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13. TROUBLESHOOTING
Problem
4. Skipped stitches at sewing
start
Thread unravelling at
sewing start
Upper
thread
5. Uneven seam
57
¡
Are the thread take-up spring tensions too strong?
Reduce the tension of the thread take-up springs.
¡
Is the thread tension spring operating range too large?
Lower the position of the thread tension spring.
¡
Is the needle too wide?
Try using a needle with a count that is one lower than the current
needle.
<7210/7230>
¡
Is the thread take-up lever at its highest position at the sewing start?
Set the thread take-up lever to its highest 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.
<7220/7240>
¡
Is the trailing length of the upper thread too short after thread trim-
ming?
Adjust the pretension.
¡
Are the threads not being trimmed cleanly?
Sharpen the fixed knives, or replace the fixed and movable knives if
necessary.
¡
Is the length of thread trailing out from the bobbin case after thread
trimming too short?
If the bobbin is spinning loosely, replace the bobbin tension spring
in the bobbin case.
If using woolly nylon thread as the lower thread, the amount of lower
thread trailing from the bobbin case will be shorter than normal, so
replace the movable knives with movable knives S (159915-001).
¡
Is the needle up stop position too high?
Adjust the synchronizer.
¡
Is the presser foot pressure too weak?
Adjust the presser foot pressure.
¡
Is the feed dog too low?
Adjust the feed dog height.
¡
Is the bobbin scratched?
If the bobbin is damaged, smooth it with an oiled grindstone or re-
place it.
SN-7200 series
Possible cause
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