In addition....I just finished working on my friend's modern Brother. It was obviously a tension problem, but one I hadn't seen before. The bobbin thread was pulling to the top, but we had loops on the bottom.
I tightened the bobbin tension and cleaned the tension disks. No more pulling the bobbin thread to the top, but still had weird stitches and loops. Then I noticed the thread she had on the machine. The 50 cent stuff you buy at the box store. No consistency to the thickness of the thread, and really linty. Pulled it, replaced it with Superior, rewound the bobbin, and VIOLA! Decent stitches...not the quality of the old vintage girls, but decent. Lesson. Don't use cheap thread... you don't have to use the most expensive stuff, but that 50 cent stuff is worthless.
The best part of the story is that she had taken this machine in to be serviced because of the problem. The guy gave her a price of $119, plus parts as needed. He then offered to sell her a Riccar for much less than the service cost....probably a 70's or early 80's model. She bought it and thought she'd sell the Brother on Craig's List with the understanding that it would need service. It no longer needs serviced...so she'll get the Riccar for free and have a MUCH better machine!