TGSL Tuning & troubleshooting

[dfbowers]

hello to all
I Collected TGSL in my Home ,Without LM308 Not gutted. I read in the Geotech forum Some participants exchanged LM308 to TL071 . My advice do not waste your time in TL071.its Weak sensitivity to detect, the maximum height from coil 20 to 22 of the coin 2.5 cm and Low Sound.
what I do need help. I do not have a Visa Card to buy lm308. I live in Sudan. If you can help please please I need your help , I put everything I have in the Collected of TGSL.
Sorry I'm weak in the English language I hope my message is clear
Best greetings

I disagree with the statement that you cannot substitute a TL071. LM308 is obsolete and if I'm not mistaken some manufacturers will recommend the TL071 as a modern replacement. I have handfulls of both and if that were your problem I would surely be able to reproduce it!! I can demonstrate that you can use TL061, TL071 or TL081 in place of the LM308 without significant issues .

The only difference I see in the TGSL application is that you should use 100nF caps between the supply and ground with the LM308s.

Don

[Dennis the Mennis]

hello to all
I Collected TGSL in my Home ,Without LM308 Not gutted. I read in the Geotech forum Some participants exchanged LM308 to TL071 . My advice do not waste your time in TL071.its Weak sensitivity to detect, the maximum height from coil 20 to 22 of the coin 2.5 cm and Low Sound.
what I do need help. I do not have a Visa Card to buy lm308. I live in Sudan. If you can help please please I need your help , I put everything I have in the Collected of TGSL.
Sorry I'm weak in the English language I hope my message is clear
Best greetings

Hi Salih ,

I think it is too early to say that the TL071 is the problem here ...
I've also tried the TL071 and the TL081 like Ivconic and compared them with the LM308 : at that moment I didn't see much difference ( must say I have a rather noisy 'workshop' )

Do you have a scope ?

kind regards ,

Dennis the Mennis

[ivconic]

I believe the main importance of the LM308 is it is a precision op amp... this means that there is very little output bias, so it is good for presenting an exact voltage to the next stage, which is a comparator that will trigger the audio at an exact threshold (adjustable by the sensitivity pot).

I feel a different op amp can be used, but you may need check the outputs for bias (offset). If both op amps have a bias but it is identical, it may not be a problem if the sensitivity pot can adjust the threshold to compensate. dfbowers showed a mod to the TGSL PCB wiring so the sensitivity pot has a wider range, which helps to compensate.

I do not believe the op amps really affect the audio strength -- but it may seem so if they have a large negative bias in the output voltage -- this can make the detection pulse shorter and so sound weaker.

In summary, I believe the TL071 can be used if you are careful to check the output bias. Basically short the inputs together a see how close the output is to zero. Hopefully less than 5 millivolts.

-SB

I tried TL081. Honestly; i haven't spotted significant differences in TGSL behavior. On contrary; i have feeling that it runs more "smooth" than LM308. It was on one of previous pcb versions, "CZ sized pcb" or something like that..
I guess other type opamps can be used there too.

[satdaveuk]

I've used the ( mylar) shielding out of a VGA cable and it worked for me , it is very strong so you can wrap it tight.

I don't know the results of the Mylar from a survival blanket or the Mylar as Waikiki_Sweep mentioned for my pcb ( also with less aluminium compared to the Mylar from the VGA cable) , maybe even better ...
Which did you use ?

I've used it in combination with Belden cable ; 4 wires in 2 pairs with each pair separately shielded

kind regards ,

Dennis the Mennis

Hi Dennis
I used the Mylar from survival blanket you can buy at most medium/large camping/sports shops for under £3, bought 2 enough to last a life time, certainly does the job, just be carefull you use the conducting side for your shield cable because both sides look identical.
Regards

[porkluvr]

Has anyone tried OP07 as a substitute for LM308? I realize that OP07 doesn't exactly grow on trees but that might be another avenue where LM308 is hard to come by.

