Hi,
I've tried all the recommendations and none of them have worked.
I've completely started over, this time using pulse encoders (to avoid the issue of compounded wiper values).
The result is exactly the same. When I rotate one of the encoders, the second motor accelerates or decelerates along with it.
I believe it has to do with the tb6600.
I've tried attaching two new others to it, with exactly the same result.
regards
Michel