What other reasons that would cause PlayStation 1 disc not to spin

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1

Hi,

     I was wondering what are some of the causes of a

PlayStation 1 SCPH - 5501Disc assembly not to spin?

I have replaced it with another assembly, and it still will not spin. The motor for the laser will reset to track 1 if the laser is moved manually (by turning the gears), but it will not spin.

I checked the switch with a meter, and it seems to be working.

       I don't know what else to check, any ideas?

     Thanks!