[simonbaker]

hello to all
I Collected TGSL in my Home ,Without LM308 Not gutted. I read in the Geotech forum Some participants exchanged LM308 to TL071 . My advice do not waste your time in TL071.its Weak sensitivity to detect, the maximum height from coil 20 to 22 of the coin 2.5 cm and Low Sound.
what I do need help. I do not have a Visa Card to buy lm308. I live in Sudan. If you can help please please I need your help , I put everything I have in the Collected of TGSL.
Sorry I'm weak in the English language I hope my message is clear
Best greetings

I believe the main importance of the LM308 is it is a precision op amp... this means that there is very little output bias, so it is good for presenting an exact voltage to the next stage, which is a comparator that will trigger the audio at an exact threshold (adjustable by the sensitivity pot).

I feel a different op amp can be used, but you may need check the outputs for bias (offset). If both op amps have a bias but it is identical, it may not be a problem if the sensitivity pot can adjust the threshold to compensate. dfbowers showed a mod to the TGSL PCB wiring so the sensitivity pot has a wider range, which helps to compensate.

I do not believe the op amps really affect the audio strength -- but it may seem so if they have a large negative bias in the output voltage -- this can make the detection pulse shorter and so sound weaker.

In summary, I believe the TL071 can be used if you are careful to check the output bias. Basically short the inputs together a see how close the output is to zero. Hopefully less than 5 millivolts.

-SB

[salih]

TL071 and TL061 Ineffective

hello to all
I Collected TGSL in my Home ,Without LM308 Not gutted. I read in the Geotech forum Some participants exchanged LM308 to TL071 . My advice do not waste your time in TL071.its Weak sensitivity to detect, the maximum height from coil 20 to 22 of the coin 2.5 cm and Low Sound.
what I do need help. I do not have a Visa Card to buy lm308. I live in Sudan. If you can help please please I need your help , I put everything I have in the Collected of TGSL.
Sorry I'm weak in the English language I hope my message is clear
Best greetings

[Dennis the Mennis]

I agree with you lot, mylar is the best so far in my experiments with these particular coils anyway, and alot easer to apply also which is a important factor.

I've used the ( mylar) shielding out of a VGA cable and it worked for me , it is very strong so you can wrap it tight.

I don't know the results of the Mylar from a survival blanket or the Mylar as Waikiki_Sweep mentioned for my pcb ( also with less aluminium compared to the Mylar from the VGA cable) , maybe even better ...
Which did you use ?

I've used it in combination with Belden cable ; 4 wires in 2 pairs with each pair separately shielded

kind regards ,

Dennis the Mennis

[satdaveuk]

mylar

I agree with you lot, mylar is the best so far in my experiments with these particular coils anyway, and alot easer to apply also which is a important factor.

[simonbaker]

I have tryed to detect pieces of aluminium tape and mylar survival blanket from Wal-Mart
with my Garrett ACE-250. It is pretty sensitive VLF metal detector.

Small piece of aluminium tape 3x3 mm. makes same signal as 100x20 mm. mylar stripe.

I can detect both from 3-5 cm. distance with my ACE-250.

You can see that Mylar is better then AL tape for shilding in (100x20) / (3x3) = 222 times

Also i have found that my Surf PI detector can't see a big bunch of mylar stripes from ANY distance.

That makes mylar ideal shield for both TGSL and Surf PI projects.

I am not sure for another mylar sources like cables because I have found mylar in the RG-6 TV cable and it contains pretty thick aluminium foil with one side of plastic film.
It is bad mylar - too much metal.

Always check your mylar with ohm meter to be sure that it conducting electricity and to
know wich side is conductive. Good contact between mylar and naked ground wire is must!

Thanks, that is good info.

-SB

[Waikiki_Sweep]

Mylar is better then AL tape for shilding in (100x20) / (3x3) = 222 times

Sounds sensible -- someday I'll also try my big bottle of graphite dust mixed with something to make a shield, see what happens.

Can you provide some actual measurements of mylar and foil and describe how you made the measurements?

