Broken garage door opener

I have a Chamberlain 1/2 HP opener. The motor runs, but the chain does not turn.
This happened about a month after the garage door spring broke. The spring was fixed, but I'm guessing the few times we tried to open the door with a non spring loaded door, probably did some damage to the opener mechanism.
I probably wouldn't try to fix it myself. The opener is about 8 years old and pretty loud. Any suggestions?
Pay for repairs? Buy a new one?
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Replace gear assembly: http://www.garagedoorsupply.com/gear-assembly.html

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Replace the gear assembly only if you are comfortable DIY. If you need a company to do this, don't. At its age and the cost to repair, you're better off buying a new one.
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