Literally! just when I was in the middle of a time pressing project [or two] which had to be interrupted for a couple of days for family, my beautiful Singer 201 decided to go over to the dark side. The top thread would be come tight, usually break, and gather loops underneath the feed dogs that jammed the shuttle.
Not to be phased, I re-did everything from new thread, new bobbin, new needle, fiddled with calibrating the tension as per internet advice and finally was driven to taking apart the whole tension mechanism, which was a very brave thing to do I might add. Fortunately I had a wonderful guide in Sid’s posts from the Old Sewing Machine Blog ,link to post here ;http://oldsingersewingmachineblog.com/2012/08/16/singer-201k-top-tension-part-one/
Mission accomplished [i.e. all back together correctly] I would like to report that all is now well, sadly it sews for a little while and then repeats error. But I have other resources to try – the only thing is that is time consuming and I want to sew. I do have other machines, [four actually, two vintage Singer 99k’s, a Singer 15-91 hand cranked inherited from Mum and my Frister Rossman modern zig zag, which I only use for knit fabrics and buttonholes] but I hate to leave a job half done and nothing sews like my ladylike 201. Everyone who owns one will know what I mean, they are as smooth as cream and so reliable [I was going to say as reliable as the Bank of England, but these days………..?]. The secret, not to get too technical, is in the hook mechanism around the shuttle. The other Singers have oscillating hooks – i.e. the little thingy that hooks up the top thread for the shuttle thread to go through – goes back and forth. Smoothish – but bound to be a little jerky as it changes direction, whereas the 201 has a rotating hook, it only goes in one direction so is infinitely smoother, simples!
Meanwhile back on the sewing front, I have a white shirt half done and a lilac blouse cut out. I could sew them on one of the other machines, and might have to eventually, but I can’t abandon my 201 and her problems just yet, bless her. I’ll let you know how the patient progresses.
lifeaworkinprogress 
I feel for you! I put off, put off, put off having my machine serviced. Then it breaks down, beyond my knowledge, in the middle of a project.
But of course it couldn’t break when you weren’t using it!
I am amazed you braved taking your machine apart… even more amazed that you got it back together again (perhaps I should stop measuring other people’s capabilities against my own!). Are any parts showing signs of wear… I guess this could lead to the problem re-occurring.
No signs of any wear at all unfortunately, the problem is well hidden. Don’t think you wouldn’t be capable to doing it – if I can do it -everyone and his dog can do it, especially with Sid holding your hand [metaphorically speaking]. The thing is that Singer vintage machines are purely mechanical so should be what you see is what you get – so I will have to keep tinkering.
I did mention to my Dad that I might treat my machine to a service… as a engineer he looked at me shocked/disappointed and just said “How hard can it be? If it moves, oil it”
A bit like the old Army adage “If it moves, salute it – if it doesn’t – paint it!” But men [especially fathers and husbands] have only themselves to blame for our ignorance in all things mechanical, because they don’t believe we could understand it and *we believe them!!*