1. 4120QDC
Service Manual
Revised: 05/09/13

3. Table of Contents Required Tools & Gauges ……………………………………………………….
Free Arm Removal .……………………...……………………………………....
Base Plate Removal ..…………………………………………………………...
Front Panel Removal ……………..………………………………….……….....
Parts of the Front Panel …………………………………………………………
Printed Circuit Board ‘A’ Removal ………………………………………………
Printed Circuit Board ‘A’ Connection Diagram ………………………………...
Touch Panel Removal & Installation ……………………………………………
Printed Circuit Board ‘A’ Installation ……………………………………………
Rear Cover Removal …………………………………………………………….
Switching Power Supply Removal & Installation ……………………………...
DC Motor Removal, Installation and Adjustment ……………………………..
Thread Tension Unit Adjustments ………………………………………………
Buttonhole Balance Adjustment ………………………………………………...
Automatic Threader Cutter Removal, Adjustments and Installation ………..
Feed Dog Height Adjustments .…………………………………………………
Presser Bar Height & Angle Adjustments …………………………………......
Needle Bar Position (Left-to-Right) Adjustment ……………………………….
Needle-to-Feed Timing Adjustment …………………………………………….
Needle Bar Height Adjustment ………………………………………………….
Needle-to-Hook Timing Adjustment …………………………………………….
Needle Bar Position (Front-to-Back) Adjustment …..…………………………
Hook Gear Backlash Adjustment ……………………………………………….
Needle Threader Plate Removal, Installation and Adjustments …………….
Rear Cover Installation …………………………………………………………..
Feed Balance Adjustment ……………………………………………………….
Front Cover Installation ………………………………………………………….
Free Arm Cover & Base Plate Installation ………………………………….....
Error Codes ………………………………………………………………… 1 2 3
4-5 6 7 8
9-11 12 13 14 15 16 17
18-20
21-23 24 25
26-27
28-29
30-31 32 33 34 35
36-37 38 39 40

5. Required Tools & Gauges
Small, Flathead Screwdriver
#PREDVR
Phillip’s Head Screwdriver (150mm)
#SD150
Phillip’s Head Screwdriver (Fine Tip)
#PHLDRVSP1
1.5mm Hexagon Key
#TLHW15
2.0mm Hexagon Key
#TLHW20
Large, Flathead Head Screwdriver
#DRIVER2
1.5mm Hexagon Screwdriver
#TLHS15
2.5mm Hexagon Key
#TLHW25
2.0mm Hexagon Screwdriver
#TLHS20
4 1/2” Curved Scissors #
2.5mm Hexagon Screwdriver
#TLHS25
Needle Nose Pliers
Multi-use Step Gauge
#860G001
5mm-7mm, Top- & Front-load Gauge Kit # 1

6. Free Arm Cover Removal This is a short screw.
1. Remove the extension table.
2. Remove the Phillip’s set screw that holds the end of the free arm cover to the machine. This is shorter of the 2 screws that hold this cover to the machine.
3. Remove the second screw that holds the free arm cover to the machine.
4. Remove the cover by sliding it outwards. 2

7. Base Plate
Removal
1. Remove the 5 small screws that hold the base plate to the machine.
2. Remove this cover by pulling on the upper left corner outwards and to the right. 3

8. Front Panel
Removal
1. To remove the front panel, start by removing the 3 set screws on the bottom of the machine.
2. Lift up the handle and remove the two large Phillip's head screws on the back of the rear cover.
3. Loosen the hidden washer screw that holds the left-side of the machine. It is located between the metal thread guide and black, needle threader guide plate.
4. Lay the machine on it’s back. Using a small, flathead screwdriver, stick the driver between the rear cover and the casting. Push down on the screwdriver in order to release the clip that holds both the front and rear covers together. 4

9. Front Panel
Removal
1. At the top of the machine, pull the front cover away to release it from the rear cover.
2. While holding the machine in place, grab the plastic leg and firmly pull towards you to release the front cover.
3. Unplug the 10 connections that hold the front panel to the machine. Be sure to grab the wires as a group and never a single wire by itself. 5

