TGSL finds gold & Euro coins but not silver & copper

[Mullihaka]

Hi folks,

I've been able to detect all Euro coins, iron nail and cap, aluminium pulltab and beer can, gold ring, pliers etc.

I have three items which do not give any kind of sound:
30mm * 2.5mm old copper coin
20mm * 3mm new copper token
23mm * 7mm silver ring

I scoped the TGSL a bit with different targets in all metal mode:
Channel 1 (green): pin 6 / U104
Channel 2 (white): pin 6 / U105
Channel 3 (yellow): pin 7 / U107a

The targets were:
1 Aluminium beer can
2 1 € coin
3 Golden ring
4 Iron nail
5 Big, old copper coin
6 Silver ring

It's evident that the last two targets cause a nice swing in the disc / all metal channels, but in opposite phase. The iron rejection works, so does the disc pot.

Does anyone have idea where's the problem?

[Davor]

It's evident that the last two targets cause a nice swing in the disc / all metal channels, but in opposite phase.

Exactly!
You learned the harder way the principles of discrimination. The discrimination channel is not your problem, but the GEB channel: your rig is not properly ground balanced.
You can picture the targets as being in some positions on the 2D plain. If you place the Tx phase on the far left, you'll find ferrites response there as well, and rotated only a bit by Rx circuitry phase shift. You'll find response to all existing metals above, iron and ferromagnetic metals on the left and coloured metals on the right. Above in the middle you find salts.
Now, a GEB phase is supposed to be aligned such that in GEB channel you have zero response to ferrites, and positive response to all metals. Disc phase further sorts the left and right, ferromagnetic metals and coloured ones. For positive response it must be both positive in GEB channel and Disc channel, and it picks only ~1/4 of the 2D plain, a single quadrant.

In your case a GEB channel is not aligned with ferrites, but instead it goes further into the 3rd quadrant, and a consequence is that you lose good conducting materials, AND you include a lot of ground response.

Simple solution: proper ground balance.
Method: use a piece of ferrite and wave in front of your coil, while adjusting your GEB setting for minimum response.

You may fine tune your ground balance for a real ground by bobbing (waving up and down) your coil to some 20cm above ground and adjusting your GEB for minimum response. Of course, on a spot without any metals in the ground.

[ivconic]

Exactly!
You learned the harder way the principles of discrimination. The discrimination channel is not your problem, but the GEB channel: your rig is not properly ground balanced.
You can picture the targets as being in some positions on the 2D plain. If you place the Tx phase on the far left, you'll find ferrites response there as well, and rotated only a bit by Rx circuitry phase shift. You'll find response to all existing metals above, iron and ferromagnetic metals on the left and coloured metals on the right. Above in the middle you find salts.
Now, a GEB phase is supposed to be aligned such that in GEB channel you have zero response to ferrites, and positive response to all metals. Disc phase further sorts the left and right, ferromagnetic metals and coloured ones. For positive response it must be both positive in GEB channel and Disc channel, and it picks only ~1/4 of the 2D plain, a single quadrant.

In your case a GEB channel is not aligned with ferrites, but instead it goes further into the 3rd quadrant, and a consequence is that you lose good conducting materials, AND you include a lot of ground response.

Simple solution: proper ground balance.
Method: use a piece of ferrite and wave in front of your coil, while adjusting your GEB setting for minimum response.

You may fine tune your ground balance for a real ground by bobbing (waving up and down) your coil to some 20cm above ground and adjusting your GEB for minimum response. Of course, on a spot without any metals in the ground.

That's correct, Davor, you are right!
Mullihaka try to adjust GEB trimmer half millimeter by half millimeter CW or ACW (depends on your perspective) to allow thin silver&copper detection.
Or replace conventional trimmer with multi-turn one. Easier to fine adjust GEB.

Cheers!

[Mullihaka]

It works, thanks!

I was in belief that at the same exact point where ferrite is rejected, the silver is accepted. However, I had to further increase the resistance.

Another twist in this is, that for some reason the trimmer seems to be logarithmic. It's been rescued from a video recorder. I'll replace it with multiturn (brand new on) one.