RX coil and sensibility

[Tony60]

Hello everyone, back with the TGSL some time ago; I'd a few questions. With the LC coil tester and a 14.7 nF cap I brought the RX coil to about 16.2 Khz but if measure the frequency on the pin 7 of U101a I read abaut 14.6 Khz, is it normal? After the nulling of coil I have a signal on U101a of 0.22mV but a 1 euro coin "sound" only 13 cm; Wath can I do? Thank you in advance and good week end.

[waltr]

The RX sees the TX OSCILLATION of 14.6kHz.
The RX Tank (LC) is Resonant at 16.1kHz but does not Oscillate at 16.1kHz.
This 'off-resonance' is to obtain proper Phase shift so that GB and Disc actually works.

There is a Huge thread on Tuning and Troubleshooting the TGSL.
https://www.geotech1.com/forums/show...roubleshooting
The one thing that came out of that thread is the GB sampling (TR5's gate) needs to be centered on the Zero crossing the the RX signal from U101 pin 7 when the GB pot is centered.
This should be the first step when checking the coils and adjusting coil null. Also in that thread is getting a Minimum Null is NOT important, what is important is getting the Phase of the RX signal matching the GB & Disc sampling.

I have been building a TGSL and also having problems getting good results. I read the above thread yesterday and readjusted coils and TX caps to obtain the RX zero crossing in center of GB sample and results are a lot better.
I tried all the commonly posted methods and in AM mode there was lots of chatter even outside away from electrical interference. Depth was not good and US Nickle disc out near the end of the disc pot. Each of these are symptoms of improper RX phase to GB & Disc sample timing.

[Tony60]

Hi Waltr,
Thanks for all the info. On this weekend I will perform all the checks that you recommend; I am sure I will have improvement in operation.
Best greetings and good weekend.
Tony60.

[waltr]

Please post back on what you did and how it worked.

I just tried a field test (back yard) on my TGSL with new adjustments and very chattery when moving.
Back in the shop and if I touch it there is chatter. Separated the coil from the shaft/box and quiet when touching the shaft/box but chatters when touching to coil.
Note: the coil is not yet full potted, only the outer round parts of the DD, so I could play with the over lap area for tuning. Seems very sensitive so I need to pot (epoxy) the straight over lapping parts of the coil. Hopefully I get to test outside again tomorrow.

This thing is touchy.

[dbanner]

Absolutely, the coil has to be potted, or at least firmly secured all the way around, otherwise any vibration will cause it to chatter. The coils are balanced such that even the slightest movement of the TX and RX relative to each other will cause chatter.
Another reason is that the sensitivity is too high, and the detector becomes too chattery, even after potting. But that is seperate issue.
Some people make poor coil(and unaware of this fact), they hack the sensitivity up too high in order to squeeze more detection distance, but the thing always chatters. The type of shielding makes big difference. Do not use kitchen foil, use graphite. The difference is several inches more(big difference in detection depth)
With graphite shield, the coil is VERY stable and silent and smooth running. No falsing, no chatter.
How did you connect the RX? Did you ground one end or did you leave it floating?
I followed Dbowers way of connecting the coils for wet grass, and it made a world of difference. But I don't think the original stock coil is connected in this way, never figured out why.

[Tony60]

Hi everuone, this morning I precisely calibrated the coils and now 16.1 Khz for RX and 14.5 Khz for TX. As soon as possible I will recheck all 13 points as the instructions 101 part_1 say. I'd like to know from Dbanner what graphite shielding is like. As soon as I have news I will dislike a Threade.
Thank you all.
Tony60.

[waltr]

Yes, I grounded coil shields using Dbower's method.

I did use kitchen aluminum foil since in general many have used this.

Have read that very thin metalized mylar is better so will try that on next DD coil.

Planning on a concentric coil on which I will use graphite shielding inside the coil shell.

Epoxy poured last night in set so and Phase still looks good so will be taking it outside for testing a little later.

