Janome 4120QDC Troubleshooting guide

SERVICE MANUAL
PARTS LIST
4120QDC

INDEX
What to do when................................................................................................................................ 1 - 3
Changing External Parts
Face Cover..............................................................................................................................................4
Free-arm Cover ......................................................................................................................................5
Front Cover........................................................................................................................................ 6 - 7
Rear Cover ..............................................................................................................................................8
Mechanical Adjustment
Presser Bar Height ..................................................................................................................................9
Needle Drop Position.............................................................................................................................10
Adjustment of Hook Timing....................................................................................................................11
Adjustment of Needle Bar Height ..........................................................................................................12
Clearance between Needle and Tip of Rotary Hook .............................................................................13
Feed Dog Height ...................................................................................................................................14
Top Tension ...........................................................................................................................................15
Replacing the Electronic Components
Circuit Board-A Connection ...................................................................................................................16
Self-diagnostic Test ......................................................................................................................... 17-22
Circuit Board-A ......................................................................................................................................23
Driving Motor .........................................................................................................................................24
Switching Regulator Unit .......................................................................................................................25
Adjusting Buttonhole Lever Position......................................................................................................26
Parts List ....................................................................................................................................... 27-41

1
Condition Cause How to fix RefeRenCe
1. SKIPPING
STITCHES
1. NEEDLE IS NOT INSERTED
PROPERLY.
2. NEEDLE IS BENT OR WORN.
3. INCORRECTLY THREADED.
4. NEEDLE OR THREAD ARE
INAPPROPRIATE FOR THE
FABRIC BEING SEWN.
5. SEWING ON
STRETCH FABRIC.
6. INAPPROPRIATE NEEDLE
BAR HEIGHT.
7. INAPPROPRIATE NEEDLE TO
HOOK TIMING.
8. INAPPROPRIATE NEEDLE TO
HOOK CLEARANCE.
INSERT THE NEEDLE PROPERLY.
CHANGE THE NEEDLE.
RETHREAD.
USE THE RECOMMENDED SEWING
NEEDLE AND THREAD.
USE A HA X #11 BLUE TIP NEEDLE.
SEE MECHANICAL ADJUSTMENT
“NEEDLE BAR HEIGHT”.
SEE MECHANICAL ADJUSTMENT “AD-
JUSTMENT OF HOOK TIMING”.
SEE MECHANICAL ADJUSTMENT
“CLEARANCE BETWEEN NEEDLE
AND TIP OF THE ROTARY HOOK”.
P. 12
P. 11
P. 13
2. FABRIC NOT
MOVING
1. INCORRECT FEED DOG
HEIGHT.
2. FEED DOG IS IN DOWN
POSITION.
3. THREAD ON BOTTOM SIDE OF
FABRIC IS JAMMED UP.
4. FEED DOG TEETH ARE WORN.
ADJUST THE PRESSER BAR LEVEL
TO MAKE THE PRESSURE STRON-
GER. SEE MECHANICAL ADJUST-
MENT “FEED DOG HEIGHT”.
RAISE THE FEED DOG.
MAKE SURE TO BRING BOTH NEEDLE
AND BOBBIN THREADS UNDER THE
FOOT WHEN START SEWING.
CHANGE THE FEED DOG.
P. 14
wHat to do wHen

2
CONDITION CAUSE HOW TO FIX REFERENCE
3. BREAKING
UPPER
THREAD.
1. INITIAL SEWING SPEED IS
TOO FAST.
2. THREAD PATH IS INCORRECT.
3. NEEDLE IS BENT OR DULL.
4. TOP TENSION IS TOO STRONG.
5. NEEDLE SIZE IS
INAPPROPRIATE FOR FABRIC.
6. NEEDLE EYE IS WORN.
7. NEEDLE HOLE IN NEEDLE
PLATE IS WORN OR BURRED.
START WITH MEDIUM SPEED.
USE THE PROPER THREAD PATH.
REPLACE WITH A NEW NEEDLE.
ADJUST TOP TENSION CORRECTLY.
USE APPROPRIATE NEEDLE FOR
FABRIC AND THREAD IN USE.
CHANGE THE NEEDLE.
REPAIR THE HOLE OR REPLACE THE
NEEDLE PLATE.
4. BREAKING
BOBBIN
THREAD.
1. BOBBIN HOLDER IS
INCORRECTLY THREADED.
2. TOO MUCH THREAD IS WOUND
ON THE BOBBIN.
3. LINT IS STUCK INSIDE THE
BOBBIN HOLDER.
4. THREAD QUALITY IS TOO LOW.
5. THREAD IS JAMMING AROUND
THE BOBBIN HOLDER.
SET THE BOBBIN THREAD
CORRECTLY.
ADJUST THE POSITION OF BOBBIN
WINDER STOPPER.
CLEAN THE BOBBIN HOLDER.
CHANGE TO A HIGH QUALITY SEWING
THREAD.
CLEAR OUT THE JAMMING THREAD.
5. NEEDLE
BREAKES
1. NEEDLE IS HITTING THE
NEEDLE PLATE.
2. NEEDLE IS BENT OR WORN.
3. NEEDLE IS HITTING THE HOOK.
4. FABRIC IS BEING PULLED TOO
STRONGLY WHILE SEWING.
SEE MECHANICAL ADJUSTMENT
“NEEDLE DROP POSITION”.
CHANGE THE NEEDLE.
SEE MECHANICAL ADJUSTMENT
“CLEARANCE BETWEEN NEEDLE AND
TIP OF THE ROTARY HOOK”.
GUIDE THE FABRIC GENTLY WHILE
SEWING.
P. 10
P. 13

3
CONDITION CAUSE HOW TO FIX REFERENCE
6. NOISY
OPERATION
1. BACKLASH BETWEEN HOOK
GEAR AND LOWER SHAFT
GEAR IS TOO GREAT.
2. LOWER SHAFT GEAR IS
LOOSE.
3. INAPPROPRIATE BELT
TENSION.
4. NOT ENOUGH OIL.
5. UPPER SHAFT GEAR IS
LOOSE.
ELIMINATE THE BACKLASH.
ELIMINATE THE LOOSENESS.
SEE PART REMOVAL AND
REPLACEMENT “DRIVING MOTOR
(DC MOTOR)”.
OIL ALL MOVING PARTS.
ELIMINATE THE LOOSENESS.
P. 24
7. DEFORMATION
PATTERN
1. INAPPROPRIATE FEED
BALANCE.
2. TOP TENSION IS TOO
STRONG.
ADJUST THE FEED BALANCING
SCREW.
SEE MECHANICAL ADJUSTMENT
“TOP TENSION”.
P. 15