Thread Tension Set for Brother Ma4-B551 Industrial Overlock Sewing Machine : Amazon.co.uk: Home & Kitchen
How to adjust the thread tension - Video Included
need to see if there's any thread caught in the tension discs, but this is as far as I can easily see to disassemble the machine to check (Brother SE600) : r/Machine_Embroidery
How To Deal With Thread Tension? | Madam Sew – MadamSew