It could be a built in slowdown for longer stitches. Check the manual and see it it mentions this.
On my SWF, as stitch length gets longer, machine slows down.
It is part of the design.
My HAPPY on the other hand does not, so if I have longer stitches and I have speed up high, it tends to shake a little.
I just slow it down and all is good again. When I say long (on the HAPPY) I mean 10mm or more.
Brother could have a similar set up.
On my SWF, as stitch length gets longer, machine slows down.
It is part of the design.
My HAPPY on the other hand does not, so if I have longer stitches and I have speed up high, it tends to shake a little.
I just slow it down and all is good again. When I say long (on the HAPPY) I mean 10mm or more.
Brother could have a similar set up.