10. Parts of the Front Panel1 2 3 4
1. Printed Circuit Board ‘L’ (LED Light)
2. Printed Circuit Board ‘F’:
3. Printed Circuit Board ‘A’
4. Touch Panel Ribbon Cable
Lockstitch
Reverse
Start/Stop 6

11. Printed Circuit Board “A”
Removal
1. In order to remove the touch panel ribbon cable, you first need to release the tab that holds it to the board. Carefully slide the tab outwards towards the right. This tab only loosens away from the board and is not supposed to come off.
2. With the tab released, slide the ribbon cable towards the right.
3. Unplug the LED light and ‘F’ board cables from the ‘A’ board.
4. Remove the 4 Phillip’s set screws that hold the ‘A’ board to the front panel. 7

12. Printed Circuit Board “A”
Connection Diagram
Built-in LCD Unit
Speed Volume 4 3 2 1 5 6 7 8 15 9 14 13 10 11 12
1. Buttonhole Sensor
2. Thread Tension Unit Solenoid
3. Presser Foot Up/Down Micro Switch
4. Zig Zag Motor Unit
5. Bobbin Winder Micro Switch
6. Automatic Thread Cutter Step Motor & Micro Switch
7. Touch Panel Ribbon Cable
8. Switching Power Supply Unit
9. Feed Step Motor Unit
10. Upper Shaft Sensor
11. Foot Controller
12. DC Motor & Sensor
13. Printed Circuit Board ‘L’ (LED Light)
14. Printed Circuit Board ‘F’
15. Not Used 8

13. Removal and Installation
Touch Panel
Removal and Installation
1. If the touch screen does not respond to touch, it will need to be replaced. First, remove the lockstitch, reverse, and start/stop buttons.
2. Using a small, flathead screwdriver, remove the 3 lock tabs that hold the decoration panel to the front panel.
3. Carefully remove the decoration panel.
4. Using a flathead screwdriver or a razor blade, carefully remove the touch panel unit from the front panel. In order to not break the glass, you need to work around all edges small sections at a time to work it loose. 9

14. Removal and Installation
Touch Panel
Removal and Installation
5. Carefully and gently peel the touch panel sheet away from the LCD glass.
6. If the touch panel sheet does not get damaged upon removal, you can reuse it. If it gets scratched, you will need to replace it. The part number for this sheet is: # If you do replace it, you need to attach 1/8” double-sided tape on all 4 sides so it will stick to the touch panel.
7. Carefully align the touch panel sheet with the LCD glass verifying it aligns as perfectly as possible.
8. If you did not replace the touch panel sheet with a new one, you can reuse the tape. If you replaced the touch panel sheet, you will need to attach 1/8” double-sided tape on the top and bottom of the unit. 10

15. Removal and Installation
Touch Panel
Removal and Installation
9. Guide the LCD unit back into position on the front panel making sure the flexible cable is inserted into the opening. There are raised marks so the LCD glass will only fit one way. Firmly push down on the top and bottom of the LCD glass ensuring it is securely attached to the front panel.
10. Replace the decoration panel.
11. Replace the lockstitch, reverse, and start/stop buttons.
12. While holding the decoration panel against the front panel, use small, flathead screwdriver to push the 3 lock tabs securely so the decoration panel does not have any ‘play’. 11

16. Printed Circuit Board “A”
Installation
1. Before attaching the ‘A’ board, make sure both the slide volume on the board and the slide volume knob are aligned. Then carefully lay the ‘A’ board onto the front panel.
2. Lift up the handle and remove the small Phillip's head screw on the left side of the machine.
3. Carefully slide the flexible cable into the cream-colored connector.
4. While holding the cable in the connector, push the small tab closed to it locks the cable in place. 12

17. Rear Cover
Removal
1. Remove the 3 Phillip’s set screws on the back of the machine.
2. Lay the machine down on it’s back and remove the 3 set screws.
3. Lift up on the rear cover to release the clip that holds this cover to the tension unit.
4. Lower the presser foot. Grab the rear leg and pull outwards towards the back. 13

18. Switching Power Supply
Removal and Installation
1. Using a scissor, cut the zip tie at the top of the machine.
2. Turn the machine around and gently lay it on it’s front. Remove the 3 Phillip’s screws that hold it to the casting.
3. Continue to lift up and bring the top cover towards the back, left-hand side of the machine to free it from the handle.
4. Replace the zip tie and cut the excess tail. Do not over tighten the tie as it can pinch the wires. 14

