Brother Sewing machine - Troubleshooting – How to adjust the tension of motor belt and timing belt

Brother Sewing machine - TROUBLESHOOTING 

Machine skips stitches
1. Improper setting of the needle.
2. Bent or blunt needle.
3. Improper threading.
4. Improper combination of needle/ thread/fabric.
5. Stretch fabric is used.
6. Improper timing of the needle and rotary hook.
7. Improper height of the needle bar.
8. Improper clearance between the needle and rotary hook. Set the needle correctly.
Change the needle.
Reset threading correctly.
Select correct combination of needle/ thread/fabric.
Use stretch fabric needle.

Fabric does not feed
1. Stitch length is set at "0".
2. Improper height of feed dog.
3. Thick fabric.
4. Feed dog is worn out.
5. The feed dog is lowered.
Set proper stitch length.
Change the fabric.
Change the feed dog.
Raise the feed dog with the drop lever.
Fabric does not feed straight
1. Uneven pressure on right/left side of presser foot.
Change the presser foot and the feed dog.
Upper thread breaks at start
1. Improper threading.

Thread correctly.
Upper thread breaks during sewing
1. Bent or blunt needle.
2. Improper setting of needle.
3. Thread tension is too tight.
4. Thread is tangled.
5. Inferior needle eye.
6. Inferior needle slot of needle plate.
7. Inferior upper thread path.
8. Inferior thread.
9. Needle hits needle plate or outer rotary hook.
10. There are scratches on inner rotary hook.

Change the needle.
Set the needle correctly.
Adjust tension control dial correctly.
Thread correctly.
Change the needle.
Change the needle plate.
Clean or replace.
Check or change the thread.
Set the needle bar and outer rotary hook correctly.
Change inner rotary hook.

Upper threadbreaks during reverse sewing
1. Fabric is pulled excessively.
2. Improper use of reverse sewing button. Guide the fabric.
Push reverse sewing button as far as it will allow.
Lower thread breaks
1. Lower thread tension is too tight.
2. Too much thread is wound on bobbin.
3. Thread is tangled.
4. Bobbin does not turn in inner rotary hook.
5. Inferior thread.
Adjust the tension by loosening the tension spring screw on inner rotary hook.
Change to other bobbin or adjust bobbin winder.
Thread correctly.
Change the bobbin.
Check or change the thread.
Needle breaks

1. Touch pattern dial while needle is in the fabric.
2. Needle hits inner rotary hook.
3. Needle hits needle plate.
4. Bent or blunt needle.
5. Improper clearance between the needle and the outer rotary hook.
6. Machine feeds while needle is in the fabric.
7. Needle flows.
8. Fabric is pulled excessively.

Operate machine correctly.
Adjust the position of inner rotary hook stopper.
Make sure the correct needle is used.
Change needle.
Guide the fabric.
Noise

1. Play of worm/pattern cam gear.
2. Play of lower shaft.
3. Play of upper shaft.
4. Noise from rotary hook.
Adjust to proper gearing.
Adjust play of the lower shaft.
Adjust play of the upper shaft.
Clean the race of inner rotary hook and outer rotary hook or replace inner rotary hook.

Forward and reverse feedings differ
1. Improper adjustment of reverse sewing lever.
Improper length of buttonhole legs
1. Improper adjustment. Adjust screw.
Improper buttonhole size against button size
1. Improper adjustment of buttonhole length.
Adjust the length of buttonhole.
Unbalanced patterns
1. Stitch length dial is not set at ''SS''.
2. Feed dog is worn out.
3. Improper height of feed dog.
4. Improper attachment of feed dog.
5. Forward and reverse feedings differ.
6. Stitch width dial is not set to "5".
Operate machine correctly.
Change the feed dog.
Attach the feed dog correctly.
Operate machine correctly.
Needle hits needle plate when pattern selector dial is turned
1. Release volume is too big. Adjust the release volume.
Fabric gathers

1. Thread tension is too tight.
2. Blunt needle.
3. Improper combination of needle/ thread/fabric.
4. Improper threading.
Reduce the tension.
Change the needle.
Select correct combination of needle/ thread/fabric.
Reset threading correctly.
The stitch shown in the pattern indication window is different from the one being sewn
1. Release volume is too small.
2. The indicate gear and pattern indicator plate are not aligned correctly.
Adjust the release volume.
Adjust the pattern indicator.

