(Sorry my German is a little rusty) Powering up an unknown DCC900 is not a good idea. It's about 100% certain that, if it's not revised, it has leaky capacitors on the read/write circuit board; the leaked fluid damages the board and may cause short circuits. And of course it will need new belts. You can get new belts and a brand new read/write board from the DCC Museum. We will need to remove the custom IC's from the old read/write board. The DCC 900 can usually be repaired to fully working status. Good luck!
Hello Jac, I know everything you write because I repaired hundreds of first-gen DCC recorders since 2016. Anyway I can explain you my point of view concerning this purchase on Facebook. We know each other very well.