My old SDR45TT seems to have reached End Of Life I think. sigh.
I've been having problems with it not wanting to move forward sometimes. You tilt the handle, hear a whirr, but it doesn't move. Tilt the handle backward, and it moves back a bit, forward and it sometimes moves, sometimes just a whirr.
I opened the hatch and watched the motor while I fiddled. When it doesn't move, and makes a whirring sound, the motor isn't actually turning. So I think the sound is just the controller noise. (no, it's not the hydraulic pump either!)
Then when it DOES move forward, I get a periodic CLUNK and you can feel the vibration.
with the hatch open, and the brake released (and unit OFF!), I turned the motor by hand. You can feel it moving the unit forward or backward, then it suddenly starts turning somewhat freely for a bit. Sounds like I broke some teeth on a gear.

THEN while I was doing that, I noticed that the plastic "tire" on the drive wheel has kind-of come off. It's about 2 or 3" longer than it should be, and the metal wheel is just rolling around in it, with it wandering around. Kind of like wearing a hoola-hoop.

Sigh, old machine, no service contract, just being used occasionally in my small shop.
I'm betting it would cost more to fix than to buy a used replacement machine.