Brother Z-8550A Service Manual page 119

Problem
3 Skipped stitches occur
while sewing.
0621M
4 Flattened zigzags appear
in the stitch during
intermittent sewing.
(8550A)
1406M
5 Skipped stitches at sewing
start
Thread unravelling at
sewing start
0622M
Upper
thread
0623M
• Is the needle tip bent? Is the needle tip blunt?
If the needle tip is bent or broken, replace the needle.
• Is the needle properly installed?
If it is incorrect, install the needle correctly.
• Is the machine properly threaded?
If it is incorrect, thread the thread correctly.
• Is the presser foot pressure too weak?
Adjust the presser foot pressure.
• Is the needle too thin?
Replace the needle with a needle that is one rank thicker.
• Is the presser foot too high?
Adjust the height of the presser foot.
• Is the needle and rotary hook timing incorrect?
Adjust the height of the needle bar.
Adjust the clearance between the needle and the rotary hook.
• Is the thread take-up spring too weak?
Adjust the tension of the thread take-up spring.
• Has the treadle been depressed backward?
Do not press the treadle backward when the sewing machine is
stopped.
(When memory switch No. 109 is set to "1", needle zigzagging
also occurs when the treadle is depressed backward, in order to
prevent a flattened zigzag from being sewn if the treadle is
depressed backward by mistake.)
• Does the treadle move too easily when it is depressed backward?
Adjust the force required to depress the treadle backward so
that it is a bit heavier.
• Is the thread take-up spring tension too strong?
Reduce the tension of the thread take-up spring.
• Is the thread take-up spring operating range too large?
Lower the position of the thread take-up spring.
• Is the needle too thick?
Try using a needle with a count that is one lower than the
current needle.
<8550A>
• Is the needle bar at the needle up stop position at the sewing start?
Set the needle bar to the needle up stop position at the sewing
start.
• Is the length of the upper thread trailing from the needle hole too
short?
Pull about 50 mm of thread through the needle hole at the
sewing start.
<8560A>
• Is the trailing length of the upper thread too short after thread
trimming?
Adjust the upper thread feeding device.
• Are the threads not being trimmed cleanly?
Sharpen the fixed knife with a whetstone, or replace the fixed
knife.
Replace the movable knife.
• Is the length of thread trailing out from the bobbin case after thread
trimming too short?
If the bobbin is spinning loosely, replace the anti-spin spring in
the bobbin case.
Adjust the tension of the lower thread presser spring.
• Is the needle up stop position too high?
Adjust the needle up stop position.
• Is the sewing speed too fast at the sewing start?
Use the slow start feature.
Z-8550A, 8560A
Possible cause
14. TROUBLESHOOTING
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