19. Removal, Installation and Adjustment
DC Motor
Removal, Installation and Adjustment
1. Remove the motor belt by sliding the belt off of the hand wheel.
2. Remove the set screw that holds the cord holder in place and the 2 large Phillip’s washer screws that hold the motor in place.
5mm deflection
3. Place the new motor into position. Be sure the wires that are fed behind the motor are not pinched, then replace the 2 washer screws.
4. The proper slack on the motor belt should be 5mm when pushing in on the middle of the belt with 200 grams of pressure. 15

20. Thread Tension Unit
Thread Tension Release Bracket & Resetting the Upper Tension
1. When the presser foot is down, the thin, black, metal wire bracket should have a very slightly amount of ‘play’. If there is no ‘play’, that means the tension discs may be slightly open causing poor or inconsistent tension. If there is too much play in the bracket, the tension discs will not open which can cause the thread to abruptly pop out of the needle after using the thread cutter.
2. When the tension dial is at ‘4’, the white gear and the black bracket directly underneath this gear should be in line on the right-hand side.
This is perfect tension.
Stitch:
Width: 5.0mm
Length: 3.0mm
3. If an adjustment needs to be made, lightly push down on the black bracket directly underneath the white gear using a flathead screwdriver. While pushing down on this bracket, turn the gear. Turning the gear towards the back of the machine will tighten the tension and turning the gear towards you will loosen the tension. Once finished, sew a sample about 5” long on a zig zag stitch (width: 5mm, length: 3mm) and adjust the bobbin case tension accordingly. You should see a tiny dot of the top thread showing underneath on both sides of the zig zag stitch. 16

21. Buttonhole Balance
1. Remove any foot you have on the machine then lower the buttonhole lever all of the way down.
2. Follow the lever all of the way up. At the top of this lever is a shield plate with a ‘slit’ in the center. At this moment, the slit should be as close to the center of the sensor as possible, although it can be slightly tweaked to make the buttonhole perfectly symmetrical if needed.
Improperly adjusted buttonhole.
Perfectly adjusted buttonhole.
3. To adjust the position of the shield plate, locate the 1.5mm hex screw sticking out of a silver lock nut. Turn the hex screw clockwise to bring the ‘slit’ of the shield plate towards the back of the sensor and counter-clockwise to bring the ‘slit’ towards the front of the sensor. Do not loosen the locknut. 17

22. Automatic Thread Cutter
Removal, Installation and Adjustments
1. Remove the 2 hex nuts on the middle of the thread cutter unit. Then, remove the hex nut on the right-hand side and the setting plate directly underneath this nut. Now, remove the thread cutter unit by lifting upwards.
2. While holding the unit in your hand, remove the 2 large Phillip’s set screws and the 2 silver posts that separate the thread cutter and step motor unit.
OIL
3. This picture shows the parts when they are untogether.
4. Clean the thread cutter from lint and debris from both the top and underside of the cutter. Put one small drop of oil on the underside of the long extending blade. 18

23. Automatic Thread Cutter
Removal, Installation and Adjustments
Cutting Blades
5. If the extending blade jams or does not slide smoothly, loosen the small Phillip’s screw that holds the setting plate in place and move the plate so it is as close to the long blade as possible without it touching.
6. Be sure to clean the step motor gears thoroughly from thread & lint. Putting a small amount of lithium grease on the gears will help the cutter run smoother and quieter.
7. When reattaching, the black post of the step motor must fit into the access hole of the thread cutter.
8. Attach the silver posts and matching set screws that hold the thread cutter to the step motor. Tighten the 2 screws. To ensure the thread cutter is properly together. Push the long extending blade all of the way out then back in again. It should move smoothly and you should only feel the tension of the step motor. 19

24. Automatic Thread Cutter
Removal, Installation and Adjustments
9. Reattach the thread cutter onto the black posts and hold it in place with your hand. Before replacing the hex nuts, notice how the wires are routed under and around the black frame. It is important these are correctly routed or the wires will get pulled and break off the microswitch and/or step motor.
10. Replace the 2 hex nuts on the middle of the thread cutter.
11. Replace the setting plate and last hex nut. The setting plate needs to be as close to the long extending blade as possible without it touching. While firmly holding the setting plate in place, tighten the hex nut. 20

