Found this advise!
1. There are tension adjustments for the upper and lower threads.

2. Most of your adjustments will be made using the upper tension knob/adjuster.

3. In theory, the tension on the upper thread should be the same as the lower thread,
though this is rarely the case.

4. What you want is the top and bottom stitch to lie tight

5. When you have looping or loose stitches on the bottom (WRONG SIDE), then you should slightly tighten the upper tension.
If your upper tension is as tight as it goes then you need decrease the bottom tension

6 When you have a top thread that lays across the fabric and a good tight stitch on the bottom,
then you should loosen the top tension (always a little at a time). If your upper tension is as loose as it goes (very rare) then you need to increase your bottom tension.

This isn't my sewing machine, I go nowadays to repair shops!