Ps2 disc reading problems!

[what]

I've got a thin Ps2, and I'm considering maybe making a portable out of it at some point when I get better. The problem is, my Ps2 has difficulty reading discs, to the point that many of my games with superficial scratchs that play on my old Ps2, they will not on this one. It wont even play unscratched games by Koei. I've opened it up to see if I can tell what's wrong, but I cant.