TDI discrimination

[Geo]

Geo,

If you added another vernier potentiometer (pot) in series with the original factory GB potentiometer, then your GB numbers will have changed as to where certain things happen such as where you will obtain the right ground balance. The GB pot increases the gain of the GB channel and if you added more resistance, you added more gain. This will increase the noise dramatically. So, to start with, turn the added potentiometer to minimum resistance and test again.

Did you adjust any pots inside the detector? Absolutely, do not do that. Some are extremely critical and others are just critical. All use special equipment to adjust properly.

Reg

Reg,
I did not adjust any pots inside the detector. I only connected a 10K pot in series with GB for fine adjusting so the total resistance is about the same......
I don't know if a bad quality potentiometer can add so much noise. I put a new cheap Chinese potentiometer. Today when i made the test i had the 2nd potentiometer at zero resistance.
At morning i will see. Now i go for sleep, time is 01:45......

Regards and thank you

btw...I think that the schematic of GS-5 who there is at forum is very close to TDI except oscillator... VCO and schematic for delays....

[Reg]

Geo,

If you added another vernier potentiometer (pot) in series with the original factory GB potentiometer, then your GB numbers will have changed as to where certain things happen such as where you will obtain the right ground balance. The GB pot increases the gain of the GB channel and if you added more resistance, you added more gain. This will increase the noise dramatically. So, to start with, turn the added potentiometer to minimum resistance and test again.

Did you adjust any pots inside the detector? Absolutely, do not do that. Some are extremely critical and others are just critical. All use special equipment to adjust properly.

Reg

[Geo]

Geo,

Just because all the LED's are illuminated doesn't mean the battery isn't part of the problem, so please try a different battery just to be sure. Like I said, almost all of the circuitry is used when you are operating with the GB off. There is just one opamp and associated circuitry that is turned on when the GB is on. Actually, there is a couple more stages that are operating but not heard with the GB off, but none of them should create a lot of noise.

I still think that some new noise making device is causing the problem and the problem is not with the detector.

Reg

Reg,
i have another one battery. Tommorrow i will charge it and i"ll give another one try!!!!

Regards

[Geo]

Reg,
tommorrow i will disconnect the second GB potentiometer and restore it to its original state. Maybe the extra cable to receives noise.... who knows

Regards

[Reg]

Geo,

Just because all the LED's are illuminated doesn't mean the battery isn't part of the problem, so please try a different battery just to be sure. Like I said, almost all of the circuitry is used when you are operating with the GB off. There is just one opamp and associated circuitry that is turned on when the GB is on. Actually, there is a couple more stages that are operating but not heard with the GB off, but none of them should create a lot of noise.

I still think that some new noise making device is causing the problem and the problem is not with the detector.

Reg

[Geo]

Geo,

Where are you testing the TDI? It sounds like you have some noise becoming a big problem. Cell phones will do that.

Try a new battery and then double check the TDI out in a field away from people and houses. I suspect you will find the detector will work ok.

Let me know if it works away from any possible noise.

Reg

Hi Reg.
I tried it at my Lab. Sure there is noise but i always test it in the Lab. As i remember now, when i made the test with the 45 coins it had very much ground noise, i had not the ability to make a good GB. Tommorow i"ll go out so to be sure about the noise, but now i am 99% sure that there is some problem. Before 1++ month i opened it and i added a potentiometer for fine GB, as it is the Pro model. Maybe i made a mistake....
About the battery.... all the leds are illuminated.

[Reg]

Geo,

Don't panic. I am sure nothing major is wrong.

Where are you testing the TDI? It sounds like you have some external emi noise becoming a big problem. Cell phones will do that.

Try a new battery (don't rely on the LED's to tell you the battery is good) and then double check the TDI out in a field away from people and houses. I suspect you will find the detector will work ok.

Let me know if it works away from any possible noise.

