laserdisc unit plays cd, but not LD

Archived from groups: alt.video.laserdisc (More info?)

Hello, I recently bought a LD player with a stack of discs, with
assurances that the player works. Surprise, it does not. The unit is a
Sony mdp-333, and yes I realize that this is a rather shoddy LD player.
When I insert a CD, it plays perfectly, but when I insert a LD, I can
hear the laser making little "rit rit" sounds as it moves and adjusts,
but after a few seconds I get a "No disc" message. Does anyone have any
clue what is going on, and how I might go about fixing it?
Thanks