25. .8 - .9mm above the surface and level with the needle plate.
Feed Dog Height
.8mm - .9mm and level with the needle plate when in their highest position.
mm above the surface and level with the needle plate.
1. When the feed dogs are in their highest position, they should be parallel with the needle plate and mm above the surface of the plate. They should not be pointing upwards or downwards.
2. Using the feed dog height gauge, set the gauge with ‘A’ facing up at the front of the feed dogs. Rotate the hand wheel towards you. If the feed dogs are too low, they will not grab and move the gauge. Now move the gauge at the rear of the feed dogs and check it again.
3. Now flip the gauge so side ‘B’ is facing up and rotate the hand wheel towards you. If the feed dogs are too high, they will grab and move the gauge.
4. This step adjusts the rear section of the feed dogs. If an adjustment needs to be made, locate the locking set screw towards the rear of the machine on the end of the free arm, directly above the thread cutter step motor. Loosen this locking screw. 21

26. Feed Dog Height
.8mm - .9mm and level with the needle plate when in their highest position.
This is a counter-threaded set screw. Turn it clockwise to loosen.
5. Turn this small adjusting screw clockwise to raise and counter-clockwise to lower the rear section of the feed dogs. Do not loosen the hexagon nut. Once the adjustment is made, tighten the locking set screw.
6. To adjust the front section of the feed dogs, locate and loosen the washer set screw set deep within this access hole.
* This is a counter-threaded screw *
Counter-threaded set screw
Empty-threaded, 2mm hole
7. To make an adjustment, locate the small, empty-threaded access hole on the feed dog height plate.
8. Temporarily borrow a 2mm hex screw from the lower shaft rear collar. 22

27. * This is a counter-threaded screw *
Feed Dog Height
.8mm - .9mm above and level with the needle plate when in their highest
position.
9. Now, insert it into the empty-threaded hole. Turn it clockwise to raise and counter-clockwise to lower the front-section of the feed dogs.
This is a counter-threaded set screw. Turn it counter-clockwise to tighten.
10. Firmly tighten the counter-threaded screw. .
* This is a counter-threaded screw *
11. Replace the 2mm hex screw you previously borrowed from the lower shaft rear collar. Recheck the height of the feed dogs using the gauge (steps #2 & #3). 23

28. Presser Bar Height & Angle
6mm when the presser bar is in it’s normal, up position with the standard
zigzag foot attached
6mm
Gauge #860G001
1. Lower the feed dogs. Now, using the step gauge, slide the gauge under the foot. The gauge should slide snuggly under the foot where it snaps onto the shank. It should not have any up or down ‘play’.
2. If an adjustment needs to be made, locate and loosen the 2mm set screw on the presser bar block. Grab the presser bar and move it up or down accordingly then lightly tighten the screw. Recheck using the gauge.
3. Before tightening the screw completely, check the angle of the presser foot. The foot should be aligned with the feed dogs. 24

29. Needle Bar Position Left-to-Right
1. With the needle bar up, turn the machine on and select straight-stitch, center needle position.
2. Locate the 1.5mm hex screw between the white needle threader lever and the black threader guide plate.
3. Now turn the large, flat-head eccentric screw to align the needle within the center of the needle plate hole, then tighten the 1.5mm hex screw.
4. This step adjusts the rear section of the feed dogs. If an adjustment needs to be made, locate the locking set screw towards the rear of the machine on the end of the free arm, directly above the thread cutter step motor. Loosen this locking screw. 25

30. Needle-to-Feed Timing
1. Turn the hand wheel towards you to bring the needle down. Stop when the tip of the needle is flush with the top of the needle plate. At this point, the feed dogs should be flush with the top of the needle plate when going in their downward rotation (sewing motion).
2. Remove both the Phillip's screw that holds the idler (which provides tension on the timing belt) and the idler itself.
3. Slide the timing belt off of the lower shaft gear.
4. Turn the hand wheel towards you so the tip of the needle is touching the surface of the needle plate. Never turn the hand wheel backwards.
* The needle bar should be going downward in its sewing motion. * 26

