If you are having trouble diagnosing your vintage Singer machine, there is no better resource on the web than the Yahoo Vintage Singer Forum.
8/2018: Update on new location
You can request membership (done to limit spam) from this page, which sometimes takes a few days.
Protip: search the archives before asking a question. Many topics come up again and again, and long-time members of this group can be testy about repeating easy to find information.
Needlebar.org contains endless archives of information on machines from the 1800s- forward to obscure makes and European models.
Hard to find parts and common replacements (at fair prices) can be found at Sew Classic, along with more free downloads and knowledgeable advice at the blog attached to the online store.