Why does my system not play discs, even with new optical lens?

My PS1 (original version, not PSOne) recently stopped playing discs. I place the disc in and hear the disc spinning, but the game never loads. I have already bought a replacement optical lens, but it did not solve my problem.

I had a similar problem a few years ago, but a replacement lens solved the problem then; this time the problem seems to be with something other than the lens.

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Is the laser moving along the track when it is reading?

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@andrewsawesome it doesn't seem to be, but I've moved the lens over to a second PS1 that I've got my hands on, and the games play fine. In my main one, I get the Sony logo screen then it goes straight to the dashboard, but I can still hear the disc spinning

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@willhough Have you checked to make sure that none of the plastic gears in the optical drive assembly are stripped?

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@andrewsawesome I've had a quick look and they all look fine, but as I said, I have moved the whole drive into a second system where it worked fine, so I don't see how the drive could be the problem?

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