Hi Stirling

Yeah, lotsa guesswork. Sometimes we have to guess!

The Homann Designs MB-02 BoB uses tristate chips for some of its I/O pins. The tristate pin is usually driven by the charge-pump detector. I have two of them.

The TIG may well be the source of the noise all right, but it has to get into the drivers somehow. Steppers or servos - both can pick up noise on the Step/Dir lines. Yep, a guess. A starting point?

Cheers
Roger