How can I fix my Tevion DVD recorder Model number MD80857?

I have a Tevion DVD recorder model number MD80857, I have tried to get a service manual with no success, the DVD will not read or record blank Rw+DVD's,it just keeps saying bad disc. I suspect the laser needs replacing as the hard drive and everything else is working. I do not know the part number of the laser or an equivalent part which you could recommend as I cannot find any information regarding this model. I would appreciate it if you could help me.