Tony,
Check Dbowers coil building for graphite shielding.
https://sites.google.com/site/dbcoil...ncentric-coils

[dbanner]

If you haven't seen this video, then you should have look, in it you will see how he made two coils, the first with ordinary foil, the second with graphite for shield. He demonstrates the very big difference in detection range between the two.
Conclusion: Do not use kitchen foil. I suspect not all kitchen foil are the same, maybe some brands are suitable but others are not. His method of wrapping thin wire around coil then shielding coil with graphite is proven to be effective. He explains why(having to do with the impedence of the shield).
My method is to dunk entire coil in thick home made graphite mix(graphite powder and glue), then hang on clothes line to dry out a little and repeat again to be sure of good layer). You must tape small section for the gap. This method works for concentric as well.
TIP: when nulling coils, do not glue down outer sections and try to bend/manipulate small section for final null, this method throws out the prestine 14.5kHz/16.1kHz resonances. Instead, you shift the entire coils with respect to one another for your final null, then pour a little potting mixture unto small sections throughout the length of the coils to hold them in place WHILE monitoring scope for any drift. When this has dried, and you are satisfied with the null, you pour the rest, leaving just a small section of the RX coil exposed for fine tuning.
https://www.youtube.com/watch?v=XnrStRklcUQ

[dbanner]

I never liked filling the whole shell with potting mix, that's why I am for having a vacuum formed bottom part of coil shell to secure in place. Seems nobody bothers to make the bottom part of the shell, only the top.

[waltr]

Thanks for video link on doing graphite.

Good point on moving entire coil instead of bending the straight sections to null/tune.

Just came back in from testing in the yard. Stable operation in AM & Disc.
Did find a few scrapes of aluminum an old shotgun head and a nickle and cent that I missed with my HH2 PI detector.
No nails, yea.

Planning on hitting a local soccer field this after noon for some coin shooting.

[Tony60]

Good morning Friends, I tested all the test points of the 101 part_1 file and the tracks are correct; only point 7 has a noise of 100mVpp for U104 and 200 mVpp for U105. According to the file these are very high values of noise (5mVpp about). I don't know how to intervene so I thought I'd give it up, patience.
Thank you for everything.
Tony60.

[waltr]

If you are testing inside there is most likely high amounts of electrical noise that will get into the circuits.
Dbower's workshop is electrically quiet compared to most. This was one of the big issues in the thread I posted a link to above (in post #2) and has been shown to drastically reduce the detection distance.

I get about 40-50mVpp at those test points in my workshop unless the heater has kicked on then its a few hundred mVpp.
Best to test for detection distance AWAY from ALL electrical noise. Try taking it outside away from your house and electrical wires.
Otherwise it sounds like your TGSL is working.
Does it reject ferrite when GB trim pot is adjusted and still detect silver?
Does it start rejecting low conductors (US nickle, pull tabs) when the the DISC Pot is adjusted?
Does it reject iron, nails, when in DISC mode and detect iron when in AM mode?
If it does all those then adjustments are probably correct and local electrical noise is reducing the distance.

I did get out yesterday with my TGSL taped to a wooden broom pole for field testing. There were lots of kids at the soccer field so went to the old 1800's farm where there is lots and lots of rusty nails and other junk. In 2 hours I didn't dig one nail but found a 1901 Indian head cent, a small working crotol bell, a very small ladies button as well as shotgun heeds and fired bullets. This was in areas that I had sweep well with my HH2 PI detector so very please with the performance of the TGSL.

[dbanner]

The one thing I am yet to try out is a "lobo" shape coil on the TGSL.
If I had to build another detector, it would be the GMT king cobra with its two tone ID and notch feature. The coils are the same.

[Tony60]

Hi Waltr,
thanks for the advice; tomorrow will be a beautiful day so I can try it in the garden. As for the three questions I can only tell you that in DISC mode rejects the nails in AM mode sees them.For the other two tests (silver / ferrite and nichel) I'll try.In recent days I thought that the neon light could disturb and so I worked without light but maybe not enough.
Thank you for everything and have a good week.
Tony60.

[dbanner]

There is something worth saying about the chip set used. Some chips seem to perform much better than others, for example, same chip number but different manufacturer etc,etc. Playing around with different chips can prove some small benefit. Some chips are noisy.