Here's an article I wrote:
When you bring your machine in for a full service, here are some things that get done:
Clean and oil throughout, which can get very deep. We remove some, or all of the outside of the machine, and use an air compressor to blow out the lint. We oil where metal rubs on metal, and put grease on certain gears and tracers.
Service the motor. Older machines have carbon motor brushes that wear out and may need to be replaced. We oil the motor bearings. In addition, we clean the motor’s commutator (or armature) when it’s accessible. Then blow the carbon and lint out of the motor when it is open.
Check the belts. Belts need to be replaced when they start to shred, get deep cuts in them, or get saturated with oil.
Check the electrical components. Foot controls and cords on older machines may be a fire hazard. On newer machines, circuit boards and stepping motors need to be checked. We also make sure the light is working properly.
Check for mechanisms that have been gummed-up with old oil. When necessary, we use a deep penetrating oil or a solvent to remove the old oil, then re-oil.
Replace or repair any broken parts.
Polish the burrs and scratches off of the hook and needle plate, if needed.
Replace the needle.
Check the timing. We make sure the needle, hook and feed dogs are working together to make a good stitch.
Balance the tensions and do a sew-off sample.
Clean the outside of the machine.
What To Bring With Your Machine When It Needs a Service
Foot control and power cord (these need to be checked, too)
Standard presser foot. This is the foot that can sew all of the stitches on your machine, and has a nice flat bottom. We need this foot to do a good sew-off sample.
Bobbin case (so we can balance the tensions)
The correct bobbin
Thread (if a particular thread is giving you trouble)
Please write what problems you are having on the service order, and if you’d like to include a full service.
What NOT to bring (because we may loose them):
Accessory case and accessories
Carry case, bag, box or crate
Walking foot or other specialty feet
Thread (unless the thread is giving you trouble)
Snap-on sew table
If you are still having problems after the service, please bring your machine back right away, so we can sit down with you, and figure it out.