Add a ground pot to an XP ADX 250

[Turboair]

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.

Hi skippy,

The pot is an Alpha one turn 10 K linear one. I do not know part number, but You know, it is very common...


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[pre9]

[thank you

[golfnut]

In the original placement the 4k7 (R1) was in series with the trimmer


This means that there had to always be a minimum of 4k7 in the circuit.




If someone attemting this mod had the original trimmer set to one end of travel AND the new pot set to one end of travel - you would have a SHORT right thru.

It not clear if the kit would survive this..



I would recommend that after removing R1 - set the small trimmer to mid travel - So when the user hits the end of travel on the new front panel pot - there is some resistance left in (like on the original design)


Steve

[Skippy]

The obvious problem with adjusting the pre-set like you describe, is that the new front-panel pot won't have it's neutral default setting at (or close to) 12 o'clock) mid-travel. I don't recall if we ever found out the preset pot's value, but I suspect that it's unlikely to end up set at either extreme. Also , the likelyhood of damage is minimal - odd behaviour, non-functioning detector, likely. Setting the ground adjust pot at the extreme is also uncommon, ground chatter will be high one way, and the other way usually seems to lose depth, so would be an unlikely setting. It's a shame these machines, or the GMP/GMAXX haven't been reverse engineered to find out their 'secrets', but I guess the microcontroller makes this task tricky

[Turboair]

Mine is still alive and kicking, and i did not touch smd trimmer. A really good mod that works flawlessy on my machine.

[golfnut]

I guess thats the point - dont adjust the on board original trimmer - it wont be at an extreme of travel (worst case a short) and so if your work is repeated by others - dont touch the trimmer - as you did and all is good.

Nice mod


Im going to do a similar thing to an ace250 soon which I run at 15kHz..

S