There is two places to adjust thread tension: the top tension device controls the top thread tension and is usually a dial or knob with numbers on the front of the machine.
Then there is some form of tension device on the bobbin; in your case I believe the Brother has a removable bobbin case with a tension spring which holds the bobbin thread firmly: the tension spring can be adjusted with a screwdriver but once set, you shouldn't need to adjust this much, just alter the top tension slightly for different weights of fabric. So start by setting top tension dial to 4-5 and test stitch a seam. Use different coloured but the same type of thread top and bottom and take a look at the seam. You want to see a balanced even seam.
Here is a link to a great info site for this:
http://www.sewusa.com/Sewing_Machine_Repair/Thread_Tension_Settings.htmIf the bobbin thread is too loose, then you probably will need to finess the tension spring on the bobbin case and adjust this slightly. The best way to set the tension on the bobbin case is as follows. Load the bobbin into the bobbin case and pull the thread tail into the tension spring, then suspend the bobbin by holding the thread tail up in the air (the dangle test). You want it to be just tight enough to hold its own weight, but still able to release thread when you pull gently downwards on the bobbin case. If it isn't doing this then take your little screw driver and adjust the screw on the leaf tension spring by ONE QUARTER TURN ONLY. Lefty looser, Righter Tighter. Then test again, and adjust again until its right.
Now test stitch again and finess your top tension to get the seam even. Remember to change the needle for each new sewing project too; this will impact tension also.
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