Add a ground pot to an XP ADX 250

[Turboair]

Hello,
XP produces an excellent metal detector. The Adventis 2, that has a manual ground rejection pot. Then its little brother is the ADX250, which has same tech specs but fixed ground rejection. Here are some pictures of my ADX250 from which it is clear that both models share same motherboad and chipset. I would like to add a manual ground rejection pot to my ADX250 and trasform it in a Adventis 2. I modded a Cibola (that is a Vaquero with a fixed GB) with excellent results, but this time I need Your help. Here are some pictures:

1) Mobo and chipset of Adventis2 and ADX250 are the same:



2) As You can see even front panel is ready for a pot:



3) Heart of the ground rejection system, with trimmer and soldering points to attach a ground rejection pot:






Do You think that is enough to remove that trimmer (it is a trimmer, right?) and solder pot in place to do the mod?

Thank you for any advice you will give me.

Turbo

[Skippy]

I would suggest removing the fixed resistor (R1?) next the L of POT SOL and then add the pot to the allocated pads (only 2 wires needed). Pot value is tricky to determine, it depends on the fixed resistor value (looks like a 4k7, 4701?) and the preset value. Try 4k7 or 10K initially, you may need to adjust the preset too, so that one end of the pot's travel is at the 'fixed GB' setting.Dont forget, the software is different, even if the main board is the same. This may affect results. You may have to devise a method of setting the GB, for example.

[Turboair]

Thank you for reply. I have some questions for you.
Pot should be soldered to the lateral points and leave the one at the center alone? Ok for the 10 k pot but 4k7 pot?
If you see that chip has written both models on label, so I guess software is the same.
Have a nice day.
Turbo

[Skippy]

I've just had a quick look at a review of the Adventis, it seems the mid position (12 o'clock) is the 'normal' position for the GB pot. This should be eqivalent to the 4k7 R1 resistor, hence your pot needs to be 10K, I think. You can connect all 3 pot wires if you want, but you can see that two pins are joined on the pcb, so only two connections are actually needed, the central pin and the left-most pin.When I said the software may be different, I was meaning 'the software operation' may be different - maybe a jumper link or zero-ohm resistor lets the software know which model detector it is. I imagine there must be more differences between the two models than a 1 dollar pot?I hope your experiments are successful - a properly ground-balanced machine will always outperform the fixed equivalent.

[Turboair]

Skippy, I took a look better at R1 smd resistor. It is written 4701 on it ...A friend of mine told me to simply replace smd trimmer with one turn pot of same value. I am a bit confused. Which method is better? With SMD stuff it is too hard to revert modifications, so i would like to strike at the first shot.

[Skippy]

R1 is 4k7, that's why it has 4701 written on it, re-read my first reply. You dont need to 'remove' it, just leave it fixed on one pad only, then you wont lose it. If you do lose/break it, you can always fit a leaded one (use a 1% part) on the pads for the pot fitting, or you should be able to find a 4k7 smd on just about any scrap board. Dont meddle with the SMD pot.

[Turboair]

I will try, Skippy. What I do not understand is why they mounted that resistor. If that circuit has a 10k trimmer it was just enough to set it at mid position to have a medium setting for normal use.

Maybe they needed just to vary its value? Or maybe they preferred to mount a fixed resistor instead to a longer process of regulation during mobo production?

Another question...I had an Adventis 2 and with ground rejection knob at twelve o'clock it was able to reject pottery and coke. Consider that clockwise you lose sens...and anticlockwise you gain depth with some false signals (it has a ground rejection system and not ground balance one). In my ground, middle setting was too conservative and i was able to make detector run with rejection knob at eleven. My question is , if trimmer is not fully opened, with this mod I will be able only to ground reject from middle position to clockwise? If so it is completely useless...what i need is less rejection, a window to the anticlockwise side of rejection.

Btw, before putting my fat hands on it i will try to get a shot of Adventis 2 mobo...

Take r.

Turbo

[Skippy]

As stated in the second reply..... remove the 4k7 resistor and fit a 10K linear pot. Then, with the pot set mid-position, it will have approx 5k resistance, ie. close to 4k7. This means in the mid position it will hopefully behave as it did before, but you now have the option of increasing or decreasing pot resistance by 5K, which should allow under and over balancing of ground phase.

[Skippy]

And since you asked, they fitted the 4K7 resistor because without it there would be an open circuit and the detector wouldn't work.

[Turboair]

Thank you for the tips. I will do it and let you know if it will take off as a Sputnik!
Take r.
Turbo

[Turboair]

Hello Skippy. I did not find any pot in my town so I have ordered one online. Today I measured trimmer value: side by side value is 7,46k (not 10k). It looks fully opened because one side measures 7,46 the other one 0. Solder spots measure resistor value 4,76k...
Is this correct? My only doubt is about trimmer value, but measure was taken with trimmer mounted on mobo...

[Skippy]

You cant just measure a pot in-circuit and get the correct value - other resistors etc may be in series/parallel with the pot. Perhaps removing R1 might allow the correct pot measurement. The only reason it is worth measuring the pot is if you meddle with it/break it, then at least you will be able to repair the detector correctly. The pot's actual value should be marked on it, possibly 103, 223, 10K, 22K.

[Turboair]

OK, let's roll.

Some shots of the how to:

1) Workbench



2) R1 resistor removed



3) A temp application of the pot with wire to test it



4) After tin in excess has been removed I insert the 10K Gound rejection pot



5) Front panel


6) Final result


It works perfectly.

I wanna say THANKS! to Skippy for His precious support!!! I could not do it without You!!!

Take r.

Turbo

[pre9]

hi im thinking of doing the same mod but do you leave the fixed ground balance in place on the 250

[Skippy]

To summarise this modification:
1) The 4K7 resistor labelled R1 is removed (or left attached by one pad only)
2) A 10K linear pot is fitted to the three pads on the PCB
Turboair: It would be helpful to others if you could tell us the make/model of the adjustment knob, and a supplier part number etc.