Brother b831 Service Manual page 84

Threads loose.
I
Needle breaks.
Thread take-up spring too
weak.
Upper thread tension too
weak.
Lower thread tension too
weak.
Needle guard maladjusted.
Needle incorrectly install
ed.
Needle bent, needle tip
crushed-
Wrong needle and feed
timing.
Wrong needle and rotary
hook timing.
Needle guard maladjusted.
Thread take-up spring ten
sion and movement.
Upper thread tension.
Lower thread tension.
Needle direction and
height.
See the section for thread
tension.
See the section for thread
tension.
See the section for thread
tension.
See the section for needle and
rotary hook timing adjustment.
See the section for install
ing the needle.
Replace faulty needle.
See the section for needle
and feed timing adjust
ment.
See the section for needle and
rotary hook timing adjustment.
See the section for needle and
rotary hook timing adjustment.