bad pi curve

[Levin]

Hello
I build a voodo metal detector. but it doesnt work. IN PI mode the damping of the coil is constantly changing.(you can see tp 2 and tp 5 in the video)

I dont know where my mistakes are. pleas help me
by Levin (sory for my bad english)

[Qiaozhi]

I doubt anyone will be able to help you with so little information.
The problem could be caused by many things; even a simple bad solder joint.
Do you have any photos of the PCB?

[Levin]

Ok I am happy to give more informations.
I have this problem wit all the coils i tried. the PCB is made by JLPCB and I only used new components. the display and the microcontroller work. tp 2 is ok too.
here are some photos in a video because I dont figure it out how i can upload photos here

[Qiaozhi]

At the bottom of the reply box, under all the smilies, there is a button labelled Upload Attachments.

Did you test the design as you went along, or simply populate the whole board?
I'm surprised that you never noticed this problem after fitting and testing the preamp stage.

Are your power supplies stable?

[Levin]

thank you
I simply populate the whole board. My power supply is perfect

[Carl-NC]

Bad solder joint? Or bad connector contact?

[Levin]

I cant see a bad solder joint and the connectors seem to be ok too

[Carl-NC]

R6 looks a little suspect, try replacing it with a short. A bad solder joint isn't always easy to see, try re-soldering joints especially right around the coil & TX. Try hard-soldering a coil to the TX pads, anything from 250uH to 1mH.

[Levin]

thanks for answering.
R6 works. On Friday I will try resoldering the joints and I will try to hard-soldering a coil to the TX pads.then I telled my results.
Could it be a problem that i use a cheap multi turn trimmer as damping resistor?

[Carl-NC]

Possibly, trimmers are not intended for such a high voltage. It could be arcing.

[boilcoil]

This is typical oscillogram of variation damp process.
But, may be, damp resistor does not change, and common capacitance changes. Check serial diode, after key mosfet. (there is something strange in starting zig-zag oscillogram)

[Levin]

thanks for you all
i found the problem. the problem was the multi turn trimmer as dampging resistor in a few hours i give more deteils

[Levin]

I first measured the value of the trimmer when the curve is critical damped. I was very confused that the value was 2 ohm. when I changed to test, the to 2 ohm adjusted trimmer with a normal 2 ohm resistor the curve was completely different. then i used a normal 500 ohm damping resistor an i had a great curve. I the original trimmer I cant find, but they were similar to this trimmer https://www.amazon.de/GTIWUNG-Potent...9&sr=8-10&th=1
What is the problem with this trimmer. I use this trimmer in other places in my voodoo detector to. will that make problems
LG Levin

[pito]

your potentiometer is for low power you need something like this one

[Levin]

ok thank you