The reason I say this is because almost all of the TDI is used when using the GB off mode. Just a small part of the detector is added to make the GB work, so I highly doubt any of that part will cause a problem.

Now, a bad battery can compound problems. Also, the GB activates one part of the circuit that is shut off with the GB off. This part that is activated amplifies the signal much more than the rest of the circuit, so any noise becomes a very loud signal with the GB on but not that noticeable with the GB off.

Reg

[Geo]

you bought a defective tdi

No, i have it 3 years. Problem is that the components are smd and my eyes are not good at this age

[Geo]

How about you use a discriminating detector from post #18 and make a new picture of your sovereign coins

Now it is not time for funny. I want help to repair my detector. If you don't know to help me then, please leave funny for other time

[Orbit]

you bought a defective tdi

[Davor]

How about you use a discriminating detector from post #18 and make a new picture of your sovereign coins

[Geo]

Hi.
Today i made a test to TDI. Results was for crying . TDI has problem.
With GB = off, delay=10us sens=7 and trshold at middle detects a gold coin 22mm at 30++ cm, it is OK.
When i turn the GB=on and GB = 8...10 without to change other potentiometer TDI beeps contineusly. I must decrease the TRHhold to zero so to stop the beeps (i have again some beeps). Now it detect the coin at 10cm only!!!!. If i"ll put the GB=6 then it detects the coin at 4..5 cm only.
So i have a big problem maybe with the one channel
Please help!!!!

Regards

btw.... i want photos from tp.... maybe Carl!!!!!

[Geo]

Geo,

The conductivity of the coins shouldn't have changed and still should be a high conductor at a delay of 10 usec and a GB at 9 to max. So, something else is wrong.

If you are looking for maximum depth of detection on those gold coins, then it appears you can gain depth by increasing the delay to maximum delay which should minimize or at least reduce the ground signal and then turn the GB off. You will have no form of discrimination this way, but will maximize depth.

With the GB off you should be able to turn the sensitivity to maximum which should help gain more depth also.

Then you can try a larger coil and gain a few more inches in depth.

BTW, what size coil are you using? Also, what type of coil are you using? The normal coil sent with the TDI here in the US is a dual field coil. Detectors sent to OZ normally are sent with a plain mono coil. A plain mono coil should gain a small amount of depth when compared to the dual field design. Also, there will be a little less ground response with the plain mono coil.

Have you ever tried to build a coil? It isn't that difficult if you can solder properly.

If you know someone who has a Minelab PI, you might try to borrow a DD coil and try it. With the GB off, there will be less ground signal if you use a DD coil, especially if the delay is advanced. A ML PI will give you odd signals on near surface iron objects, so there is a little discrimination, but not much.

Reg

Hi Reg.
I am interesting for max depth but with a kind of discrimination. If the GB is off then i must keep the disk at least 15 cm over the ground, and so i lose again the depth. From my experiences i saw that i had the best results when GB was on and the disk was closed to ground. I have the 12" dual field coil, also i have a homemade 60cm coil and another 105cm coil. The 60cm work very good after the 15us and the 105 cm after the 17us. When i tried the TDI with the gold coins i had the 12" coil. I have a friend with Minelab GPX so i will try the DD coils of it.

Regards

[WM6]

This problem of low conductor or high conductor becomes worse when it comes to gold nuggets. Some places gold nuggets are very solid but more importantly are very pure, while other locations the gold has a higher silver, copper or other metal alloy content. Alloy gold with other metals radically alters the conductivity which then alters the way it will respond.


Reg

Excellent explanations, Reg.

Not only nuggets, all "artificial" gold are made out of some sort of alloy too. Pure gold are very rare to find.

[maikl]

You understand that if has not the ability to discriminate between gold and iron then no reason for discrimination.

Hi Geo. Qiaozhi well explained in a few words "discrimination" for PI detectors, read post # 630 http://www.geotech1.com/forums/showt...egend-1/page26