PS2 Slim (77001) disc motor not working

I have a PS2 Slim (77001 series) in which an inserted disk does not spin. I originally thought it was a laser problem, so I bought a "new" laser/motor assembly and installed it, but the problem persists. I’ve tried swapping the original laser with the new motor assembly, but that didn’t fix the problem. Based on a random Google search, I also tried inserting a music CD but it also did not spin.

The laser does move back and forth and the lens focuses (moves up and down) as would be expected, so that motor is working and the unit is responding to sensors.

Any suggestions on how to approach fixing this problem? I realize this is a very old machine and it may simply be beyond repair, but if I can hack a fix, I’m willing to try.