building and nulling activity, because it means you don't want to tolerate any drift of the null from your design point. That's all I was musing in my last post.



Theoretically the outputs of the LM308 that feed the pins 3 or 5 are zero all the time except when a target makes a pulse, regardless of the null point. So not sure what you mean there.

But if you mean no "pulse" at pin 5 while ground balancing by waving your brick, yes, theoretically that should happen when you center the GB sync pulse over the null signal zero crossing -- meaning you are ground balanced. (I'm aware there are some additional tweaks to take care of secondary effects.). But this will only be true if you use a true-blue hunk of ground material that affects the coil like real ground. My wannabee ferrite chunks don't seem to act like that -- they don't give a darn where my RX null zero crossing is.



I've got my Euro. Now the hunt is on for a realistic ground object to use. Although I'm questioning how important it is to do that step if you have a scope to ground balance with by setting the GB sync pulse phase over the null RX zero crossing.

Don: when you get a chance to use your ferrite and brick, try ground balancing with and without the null signal zero crossing under the GB sync pulse center and let us know what happens.

I would actually be interested in someone testing how the TGSL performs when it is not ground balanced. Play with the GB in the field and report what they observe.

Regards,

-SB

Hey Simon,

Disregard my question from earlier.. it was a total brain fart and I think I'm just getting old. (I even passed for a senior citizen at a place yesterday)..

I just need to double check some of my observations from last week when I get home, on what test point it was I was monitoring when nulling my coils.. it may have been U103 pin 7 now that I think about it.. All DC levels get filtered out downstream. Probably the same as monitoring at C12.

As far as Rx zero crossing goes, I have not found that it matters but Qiaozhi's argument makes sense, particularly from a quality control perspective. I may just pay particular attention to that when making subsequent coils just to help eliminate variables between coils and allowing for a full range of adjustment.

I will let you know about balancing with and without the null signal zero crossing under the GB sync pulse center next week when I have my scope to set it up.

I have not gotten to Spice simulations yet, but that's something I want to play with soon. I was wondering.. how would your simulations behave NOT grounding the Rx coil at all? Based on all my field test, the ONLY coil I have made that is suitable to be used in wet grass is the last one - Shields only connect to ground on the PCB and nowhere else.. Both leads from the Rx coil float free. Performance is not noticeably different is actual use and the benefits of a noise free coil on wet ground seems to be the way to go..

BTW.. I found enough change on the beach in a few hours to buy my next set of batteries. I even found a euro! Finding coins in loose sand has it challenges as coins are more than likely on edge.. No gold yet, but I have a few days to go..



Don

[pebe]

Monk,
No, I meant C13. I missed C12 when I looked at your circuit, but I see now that C12 and C13 are in parallel, so both tune the coil. I really wanted to know what the loss in amplitude is when the Rx coil is untuned, and that means leaving both caps out.

They don't seam to shift the phase appreciably, but C12 certainly increases the gain - although the tuned circuit is way off resonance.

The Rx coil of Micronta has a much higher inductance, which I suppose is why they left it untuned, but it would probably benefit from tuning.

[simonbaker]

I think you mean C12 (not c13).



The following simulation showing C12=15nF and C12=0nF. measurement taken at node 4 of schematic (output of U101A).



I'm also applying a simulated ground response (90° phase lag with respect to transmit waveform).

I suspect that some who have experienced problems getting TGSL proper sensitivity may be due to an exact component values i.e. it doesn't take much variation in the tuning capacitor to make a significant difference in Rx response (amplitude and phase angle).

I'm sure that's true in some cases, but that's an extreme change from 15nF to 0 nF. But my 15 nF caps often measure at about 13 nF, so it is a concern. Also you can get your coil inductances wrong. It is definitely a tuning area to keep an eye on.

My instinct is that you want to get your RX resonant frequency as close to the TX oscillator frequency without introducing more than about 5 to 10 degrees phase shift in the filter. That seems to be the nominal design point. If you get them too far apart, you lose gain; too close, phase problems. That's what I think from looking at it with a frequency analysis in LTSpice simulator.

Regards,

-SB

[simonbaker]

If in fact the ground never contains anything that could produce a signal like my ferrite targets, then I'd just pick the center pot position and not worry if the null slips to a lower position.
-SB

Editing myself: that sounds misleading, so it should read:


If in fact the ground never contains anything that could produce a signal like my ferrite targets, then I'd just pick the center pot position and not worry if the null slips to a lower position because I can adjust the GB pot for the new null position.

[Monk]

Monk, Have you tried a simulation with C13 removed, ie. with an untuned Rx winding.

I would be interested in its phase and amplitude (because I am currently looking at the best way of modifying the front end of a Micronta 4003, which is untuned).

I think you mean C12 (not c13).



The following simulation showing C12=15nF and C12=0nF. measurement taken at node 4 of schematic (output of U101A).



I'm also applying a simulated ground response (90° phase lag with respect to transmit waveform).

I suspect that some who have experienced problems getting TGSL proper sensitivity may be due to an exact component values i.e. it doesn't take much variation in the tuning capacitor to make a significant difference in Rx response (amplitude and phase angle).

[simonbaker]

Hi Don! That's great you can get quarter at 10 inches at the beach -- can't wait to get out on some sand. Beach seems like a place where targets would sink easily down, so a really deep detector would be interesting.

If the GB trimmer never needs to be changed while adjusting the null, then what purpose would it serve to seek the Rx zero crossing? Other than to possibly allow for maximum range of adjustment and/or maximum sensitivity?

