RX coil and sensibility

[chuubaba]

Dbanner

That looks like a rapidly decaying pulse.

[dbanner]

My logic is as follows: You do not know if there is such noise when you go outside with the detector, since you are not connected to the scope. However, you are getting poor distance both inside and outside. You could have two problems. A poorly constructed/balanced coil and secondly, noise from interference.
The chips I used were TL071.

[dbanner]

Which FET are you using to "gate" the samples?(TR4,TR5)

[dbanner]

Remember, although everything is off, except for the scope! There are "SYNC' pulses present in old analog type scope. Could be noisy??

[waltr]

My logic is as follows: You do not know if there is such noise when you go outside with the detector, since you are not connected to the scope. However, you are getting poor distance both inside and outside. You could have two problems. A poorly constructed/balanced coil and secondly, noise from interference.
The chips I used were TL071.

I am thinking the same. I did expect the noise to decrease with C6 shorted.

The noise I get at U104 & 105 is not a singlur pulse but a steady Waverly at low frequency (in filter pass band).

I use the TL081 which are nearly the same as the TL071.

Next was to see what your RX to GB sample looks like which is set with coils. This is where I had a hard time and reduced distance.

[dbanner]

The scope shot shows a distinct pulse which has a decaying oscillation. It looks strangely similar to oscillation of op amp due to capacitive loading. The 308 chips may be the culprit. Check to make sure you have correct component values in the feedback loop of these op amps.
Maybe try 072 or 082.
There is always the possibilty that you have fake 308 chips. So they are op amps, yes, but they're not 308. See?
Here is a pic of what it looks like(oscillation due to capacitive loading). Look familiar? Maybe, Maybe not. Anyway, exploring possibilities from afar is fun, I get to read some stuff for research.

[dbanner]

If you zoom in, you can see on scope that at U105 output, the oscillations continue on much longer. It looks like RINGING. Op amp oscillation due to capacitive loading? What do experts say?????????
All this is just throwing things out there, like guesswork. But it does make for interesting learning.

[Tony60]

Hello Waltr and Dbanner,
thank you for all the advice; the FETs I've mounted are J310 also used in UHF. The coils seem to me to be in place even the screen is made of aluminium from kitchen. I will try the TL081 and also get TL071 we se the results.
Thank again and have a goo day.
Tony

[dbanner]

Correction: above I stated 082, 072. This is wrong. I meant to say 081, 071.(single op amp)

[Tony60]

Hi guys,
I tried to replace the LM308 with TL081 and I also replaced C14ab and C17ab with 22 uF electrolyte capacitors. I rebalanced the coil and now 1 euro coin has gone from 13 to 18 CM!!! Everything here thank to YOU. Now I would like to try to change the shielding of coils using graphite paint but I do not have the material at home; as soon as possible.
A hug.
Tony.

[dbanner]

What has happened to the noise????? Any change?????
Also, be careful to adjust GB trimmer to the EDGE of ferrite detection. It is important not to over- trim the GB trimmer, but just on EDGE so you hear a tick tick as though the detector is almost trying to "see" the ferrite. Over trimming the GB leads to reduction of sensititvy.

[Tony60]

Ok Dbanner,
The noise track has changed but I can't see it, the scope sweep can't stop it. The GB adjustement is always at the limit and in experiments I use a ferrite to adjust it. In the afternoon I will perform more tests.
Thank you and soon.
Tony.

[waltr]

I agree with Banner, good catch.

Since LM308's are hard to find and expensive there are many Fakes out there. I and many others use TL081's and they work fine.

Sounds like some Progress.

Adjustment of Coils is very difficult.
What I found is first set the GB pot to the middle.
Then with scope on pre-amp out (U101 pin 7) and on gate of TR5 adjust coil over lap (null) to have the positive part of TR5's gate centered on the Zero crossing of U101-7.
In theory this can be the rising or falling part of the sine but my TGSL liked the falling part better (more sensitive and disc worked better).
Do not worry about getting the minimum peak-peak Voltage, 200mVpp to 1Vpp works fine.

Now use the ferrite and adjust the GB pot as Banner posted.
With the GB pot properly adjusted Silver will still be easily detected.

[dbanner]

Ok Dbanner,
The noise track has changed but I can't see it, the scope sweep can't stop it. The GB adjustement is always at the limit and in experiments I use a ferrite to adjust it. In the afternoon I will perform more tests.
Thank you and soon.
Tony.

If the scope sweep can't stop it, then it's because the triggering threshold exceeds the noise threshold.
That means the amplitude of the noise you were seeing before(which I suspected to be op amp ringing) is now greatly reduced or gone.
Changing the chips has made the difference.
KITCHEN FOIL IS NOT BEST FOR SHIELDING COIL.
I used mylar salvaged from cable company coax coming from the road. It had a greenish tinge to it. It has the width to go around cross sectional circumference of the coil. It worked great!!!

[Tony60]

Good morning Waltr and Dbanner,
thank you for the suggestions; as for the shielding of the coil I found a paint spray of the KONTAKT CHEMIE and it is called GRAPHIT33. When it is dried it forms a highly conductive thermoplastic layer. I have a link but it's in italian.
That's all for now. Have a nice day.
Tony.