JUKI MF-7900 Series Engineer's Manual page 202

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine
Trouble (1) Trouble (2)
6. Stitch of ravel- 6-1) Adjustment of thread
ing prevention hook and needle
is not possible.
6-2) Parameter settings
Cause (1)
From the previous page
14-D) Top covering thread
take-up
14-E) Needle thread tension
14-F) Threading
14-G) Top covering thread ten-
sion
14-H) Stitch tongue of presser
foot
14-I) Needle
14-J) Feed dog
1-A) Thread hook does not
hook needle thread
because of incorrect ad-
justment of thread hook
and needle.
2-A) Incorrect settings
Cause (2)
D-1) Drawing-in amount of top covering
thread is too much or too little.
E-1) Needle thread tension is too weak.
F-1) Incorrect threading
G-1) 1st thread tension is too weak.
H-1) Thread threading is not smooth due to
incorrect shape of stitch tongue.
I-1) Worn out needle point, needle bend,
wrong needle
J-1) Flaw, surface in feed dog
Check and corrective measures
Refer to the standard adjustment figures.
Increase tension.
Refer to threading diagram.
Increase tension.
Replace when its shape is modified or
changed too much.
Replace needle with new one.
Use UY128GAS.
Modify or replace
Refer to "12.-(10) Adjusting the thread
hook".
Refer to "12.-(9) Setting the raveling
prevention parameter".