Melzi USB interference

Hi all,

I've got some strange problems with my Melzi board. I've tried several USB cables but I only got a connection with a 20 cm cable.
I can connect to the printer and everything works as supposed to. The strange part is that as soon the shield/cover of the USB cable touches the computer my touch pad starts behaving erratically...
Of course this only applies when the 19v brick is connected - it doesn't matter if the board is powered by the computer or the 19v, in both cases there are problems.

Is there a short somewhere? How can I measure to find more info?

The USB cable is probably creating a ground loop when you connect it. Here are a couple of things to try:

1. Put a ferrite bead around the USB cable.

2. Power the printer and the PC from the same mains power outlet, if you are not already doing so.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thank you for your suggestions

1. The short 20 cm cable got a ferrite bead, the other doesn't.
2. I've only tried another outlet but they are using the same fuse and that made no difference.