31. Needle-to-Feed Timing
5. Turn the lower shaft towards you so the feed dogs are level with the top of the needle plate when going in their downward rotation. Never turn the gear backwards.
* The feed dogs should be going downward in their sewing motion. *
6. While firmly holding the hand wheel in place with one hand, slide the timing belt onto the lower shaft with the other hand.
7mm deflection
7. Place the idler back into position and tighten the screw. Recheck feed timing by turning the hand wheel towards you to bring the needle up. Continue to turn the hand wheel to bring the needle down. At this point the feed dogs will also be going in their downward motion.
8. While pushing on the middle of the belt, there should be about 7mm deflection. 27

32. The tip of the needle should be 8.8mm above the surface of the needle
Needle Bar Height
The tip of the needle should be 8.8mm above the surface of the needle
plate when the needle bar is in its highest position using a Janome needle.
1. Turn the machine on and select straight-stitch, center needle position. Now remove the needle plate. If you have the front panel off of the machine, push the needle bar so it is centered within the feed dogs.
2. Attach the needle bar height gauge with the black post sticking out towards you. Be sure the gauge is all of the way up into the needle bar.
Make sure the tip of the hook is touching the flat spot of the gauge at all times.
3. Turn the hand wheel so the tip of the shuttle hook is directly underneath the right bobbin stopper.
4. Insert the 5-7mm radial timing gauge. Make sure the tip of the hook is touching the flat spot of the gauge at all times. This is necessary for both proper timing checks and adjustments. 28

33. The tip of the needle should be 8.8mm above the surface of the needle
Needle Bar Height
The tip of the needle should be 8.8mm above the surface of the needle
plate when the needle bar is in its highest position using a Janome needle.
5. Turn the hand wheel towards you to bring the needle bar down.
6. As you continue to turn the hand wheel towards you, the needle bar height gauge should very lightly skim the top of the gauge. There should not be a gap between the gauges and the needle bar height gauge should not hit the radial gauge enough to shift the needle bar sideways.
7. If an adjustment needs to be made, locate and loosen the 2mm hex screw on the block that holds the needle bar in place. This is the larger of the two blocks on the needle bar. Raise or lower the needle bar accordingly. Do not twist the needle bar left or right. Recheck the adjustment. 29

34. Needle-to-Hook Timing
The tip of the hook should be 1.6mm-2.0mm above the top of the eye of the
needle when the hook touches the right-side of the needle in its sewing motion.
Make sure the tip of the hook is touching the flat spot of the gauge at all times.
1. Remove the needle bar height gauge and install the needle-to-hook timing gauge. Make sure this timing needle gauge is fully inserted.
2. Make sure the tip of the hook touches the flat end of the gauge at all times. This is necessary for both proper timing checks and adjustments.
3. Slowly turn the hand wheel towards you to bring the needle bar down.
4. When the needle touches the top of the radial gauge, it should touch the middle of the smaller, red line of the gauge. If the needle hits before or after the line on the gauge, an adjustment must be made. 30

35. Needle-to-Hook Timing
The tip of the hook should be 1.6mm-2.0mm above the top of the eye of the
needle when the hook touches the right-side of the needle in its sewing motion.
5. Locate the two 2.0mm hexagon screws holding the timing gear to the lower shaft.
6. Using a 2.0mm hex screwdriver, loosen both screws. While holding the hand wheel, use your screwdriver as leverage to adjust hook timing and tighten the 1 screw.
7. Make sure the tip of the shuttle hook is touching the flat spot of the radial gauge and recheck timing. If the adjustment is correct, tighten the second hex screw on the timing gear. Remove the gauges. 31

36. Front-to-back. Alternate adjustment to hook clearance
Needle Bar Position
Front-to-back. Alternate adjustment to hook clearance
1. Attach a new, size 14 needle. With the machine off, push the needle bar to the left. Turn the hand wheel to bring the tip of the hook directly behind the needle. The distance between the back of the needle & the tip of the hook should be mm.
2. If an adjustment needs to be made, locate and loosen the 2 Phillip’s set screws that hold the needle bar supporter in place.
3. Now pivot the needle bar supporter towards the front or back to get the proper needle-to-hook clearance adjustment. Once you have the adjustment, hold the needle bar supporter firmly in place and tighten the 2 set screws. Recheck the needle bar position. 32

