Brother b831 Service Manual page 82

Trouble
Bobbin races
during thread
cutting.
Stitches skip or
thread casts off at
start of sewing.
After thread cutting,
thread trailing from
needle hole is short.
Needle hits the thread
wiper due to scattered
stopping in the needle up
position.
Cause Check
Remedy
Lower thread not running
in normal direction.
Adjust so the lower thread
winds in the same direc
tion the rotary hook
rotates.
Bobbin wound over 80%.
Bobbin thread.
Adjust lower thread
amount to 80% of bobbin
capacity.
Racing prevention spring
tension is too weak.
Replace the racing preven
tion spring.
Needle up stop position is
Height from needle tip to
needle plate after thread
cutting.
Refer to needle position
too high.
detector adjustment.
After thread cutting, the
lower thread is not held by
the lower thread tension
spring.
Replace the lower thread
tension spring.
Improper feed dog height.
Feed dog height.
Adjust feed dog height to 1
mm.
Thread take-up spring ten
sion IS high, or the stroke is
too large.
Thread take-up spring.
Decrease thread take-up
spring tension, or stroke
length.
Upper thread from the
needle hold is short.
Decrease pre-tension
strength.
Needle is too thick.
Replace with finer needle.
Presser foot for thick mate
rials (slotted) is being
used.
Replace with slotless
presser foot.
Thread does not come
easily off the spool stand.
Adjust so thread feeds
easily.
Pre-tension is too high.
Decrease pre-tension
strength.
Thread take-up spring
stroke is too large.
Thread take-up spring
stroke.
Decrease thread take-up
spring stroke length.
Improperly polished knife
hook.
Polish or replace the
movable knife.
Tension disc does not
engage during thread cut
Tension release move
Adjust the tension release.
ment.
ting.
Improper wiper position.
Adjust the thread wiper.
Needle up stop position is
Height from needle tip to
needle plate after thread
cutting.
Refer to needle position
weak.
detector adjustment.
V-belt tension is weak.
Adjust V-belt tension.
Feed dog needle is too
small for the needle and
thread.
Feed dog needle hole.
Replace the feed dog.