It always starts with cleaning the machine. I take apart everything, clean with rags and Qtips, and oil liberally.
Click here for a great article on how to oil a sewing machine.
I am very careful about keeping track of tiny parts like this bitty screw. I look for possible issues, like threads caught in places like the bobbin casing and fix them up. Do you see the thread caught in this one at the 12 o'clock position?
I make notes about threading and bobbins that are particular to the machine. Each vintage machine takes different kinds of bobbins and I try hard to keep all of these bobbins with their own machines. They also have varying positions for needles (flat to the back, flat to the side, etc) and different methods of threading the needle (front threading, side threading, etc.)
This is how my new baby was looking. I had to take apart the whole thread tension assembly. I discovered it needed a new spring... see the weird shaped coil on the left of all the pieces? It shouldn't look like that!
My trusty mechanic ordered me a new one, so stay tuned to see if I can get this baby going again!