Brother LH4-B800E Service Manual page 131

5. Troubleshooting guide
Folding clamp is not raised (2) Pulse motor stepping sound can be heard
Cause
Machine Rotary hook touches
stopping lower thread retainer at
position the stop position
Presser foot Memory switch No. 02.
rise speed is (presser foot rise
too fast speed)
Upper thread Interference between
scissors upper thread scissors
and cloth presser
Upper thread scissors
operation
Lower thread Interference between
trimmer fixed knife set, lower
thread clamp, lower
thread presser and
lower thread retainer.
Rubbish, dust or
thread scraps present
Loose screws Check for loose screws
in the upper thread
trimmer, lower thread
trimmer and presser
lifter mechanisms.
Presser foot Memory switch Nos.
lifter height 04., 05. and 06.
threading Bobbin thread
threading
Lower thread tension
(if the lower thread
tension is too strong,
step errors may occur)
Presser foot Presser foot lifter home
lifter home position sensor
position position
sensor
5 - 16
Check
Adjust the needle up
stop position.
Reduce the presser
foot rise speed.
Check the standard
adjustments,
particularly for joint
sewing.
Carry out upper thread
scissors standard
adjustments.
Repair or replace the
parts.
Remove all rubbish,
dust and thread scraps
from the lower thread
trimmer.
Securely tighten all
screws.
Adjust so that the
memory switch setting
value matches the
presser foot rise height.
Insert the bobbin
correctly and pass the
lower thread through
the bobbin case
correctly.
Reduce the lower
thread tension.
Reduce the setting for
the presser foot lifting
speed (memory switch
No. 02.) to reduce the
lifting speed.
Adjust the presser foot
lifter home position
sensor position.
Solution
Change using memory switch No.
02..
Refer to "4-13-6. Adjusting the installation
height of the upper thread scissors"
(Remove the upper thread scissors to check.)
If the presser foot rise height is too
high, the lower thread trimming and
presser foot rising will become
harder and the pulse motor may go
out of step.
If the bobbin is inserted back-to-
front, it will spin loosely and the
presser lifter may go out of step.
Buff this section to reduce the
thread retaining resistance.
LH4-B800E
Retainer
Inner rotary
hook presser
Lower thread
trimmer link A
Lower thread
trimmer
connecting rod
Remove the shoulder
screw to check.
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4-12
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(Instruction)
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(Instruction)
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4-5