Sony DVP-NC600 Multi-disc DVD Player No Disc Error

The only other thing I can suggest, having repaired many Sony CD/DVD players, is to get a can of compressed air and try to blow any dust out from the laser assembly. Since you've already cleaned the lens (the most common problem), you also might have dust down inside the assembly. Try blowing with the air nozzle down inside, around the gap between the lens and the cover surrounding it. Do it from at least a couple of inches away, if the air is too powerful you can damage the wiring inside the assembly.

If this fails to correct the problem, your laser output may have diminished over time and the only repair is to install a new laser assembly. The cost of this part will most likely not be worth it to you since the unit is quite old now.