you may have the laser pickup head bad, over the time, sony had theses problem on lots of cd players, the lens suspension tend to fall down in the laser head assembly, making it harder to move to an zero position before doing the focus ok (meaning checking if an cd is in there), you cant adjust or compensate this default. You may have dust in the lens assembly, or the laser led has gone weak with time.
The cdp-30 is an old model, very old, the best players around where the cdp-291 cdp-292 models, cheap but easy to repair
The laser part begin with : KSS .... in some case you can have the laser with the mechanical kit assembly.
I would not loose time on this ... trash it
I have repaired lots of cd players before, a new laser alone may go around 40$ usd and the cd player must be re-adjusted calibrated by professionals after the change.