It looks like the turntable platform is meant to be slid out from the back. It looks like there is a rail supporting the shelf into which the turntable is mounted. There might just be a clip that locks it in place. If you have any Carpenter or cabinet making friends have them take a look at it. It looks pretty straightforward from the photos. The wooden rails which support what looks like a black metal track can also be unscrewed and then the turntable shelf should just drop down.
it’s a cool looking piece of furniture. I’m always impressed with how the designers used to try to integrate audio systems into a lifestyle type of living room setting.