Wood lathe problems, speed doesn't adapt?

Hi, I've been turning for about a year immediately. I'm 18 and at about 16 started getting into woodworking because my stepfather was other working out in the garage. Anyway about a week ago my stepfather brought home a lathe by Central Machinery (I know cheap..I told him not to buy it) that he found on Craigslist. His heart be in the right place but his brain wasn't. He didn't turn it on or anything before paying for it and in a minute it sits in our garage because we cant get it to work. It turns on fine but the speed doesn't seem to be to change. It's very unyielding to turn the speed knob and the v belt seems very loose when turning it to a greater speed (and it looks like its spinning at the same speed) but become tight again at the lowest speed. I'm not sure what the model number for the lathe is but its a 12X36 JET clone. Please if anyone knows about it, anything help. Thank you for your time reading all this.
Answers:
The variable size pulley may be stripped or in recent times frozen. You will need to disassemble and examine, and possibly replace the pulleys.

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