Hey all, my prusa mk3s (mmu2s) blew a fuse the other day mid print. I replaced the fuse and it instantly blew again. The heat bed and other wires seem ok, although one of the psu wires that connect to the einsy board looked like this with a little charing. Any suggestions on how to proceed with figuring out and narrowing down the problem?
Which fuse is blowing? This page should give you an idea of which fuse protects which part of the printer: https://help.prusa3d.com/en/article/blown-fuse-mk3-mk3s-mk3s_1925
If it's the extruder or bed heater fuse that is blowing, this page should help you troubleshoot them if you have a reasonably accurate multimeter: https://help.prusa3d.com/en/article/multimeter-usage_2117 I wouldn't bother trying to measure voltages at this point, only resistance. They should be non-zero, and close to the numbers on the Prusa knowledgebase.
The fuse that is for sure blown is the glass fuse right inside the PSU above the power cord plug.
That's worse... If it's blowing that fuse, the power supply would've been pulling a lot more power from the wall than it should be, and far more than its rated for. There's probably a very good chance the PSU is done, and maybe the Einsy, too. Might want to contact Prusa support for this one, especially if you're within the warranty period.
I would agree with contacting support, however.. (Ignore me if this is below your experience level)
Are you sure you did not have a slack connection somewhere, this could certainly cause a fuse to blow if connections were loose, this generates heat, which results in charred connections, which pulls more current and blows the fuse?
Also, are you also sure you replaced the glass fuse with the correct one, did you check the code etched onto the metal part of the fuse and replace with EXACTLY the same fuse? There are many many ratings of Fast and Slow blow glass fuse.
Unfortunately I am out of the warranty period, but I will likely have to reach out to Prusa if I cant diagnose it myself... I checked the fuses on the einsy board, and they seem fine. Additionally, the resistance and the continuity seem ok on the hotend/heatbed and thermistor seem ok. I'm not sure how to determine if the PSU is shot...