I've fixed duos that had trouble reading by adjusting the potentiometers.
Here's how I have done mine....
Removed the top half of the system shell. Remove the magnet from the disc lid.
Tape the drive closed switch so that the system thinks the drive is closed.
I use an audio CD since it is always reading and you can easily test the full range of the laser, plus the audio feedback is useful. Place the audio cd in the unit and put the magnet on top of the spindle to hold the CD in.
Once the CD is running (or at least trying to) I go about adjusting the Pots. Some might say not to adjust them while the unit is running, but I didn't destroy two systems doing it this way.
I used a small phillips head screwdriver to adjust the pots.
I started with the VR102 and VR104 pots. Those two seem to have a narrow adjustment range where the CD drive will spin at all. Once those are adjusted such that the CD is spinning and the laser is at least attempting to read, I move on to the VR101, 3 and 5. You may have to do minor tweaks to VR102 and VR104 after adjusting VR101,3 and 5 to ensure that the laser will read the entire CD. Sometimes you'll get the unit to work but only up to about halfway through the CD before the laser has trouble finding the track.
VR101 seems to help with the noise of the unit (and when its out, the laser will be very loud and audio will cut out)
VR102 has a small (maybe +/- 15 degrees) range in which the laser will read a disc at all.
VR103 seems to also help noise level, but isn't as extreme as VR101.
VR104 similar to VR102, but has a slightly higher range, and affects laser focussing on tracks further out on the disc.
VR105 seems similar to VR103 in results, but if its too far off will cause read errors.
Here is a picture of the final adjustment positions of the 5 pots in one of my duos. It seems to be different for each laser. There is one side of the pot adjusters that seems different, I use that as a reference. Any other questions, please ask.