Since yesterday, I had the Z axis behaving very weirdly, suddenly lots
of chatter and vibration, extremely erratic behavior, then no motion
at all, then some motion etc.

Attempting to diagnose it (isolate the issue) pointed to a servo drive.

Since I have several additional drives on the shelf (they were cheap),
I swapped one for another. Now the Z axis is back to working. I opened
up the drive that I relpaced and saw one chip blackened.

Not sure what to make of this.

i