The light does not turn on even after the power is turned on (B375, B376 and B377)
1. The voltage on both ends of connector SOURSE is not 120V or 230V AC when the AC cord is inserted.
2. The resistance on both ends of the power switch is not less than 1 W when the power is turned on.
3. The fuse is broken.
4. The light does not turn on. (The light may be damaged or deteriorating or it may have a bad connection.) Remove the light and measure the resistance on both ends. It should be less than 30 W.
5. The output from the transformer is incorrect. When connector CN1 is disconnected from the motor control unit and the power is turned on, the voltage between pins 1 and 2 of connector CN1 should be approximately 13 V AC.
Replace either the AC cord or the motor control unit.
Replace the motor control unit.
Replace the fuse or the motor control unit.
Replace the light.
Replace the motor control unit.
Replace the motor control unit.

The motor does not run. (B375, B376 and B377)
1. The pulley is turned too tightly.
2. The resistance between the two ends of motor connector MOTOR is not between 30 and 40 W (120V), 120 and 130 W (230V).
3. Inferior SS switch.
The resistance between pins 3 and 4 of the SS-VR PCB assy. connector should be less than 1 ohm  when the SS switch is turned on and infinity when the SS switch is turned off.
4. Inferior BT switch.
The resistance between pins 1 and 2 of connector CN3 (red) should be less than 1 Ohm when the BT switch is turned on and infinity  when the BT switch is turned off.
5. The resistance between pins 1 and 2 of the pin jack assembly on the PCB (F.C.) should not be less than 1 W when the foot controller is not inserted.
6. The fuse is broken.
Adjust the mechanism so that the pulley rotates easily.
Replace the motor.
Replace the SS-VR PCB assy..
Replace the BT switch.
Replace the motor control unit.
Replace the fuse or the motor control unit.
Replace the motor control unit.

The motor does not run smoothly or it does not run at a high speed. (B375, B376 and B377)
1. The pulley is sometimes easy or sometimes difficult to move when it is rotated by hand.
2. Accumulated lint or thread under the needle plate.
3. Improper tension in motor belt and timing belt.
4. Tight gearing of the pattern cam gear and worm gear.
5. The rotary shutter is dirty.
6. The motor control unit is installed incorrectly. It is loose or slanted.
7. The voltage between pins 2Å and 4 - on the SS-VR PCB does not stay between 0 and 5 V DC when the power is turned on and the speed adjustor is slid to the maximum.
Adjust the mechanism so that the pulley rotates easily.
Clean inner rotary hook, outer rotary hook and feed dog.
Clean the rotary shutter.
Install the motor control unit correctly.
Replace the SS-VR PCB unit.
Replace the motor control unit.

The speed at which the machine operates cannot be adjusted. (B375, B376 and B377)
1. The output from the transformer is incorrect. The voltage between pins 10 and 12 of the transformer should be approximately 12 V AC.
2. The voltage between the 5 V and GND on the motor control unit is incorrect. It should be 5+/- 0.25 V DC.
3. The voltage between pins 2+ and 4 - on the SS-VR PCB does not stay between 0 and 5 V DC when the power is turned on and the speed adjustor is slid to the maximum.
Replace the motor control unit.
Replace the motor control unit.
Replace the SS-VR PCB unit.
Replace the motor control unit.

The machine does not operate if the reverse sewing button is pressed or does not operate at a slow speed if the button is pressed while the machine is running. (However, the machine does not operate at slow speed when the foot controller is being used.) (B375, B376 and B377)
1. The BT switch does not work.
2. Inferior BT switch.
The resistance between pins 1 and 2 of connector CN3 (red) should be less than 1 Ohm when the BT switch is turned on and Infinity when the BT switch is turned off.
Check and adjust if necessary.
Replace the BT switch.
Replace the motor control unit.
ADJUSTMENT OF THE MOTOR BELT AND TIMING BELT TENSIONS
STANDARD
There should be 4 - 6 mm of slack on the motor belt and 3 - 4 mm of slack on the timing belt when the center of each belt is pushed with about 200 g pressure.
ADJUSTMENT
Timing belt
1. Loosen the screw on the tension pulley.
2. Adjust the tension pulley position.
3. Tighten the screw on the tension pulley.

Motor belt
1. Loosen the two motor holder screws.
2. Move the motor holder to adjust the belt slack.
3. Tighten the two motor holder screws.