Can you elaborate that question? I'm between models here so need to know what assumptions you're referring to. In the past I assumed that ground balancing had nothing to do with the RX zero crossing because I treated ground as a target with a specific phase. This is how my ferrite targets seem to behave. In such a case, you wouldn't care about the RX zero crossing (which essentially is the null signal zero crossing). And that was my advice.

But I'm going with Qiaozhi's model of how ground influences the RX signal, which is a pure amplitude modulation, so the ground has no distinct phase of its own. In that case, we just want to zero out the null signal (with the addition of any steady signal from the real ground) at the output of the Synchronous Detector (capacitor C12). To do that we have to make sure the null zero crossing is in the range of the GB control (by shifting our coils until the zero crossing is under the GB sync pulse center), and we might as well choose the middle position so if the null drifts either way, we can readjust the GB pot.

However, I'm still musing if there is a possible issue with ferrite targets that do act like a real target and create a signal with a distinct phase. If so, it can still be dealt with I think, but it puts a tighter constraint on your coil building and nulling activity, because it means you don't want to tolerate any drift of the null from your design point. That's all I was musing in my last post.

I did notice that when setting up my coils, 0 Volts at pins 3 or 5 of U106 coincided closely to a null at pin 7 U101. Would the 0 Volt crossing at pins 3 or 5 of U106 coincide with the Rx zero crossing? (I don't have my scope with me this week).

Theoretically the outputs of the LM308 that feed the pins 3 or 5 are zero all the time except when a target makes a pulse, regardless of the null point. So not sure what you mean there.

But if you mean no "pulse" at pin 5 while ground balancing by waving your brick, yes, theoretically that should happen when you center the GB sync pulse over the null signal zero crossing -- meaning you are ground balanced. (I'm aware there are some additional tweaks to take care of secondary effects.). But this will only be true if you use a true-blue hunk of ground material that affects the coil like real ground. My wannabee ferrite chunks don't seem to act like that -- they don't give a darn where my RX null zero crossing is.

A word about ferrite.. Obviously we all have different samples, but several of mine are nearly the same. A clay brick can closely approximate the ground as well (obviously all bricks can be different too). I have found that some of the clay bricks that I have closely approximate the soil where I live, so I can GB indoors. My ferrite slugs tune out slightly clockwise on the GB trimmer from where clay bricks do. So.. probably a safe bet to GB on ferrite.

I've got my Euro. Now the hunt is on for a realistic ground object to use. Although I'm questioning how important it is to do that step if you have a scope to ground balance with by setting the GB sync pulse phase over the null RX zero crossing.

Don: when you get a chance to use your ferrite and brick, try ground balancing with and without the null signal zero crossing under the GB sync pulse center and let us know what happens.

I would actually be interested in someone testing how the TGSL performs when it is not ground balanced. Play with the GB in the field and report what they observe.

Regards,

-SB

[dfbowers]

Isn't U106 the LM393 comparator? In which case you are looking at the individual outputs of the GEB and DISC channels. The correct point to measure, for detecting the position of the zero-crossing, is the source pin of TR5.


This is because salt water is conductive, and the transition area between dry and wet sand is the domain of the PI detector. A VLF will always have problems there. Once you get into the totally wet sand, the ground conductivity becomes consistent, and (as you have found) you can use the DISC control to knock out the salt.

Yes, I guess my question would be: Should the Rx zero crossing (Source pin of TR5) coincide with the 0V crossing at the outputs of the GEB channel?

[lunamay]

hi
i have a problem and i'm open for any sugestions
once i connected wrong my tgsl to power suply and i broke it..some time ago i replace broken parts and it works again ..but not fully
today i made a coil with excelence parameters and i noticed that it works only on all metal mode..i checked it with scisors and a silver ring but when i switch on secound mode there's no response

now i'm trying to find where is a problem but i'm wasting a lot of time to find it..maybe someone could help me and tell which parts are responsible for switching mod from all to disc by switch..maybe some ic are broken..but it's strange because on all it works

best regards

[Qiaozhi]

Hi Simon,

My preference would be for the ferrite on the left. The eddy current response will be enhanced for the ferrite ring.

[Qiaozhi]

I did notice that when setting up my coils, 0 Volts at pins 3 or 5 of U106 coincided closely to a null at pin 7 U101. Would the 0 Volt crossing at pins 3 or 5 of U106 coincide with the Rx zero crossing? (I don't have my scope with me this week).

Isn't U106 the LM393 comparator? In which case you are looking at the individual outputs of the GEB and DISC channels. The correct point to measure, for detecting the position of the zero-crossing, is the source pin of TR5.

Now, this week, I have been hunting the eastern shore of Maryland. The sand on the beach is difficult to GB to.. probably very little ferrite. But, as I walk closer to the salt water, my TGSL will false until I turn the DISC up a little. Then I can walk right up to the edge of the water (wet sand) and it's quiet. Salt drops out around where thin foil (like a cigarette pack) does. I still have been getting decent depth in wet sand.

This week I have dug several deep quarters in the 10"+ range, so detection range is not so limited in sand, like hard packed clay.

Don

This is because salt water is conductive, and the transition area between dry and wet sand is the domain of the PI detector. A VLF will always have problems there. Once you get into the totally wet sand, the ground conductivity becomes consistent, and (as you have found) you can use the DISC control to knock out the salt.