Hello,
It's true, a spring is missing and I think this is a design mistake: it is indeed present on CDX players.
As for the question mark, I solved this problem by resoldering a few dry joints on the PCB (in the servo area), you have to inspect it with a slide magnifying glass.
Also I had a bad PSU relay (the one that links AC to the main PCB). My CD3300 even reads CD-RW, provided they were correctly (slowly!) burnt.
It sounds very good too, because the OP amps are NOT NE5532 chips, but better ones!
A few pics (thanks to Ville Laustela) and a video (the proof of the pudding) to show how this can be fixed. The spring you can see is the one from the CDX (checked on mine).