37. Hook Gear Backlash .8mm of “Play”
1. Hold the hook race with your hand and wiggle it left & right. The hook race should have about .8mm of backlash, or “play”. Check it in 3 different spots by turning the hand wheel so the hook race turns 1/3 each time.
2. If an adjustment is needed, locate and loosen the Phillip’s set screw that holds the lower shaft eccentric bushing in place.
3. Using a small flathead screwdriver, move the bushing either upwards or downwards. Moving the bushing upwards will give the hook more backlash, or “play”, and downwards will give it less backlash.
* Do not move the bushing upwards or downwards too much as this will also change feed dog height which will also change needle-to-feed timing * 33

38. Removal, Installation and Adjustments
Needle Threader Plate
Removal, Installation and Adjustments
1. To remove the needle threader plate, first remove the needle and then pull down the threader. Push down on the threader plate to remove it.
2. To replace it, place the new threader plate under the threader shaft. Push up on the threader plate until it snaps into position. Now, insert a size 12 or 14 needle.
3. If the pin does not line up with the needle eye left-to-right, loosen the screw on the front of the threader plate. Move the threader plate to the left or right until the pin fits into the eye. Then tighten the screw.
4. To adjust the height of the threader pin, loosen the 1.5mm hexagon screw in the threader setting block. This is the smaller of the two blocks. Raise or lower the block accordingly. Do not rotate the this block left or right when you make this adjustment. Tighten the screw. 34

39. Rear Cover Installation
1. Lower the presser foot. Start by hooking the rear cover around the switching power supply then swing the rear cover into position.
2. Lift the top of the rear cover as you are pushing in so it will clip into the top of the tension unit.
3. Replace the 3 Phillip’s set screws.
4. Gently lay the machine down on it’s back and replace the 3 set screws. 35

40. Feed Balance Adjustment
1. Align the front panel with the machine and reconnect the 10 plugs. Do not install the front panel yet.
Perfectly balanced stitches
Stitches are too far apart
Stitches are too close together
2. With the front panel hanging off of the machine, turn the machine on. 36

41. Feed Balance Adjustment
3. Before making an adjustment, be sure the customer’s balance screw is vertical.
4. Locate and loosen the 1.5mm hex screw that holds the silver, pinion gear onto the step motor.
5. With the screw loose, use your screwdriver as leverage to turn the gear. Turning the gear upwards will make the stitches longer and shorter if you move the gear downwards. Do not move the gear to the left or right. Turn the machine off, then back on again and recheck the feed balance.
* These are very fine adjustments. If you move the gear too much either way, the machine will sew backwards * 37

42. Front Cover Installation
1. Gently lay the machine on it’s back and replace the 3 Phillip’s set screws.
2. Lift the machine upright and turn it around. Raise the handle and replace the 2, large Phillip’s screws.
3. Lay the machine on it’s back. Using a small, flathead screwdriver, stick the driver between the rear cover and the casting to reattach the clip that holds both the front and rear covers together.
4. Tighten the hidden washer screw that holds the left-side of the machine. It is located between the metal thread guide and black, needle threader guide plate. 38

43. Free Arm Cover & Base Plate
Installation
1. Reattach the base plate and reinstall the 5, small, self-tapping set screws.
2. Reattach the free arm cover by sliding into position. Be careful not to scratch, pull, or damage the auto thread cutter wires.
This is a short screw.
3. Replace the short Phillip’s set screw.
4. Replace the 2nd set screw on the bottom of the free arm cover. 39

44. Error Codes Thread may be caught in the gears of the step motor
The thread cutter step motor and/or microswitch are not plugged into the ‘A’ board
See pages #18-20
The customer may have jammed the machine.
The foot controller may need to be replaced.
The DC motor or upper shaft sensors may be dirty or need to be adjusted.
The upper shaft sensor or DC motor (in this order) may need to be replaced.
The buttonhole lever is not lowered all of the way down before sewing a buttonhole
The ‘slit’ of the shield plate is not in the center of the sensor.
See page #17
The bobbin winder is pushed to the right.
The bobbin winder micro switch plug is not connected to the ‘A’ board. 40