I had one out of the trash that was covered in leaves and smelled like some of it was dog poo... well at least I thought it was dog poo..... it was a 401G though...
You are so right... usually I see lint, broken needles, backward needles, tension that is maladjusted, sprung tension springs, dried up oil... etc. You do have to know what to look for though. And I have seen people's eyes glass over at the thought of sewing anything let alone maintaining...
I just repaired the bobbin area in an Elna - somebody put it together wrong. It sews beautifully now. Miss L sewed her first quilt for her baby sister's doll today on that Elna. I fear I have spoiled her for life. She was used to old Singers. That Elna can go so slow it seems like one stitch at a time. She was thrilled. What I wonder is why somebody took the bobbin area apart in the first place?
As some know, I take care of my parents these days. I taught Dad to use the blender so he can make his own smoothie when he gets up in the morning. We had success for the first month or so. Then yesterday he used the blender, made a smoothie, turned it to take it off the unit and the top came off but not the bottom... Yup a mess. He did it again today only I just watched him clean it up... I tried not to laugh or swear... He knew how to use it. He just had a user problem. He's ready to quit... Maybe that is how it goes with sewing machines. People learn how, get familiar, get comfortable and forget what they know to do? One little glitch and they are through.