This just as a note, if someone's interested...
My DVD player, a Pioneer DV-535, had problems recognizing DVDs. Some would work, some would not. Sometimes a little open-close-open-close workout would be necessary with the tray to get a disc working.
So I took it apart. Observing the lens while loading a disc, it was obvious that focus was not being obtained. The lens made two passes with the red laser and then two passes with the IR one, then gave up. There were not many adjustments, the only ones being for the laser powers (there are two). They didn't help anyway.
Fumbling around in there, I noticed that lifting the disc edge with my finger allowed the pickup to focus every time. I guess the suspension (or something) is beginning to sag and is causing this problem. I fixed it by removing the spindle motor assy (three screws) and installing it with cardboard washers, cut out of a business card. This raised the disc just enough (less than 1 mm) to allow focus to be obtained. I did this about six months ago and the player hasn't failed me since.
The player still makes the noise that other users have also been complaining about. It appears that the disc clamp doesn't quite center on the spindle and this causes the whole thing to wobble a bit. Any easy fix for this?
BR, Ilkka