Potentiometer Screwheads broke off!!

Hi,

I was setting the motor currents to their required values, following the instructions which said to turn the potentiometers on the board Melzi v2.0 till the voltage (between the - on the power connector, and the left-hand side of the potentiometer) was about 0.8V. The screws were really stiff, but i managed to turn them slowly. Near the end of their travel, the voltage had only risen upto about 0.45V, so i turned the screws a but more. When I felt some resistance i turned in the opposite direction but the screws had taken too much stress and the head simply dropped off.

Is there anything I can do now except for ordering a new board? And why do the voltages not go up to what they are supposed to? I was thinking of desoldering the surface-mounted potentiometers and putting some of my own in their place, but I do not know if that is a good idea or if so, then what voltages to set to.

Can anyone please help?

Thank you.

If you can take the pot off and re-solder a new on in its place then do that, just look at the boards schematic and find the value and order a new one from somewhere like RS or Farnell.

If you have a decent iron and some solder just put a big blob of solder on each of the three pads and keep touching each one in turn until all three are eutectic (wet / fluid / molten, however you want to describe it.) and lift off with tweezers.
Clean up pads with solder brade and flux, then carefully solder the replacement in.

Job done!

Thanks, I'll try and replace the pots, although the resistances of the existing ones don't seem to make sense....

Hi Epmac,

which Vref can you measure now with the broken poti still in place? You might be lucky that it broke in a position that it is possible to simply add a resistor in parallel to the poti as a kind of one-time-setting. Of cause you can't adjust the value anymore, but as a workaround it might be easier than changing one of these SMT potis.

Michael

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