Belt snapped, cannot pass homing with new one
I have been using my core one for past three months with almost no issues.
I've passed all calibrations and printed some owesome stuff. Today i had two prints to fail on me, with printer 'skipping' and out of nowhere printing somewhere else.
So i have a look, try to move gantry by hand and i feel a click when moving it. Soon i find that belt is snapped in one place. 
So i replace the belt and now after tensioning, i cannot pass homing calibration, no matter what i try (i pass Y,X,Z calibrations).
Ive made a video of failed calibration:
and it seems the top belt is moving up-down, but i have no idea what is the issue.
Gantry is squered, flush with the body. Belts tension is allright. I checked it via app on the phone and with the strobing effect.
Ive checked the motors for the screw is alright and belts are not 'slipping' but everything seems ok.
Thanks for any ideas and help.
RE:
So i pulled out X axis motor to check grub screws, same on Y. Both were fine.
Now i do not pass even X,Y calibration.
When doing the manual tuning via strobing efect, both (upper and down) belts are moving.
Completly at loss, what is wrong
Prusa support did not know what to check next either.
RE: Belt snapped, cannot pass homing with new one
I see the belt in the back is doing some strange motion, like a pulley is binding or it's hanging up on something. It should move in a straight line, not deflect, nor should there be a significant change in curvature near the pulleys/motors as the forces/direction change. I wish I could be more specific, but something's not right back there. It's also very odd that a belt would fail unless some abnormal stress was being put on it.
RE: Belt snapped, cannot pass homing with new one
Hello,
I just finished assembling my Core One two days ago. The only issue I encountered was with the NFC; after re-seating the nano antenna plug, it worked fine.
I agree with @Conrad that there is something wrong with the lower left mechanics. It’s possible the M3 screw holding the pulley bent from excessive tension, which then scraped and damaged the plastic block. I advise dismounting it to check if either the plastic or the screw is damaged. Even the lower back fixing screw may have eroded the hole (since it screws directly into plastic) and is now sliding out. This would explain the belt moving upward, as the plastic box would be lifting from the back.
The issue was probably caused by too much tension initially applied to the belts, which broke the plastic and then even broke the belt 😮.
I did not rely on the Prusa app or the firmware method to tune my belts. Instead, I used my reliable mechanical belt tension gauge that I’ve mentioned here in the forum. I set the belts to a value of 2.2, a value that is commonly used on Voron builds as well as my MK4S and MK3.5 with no issues for years. I tried the PRUSA app on my new iPhone, but I got much more random values than I did on my old iPhone, so I went back to the mechanical gauge 😉.
I think this gauge is the real way to fine-tune belts without any surprises.
Regards
RE: Belt snapped, cannot pass homing with new one
Wow, that's a really cool gauge. It reminds me of similar gadgets for tensioning bicycle spokes.
IMO, the strobe utility in the new firmware works very well if used correctly. For some reason a lot of people have a desire not to use it the way it was intended, leading to all sorts of errors. I agree, the problem seems to be in the left rear. If something can be found there, I'd still keep checking, as I've been bitten by multiple problems in the past!
RE: Belt snapped, cannot pass homing with new one
Thinking more, I've changed my mind. The belt would be doing that motion if the left upper belt is being driven by the motor and there were binding anywhere else. If you disable the motors (menu) and move the print head by hand, how does it feel and what do you see in terms of belt deflection?