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

When it comes to the hotend: first read and follow the “things to do first with a J1” topic if you did not already do that. Did you check/ add thermal paste?

If that does not help, the filament you use is probably one of those that causes issues with your retraction settings. Read the long thread about the J1 clogging issues…

The z motor thing sounds like some issue with the z endstop to me. Turn off the printer and slowly (to avoid induced currents getting too high) hand-crank the z leadscrew that the bed is close to the top. Search for the metal flap of the micro switch in the bottom plate near the z leadscrew. Does it click if you press it? If not, the switch is mechanically damaged.
If it clicks, turn the printer on, leave your hand on the power switch (!) and try to stop the bed moving down by pressing the switch with your finger. If pressing the switch does not do anything, immediately shut off the printer. Take care to not let the bed come down too far in that process, otherwise your hand might be in serious danger! Use a ruler or such to push the switch if you do not trust your reactions - the z motor and the stiff bed frame are capable of breaking bones.

When it comes to the gear: please post a close-up picture of the gear. If you mean the gear in the lid of the extruder, I suspect the DIN 6799 retaining ring that should keep the axle in place is missing there.

The unresponsive motor may have several reasons - motor defective due to blockage because of the “axle issue” you noted, or a fractured hotend flex wire. The latter can be found easily - if there is a sharp bend, it is defective.
Again, pictures might help.

Strange, this is with J1S printer? The “new version” ?