Sewing Machine Repair Checklist

From the Repair Bench

This is the checklist my dad, Dan Schoenberg, gave me when I first started servicing machines in high school: everything a trained technician does when they service your machine.

He called it the COA Checklist: Clean, Oil, Adjust. I hope you find it interesting, and that it demystifies what happens when your machine is on our bench.

— Cale

Why People Bring Machines In

The problems that start it

When someone brings their machine to our shop for service, it’s usually because it has started having a particular problem. Common ones include:

Poor stitch quality
Skipped stitches
Unusual sounds (squeaking, growling, etc.)
Electronic issues (no power, error messages, etc.)
Fabric feeding poorly
Needle threader not working

Order matters

The most important thing a technician must remember is to work in a precise sequence, because one adjustment affects another. For example, you must always check and adjust feed timing before hook timing. Changing feed timing always changes hook timing, but changing hook timing doesn’t change feed timing. In fact, most adjustments have a direct effect on every other adjustment.

There are two main categories of adjustments: the feeding system and the stitch-forming system. Then there are minor areas like bobbin winding, the needle threader, and electronic calibrations.

The Checklist

Clean · Oil · Adjust

Teardown, cleaning & lubrication

Remove all external covers to expose all inner workings.

Sewing machine with external covers removed

Disconnect key electrical connections and remove circuit boards as needed.
Clean the machine with compressed air and remove all shredded or caught-up thread.

Lint and shredded thread inside an old serger.

Inspect for worn or broken parts.

Broken gear on a Pfaff 1222.

Take up shaft play and gear lash (translation: make loose parts tight again).
Lubricate the machine. As a general rule, we do not use any grease. More on sewing machine oil.

Feed system adjustments

Motor belt tension

Feed dog position in the needle plate
Timing of feed motion

Feed timing: needle down, feed dogs down

When the needle is up, the feed dogs are above the plate feeding fabric. When the needle goes below the plate, the feed dogs must drop below it too. You never want the machine feeding fabric while the needle is in it. More on timing.

Feed dog height

Checking feed dog height above the needle plate — When checking feed dog height, we should see both the “peaks” and “valleys” of the feed dogs, though only just barely the valleys.

Synchronizer (you wouldn’t want the needle moving right and left while in the fabric)
Presser foot alignment and height

Stitch-forming parts

Position of needle in needle plate (left to right and front to back)
Hook-to-needle clearance (must be close, but not touching)

Proper and too-tight needle-hook clearance — Left: proper needle-hook clearance. See the little bit of daylight between needle and hook? That’s necessary. Right: clearance too tight. The needle deflects off the hook on every stitch, which can cause skipped stitches and further problems.

Hook timing

Needle-hook timing: hook point just above the needle eye — Needle-hook timing: the hook point should pass just above the needle eye as the needle rises 2–3 mm above its lowest position.

Needle bar height
Bobbin case positioning finger or bracket

Bobbin case positioning bracket — Positioning bracket circled in blue. The bobbin case’s triangular nipple (circled in red) must bump up against it, which keeps the bobbin case out of the needle’s path.

Sew-off adjustments

Bobbin winder and bobbin winder auto-stop
Bobbin case thread tension
Check spring travel

Check spring properly threaded on a Bernina 770.

Top thread tension
Forward and reverse feeding (the machine should feed forward the same as in reverse)
Foot control speed calibration

There are probably 20 other secondary adjustments I haven’t listed. On top of the checklist, a technician also addresses the specific issues that brought you to us in the first place.