I run a Cyrus II in The bedroom. Hums like a mother****er
Mechanically or electrically? Could be a loose transformer bolt or dodgy earth somewhere.
There’s plenty of info available online on the CD6xSE modding. Basics would be swopping out the op-amps in the O/P stage for better ones (with the same pinout, because I’m lazy!). I’d be replacing electrolytics at that sort of age as well. If you can wield a soldering iron with any sort of competence it’s worth a go.
Units that won’t read discs could, as Elfin discovered, simply require laser diode cleaning, especially if you’re a schmoker 😉 The laser mech in them can go faulty, or just require lubricating if you’re lucky. The decoder that controls the laser can die, but that isn’t as common. If the unit has been moved around a bit ribbon connectors can come loose as well.
The DAD3 buttons fault could be a few things similar to above, it can’t be complicated. Does it still work by remote? Haven’t seen the internals of those but would assume direct IR into the PIC with all buttons having a separate control line. If none of them work it could be the common supply rail on the display board or just a loose connector. Take it apart and have a look! Just unplug it first 😉