Hi So-Sew
I just had my mother's old Singer refurbished. The repairman charged me $13 to replace the wiring and $79 to do the tune-up. His son who was in when I picked up the machine (it's about 70 years old) showed me how to thread it and fill in the bobbins. He also told me to clean up the grime with rubbing alcohol, nothing like paint thinner and to go lightly over the painted areas.
I hope that helps a little.
Can someone post the link for the instructions on how to service these old machines ourselves?
Thanks.