Thank you both. Yes, the spring is there. It doesn't drop back down like it should, so the thread just runs straight through like there was no spring at all. Joe, I thought the bottom tension was just the screw setting in the bobbin case? Or are you saying to clean - degunk - the whole underside of the machine? I haven't disassembled anything yet, but it sounds like that's where I should start.
I don't think I'll degunk everything at this point, as Mom and the machine are 300 miles away. She told me I could bring the machine home with me next visit since it doesn't sew right, but she said it with such longing in her voice that I'd really like to get it fixed so she can use it for what sewing she does now, which it not much. I think I should buy a spring and take with me just in case I need it. Does this sound reasonable?