Z-rod/motor vibrating loudly / left extruder doesn't work / hobbed gear came out

Endless problems since I got the J1, and I’ve been 3D printing for many, many years with various low-end printers, with very few problems.

currently, something is wrong with the z-axis motor (or controller?). when I turn the printer on, the z-rod vibrates loudly and doesn’t stop. the build plate is at the lowest position. the printer is completely unusable because of this. this just started out of nowhere - I hadn’t changed anything.

something is also wrong with the left extruder. the extruder motor doesn’t turn at all when instructed, such as when I’m printing or loading/unloading filament. the hot end heats up just fine. bad wire/connection somewhere?

the left extruder hobbed gear keeps coming loose - the axle has already fallen out of the support holes twice. I took it apart and pressed the pin back into place both times and it worked for a while, and then fell out again. I think that whole piece w/gear is defective and needs to be replaced.

The right extruder is suddenly getting terrible nozzle clogs randomly while printing very simple models, using Cura settings that I have used many times in the past with no problems. So, I guarantee my settings are not the problem. The clogs are bad enough I have to take the hot end assembly out, remove the nozzle, and burn the filament out from the top, since it’s melted within the cold end. I haven’t noticed the cold end cooling fan not running, so I’m not sure what is going on.

Any suggestions for what I should try? This thing has so many problems I should have sent it back right away. Tons of other/smaller issues I’m having as well, which I guess I will post separately.

The maximum speed of the Z-axis is 10mm/s, otherwise it will vibrate