Regards,

-SB

I have tryed to detect pieces of aluminium tape and mylar survival blanket from Wal-Mart
with my Garrett ACE-250. It is pretty sensitive VLF metal detector.

Small piece of aluminium tape 3x3 mm. makes same signal as 100x20 mm. mylar stripe.

I can detect both from 3-5 cm. distance with my ACE-250.

You can see that Mylar is better then AL tape for shilding in (100x20) / (3x3) = 222 times

Also i have found that my Surf PI detector can't see a big bunch of mylar stripes from ANY distance.

That makes mylar ideal shield for both TGSL and Surf PI projects.

I am not sure for another mylar sources like cables because I have found mylar in the RG-6 TV cable and it contains pretty thick aluminium foil with one side of plastic film.
It is bad mylar - too much metal.

Always check your mylar with ohm meter to be sure that it conducting electricity and to
know wich side is conductive. Good contact between mylar and naked ground wire is must!

[simonbaker]

If we building metal detector then we have to use as less metal inside search head as we can. Otherwise this metal becomes a target or masking the target.

Mylar contains 100 times less metal then aluminium foil and it is not so conductive.
So we have:

lower phase shift
lower eddy curents
lower inductance
lower capacitance

Also other elements like coaxial cables should be lighter too. (less metal, lower diameters)
Similar effect was with Surp PI project. Aluminium foil shild, bulky coaxial cable were masking targets, decreasing detecting range, prolonging delays, masking gold and small targets.

If someone have wires with lower diameter AWG 34, 40, 44 may be it is good idea to try it for TGSL because by lowering amount of metal we may have more sensitivity and detection distance, more stronger phase shift and discrimination for smaller targets.
Instead of PI detectors (several Ampers in pulse) TGSL using much less current (miliamperes) so higher resistance of wires is not so impotrant.

Sounds sensible -- someday I'll also try my big bottle of graphite dust mixed with something to make a shield, see what happens.

Can you provide some actual measurements of mylar and foil and describe how you made the measurements?

Regards,

-SB

[simonbaker]

I ment it doesn't seems to influence the phase shift of the target signal
pls correct me if I'm wrong Simon

Yes, I believe the phase of the null signal does not directly affect the phase of the target signal. I'm fairly certain of that.

I also suspect the following: that the phase of the null signal may have two main components:

1. Magnetically coupled component.

2. Non-magnetically coupled component.

(for want of better terms)

We see the sum of both those components on our oscilloscopes when looking at the "null" signal. As we shift our coils while nulling, the phase of the null moves around smoothly from one phase to an almost opposite phase (approx 180 deg movement).

However, my feeling is that the "Magnetic" component only has two phases, and as we null, the amplitude grows and shrinks and suddenly flips phase 180 degrees.

The "Non-magnetic" component adds in -- I don't know if it changes while nulling -- but by adding in, it creates a smooth total phase shift that we see.

My feeling is that ferrite will only modulate the "Magnetic" component of the null, which only has two distinct phases. That is why I think the GB pot generally is set in virtually the same position for all our MDs regardless of the null phase we see.

That's my theory for the time being!

It's good to see everyone helping with the troubleshooting -- better chance of success.

Cheers,

-SB

[Dennis the Mennis]

=
I think Simon is right ( once again ) about his beleave concerning the phase shift between Tx and Rx : even with ridiculous values for the phase shift it seems not to affect the target signal .

I ment it doesn't seems to influence the phase shift of the target signal
pls correct me if I'm wrong Simon

[Dennis the Mennis]

Mylar

Good info about mylar. But why better than aluminum tape? Mystery of coils...

Well it worked perfect for me , especially the phase shift , and I really don't know why I wasn't able to get a good shielding results with aluminum :
I've tried a dozen ways , thickness , overlaps , other brands untill I've tried the Mylar shielding.

I think Simon is right ( once again ) about his beleave concerning the phase shift between Tx and Rx : even with ridiculous values for the phase shift it seems not to affect the target signal .

kind regards

Dennis the Mennis