Troubleshooting - Brother T-8420C Instruction Manual

11. TROUBLESHOOTING

11. TROUBLESHOOTING
y Please check the following points before calling for repairs or service.
y If the following remedies do not fix the problem, turn off the power switch and consult a qualified technician or the place of
purchase.
Turn off the power switch and disconnect the power cord before carrying out troubleshooting. The motor will keep
turning even after the power is switched off as a result of the motor's inertia. Wait until the motor stops fully before
starting work.
The machine may operate if the treadle is depressed by mistake, which could result in injury.
Items with a "*" in the "Page" column should only be checked by a qualified technician.
Problem
1 Machine does not operate
when power is turned on
and treadle is pressed.
2 Upper thread is not tight.
Lower thread is not tight.
0573M 0574M
3 Loops appear in seam.
45
y Is a breaker or fuse blown?
y Is the power cord disconnected?
y Is the V-belt installed?
y Has the thread been threaded incorrectly?
If the thread is threaded incorrectly, thread it correctly.
y Is the upper thread tension or lower thread tension too weak?
Adjust the upper thread tension or lower thread tension.
y Is the thread take-up spring operating range too small?
Lower the position of the stopper.
If using a sewing machine with medium-weight material
specifications, poor thread tightening may occur depending on the
type of material being sewn.
In such cases, replace the presser foot with one that has slots on
the underside, or replace the feed dog with a grooved feed dog.
y Is the thread path not smooth enough?
Use a file with a fine grain or sandpaper to polish smooth the
thread path.
y Is the bobbin not turning smoothly?
Pull out the lower thread to check that there is no slackness in
the thread tension, or replace the bobbin or bobbin case.
y Is the thread take-up amount too large?
Adjust the thread take-up amount.
0977M
T-8420C, 8450C, 8720C, 8750C
CAUTION
Possible cause
Slot
Page
*
7
16-20
25
35*
Grooves
2944M
28