It's likely that, as Janey stated, it's just stuck somewhere. Start with pushing the needle bar back and forth and see if it moves easily. There are several oil spots in the "nose" of the machine. Then, take off the top of the machine and follow the path of movement from the needle bar to the spot where the cam sits and oil everything that moves (see third paragraph). Also, while you have the top off, look behind the lever to see if anything is moving when you move the lever on the front.
Also, the 401, 501, 503 and 403 are pretty much the same, so what applies to one usually applies to all. The cam stack on the 01 models is built in and just gives you more places to look for the "stuck" spot. Pretty much, the bobbin winder position on the 500 is the only change besides the looks. The 601 is pretty much the same, too.
If you have a bunch brown dried up oil in the "joints" of machine, you may have to go with clean and oil. If it's not too "brown", you may be able to get by with just oiling it a couple of times, a day or two apart. I didn't follow Janey's links, but there is a ton of information on how to get those 400 and 500 series machines moving correctly on this site. You will probably have to "refresh" the page to get anything to show up on the old posts. It's a long time glitch on QB that they haven't fixed.
Come back if you have a problem. It's unusual that anything is broken, but the "clean and oil" takes a few days, usually. It's not always instant gratification, but it usually works and is cheap, as opposed to a $100. or more shop charge.
bk
Last edited by bkay; 05-06-2019 at 06:25 AM.