Fantastic tutorial. I have been sewing for over 50 years and maintaining a sewing machine is so important. Most people are clueless and like you said pay an outrageous amount of money to have it done. I made the horrible mistake of sending my very expensive machine to the repair shoppe. This place had been in business for a very long time and I never read/heard anything negative stated. I had attempted to fix the issue myself having done so many time over the years. Oh my god worst mistake every. My daughter picked it up for me and all looked good to her but she had never used the machine. She left and I set up the sewing machine and much to my horror he had replaced expensive metal parts with cheap plastic parts that he must have gotten off of a trashed machine. It lend me to test his "fix" and the machine still didn't work and the part he put it was I was/am not able to adjust and it should as it's for tension. I was livid and a call to him ended with him being nasty. Five years later the machine is still not working and I love to sew.
So your advice on maintaining your machine is good info and I am book marking this one.
Can't wait to see what your next tutorial will be.