When the unit is saying "Checking discs" and neither goes further nor turns off (just says "Off" but doesn't), the reason is still the main belt (accessible from the top as soon as you lift the cover) that is too loose.
When the belt is way too loose, the unit doesn't move at all, if the belt is little too loose, the player-unit will move a bit, but there is not enough torque on the gears to reach the end position (where the player unit is secured against movements).