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CHECK before printing - Prusa Mk2 - Bed Thermal Runaway (Bed heater wires)

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  • CHECK before printing - Prusa Mk2 - Bed Thermal Runaway (Bed heater wires)

    the Prusa is down again. It was 93% into a print when it auto aborted with the above error.
    Looks like the screw for the strain relief on the power harness for the bed fell out and the wires broke. I am too frustrated with this to bother repairing it tonight so I have put it in the logbook for the hackroom

  • #2
    Thanks John. The wiring on the bed gets a lot of strain. Not the first time it's broken, and I'm sure it won't be the last.


    • #3
      Almost time for a drag chain? I plan on using a drag chain for my 3d printer.


      • #4
        Repaired the bed this evening and completed the writeup in the logbook. A print is running right now.
        I had to restrip the wiring and clean the pads on the bed as a previous repair looks to of used an iron not hot enough. The blue Weller typically cannot handle large traces and planes but the red Weller if you are careful about it will work even though it thermally runs away when you use it.


        • #5
          Lasted about a month before failing again.
          Came in last night and found the printer was "not working". This time the top strain relief had broken around the screw, the zip straps had all broken and the wiring to the heating bed had again broken free. This might have something to do with adhesion problems that reportedly started the day before - https://community.kamloopsmakerspace...esion-problems. We can now use the other 3D printer to print a replacement strain relief but what's going to stop it form failing again? I can only resolder the cabling back to the bed one or two more times before the heater bed wiring becomes too short.


          • #6
            Thank you so much MIPS for finding this problem and fixing it. I had heard rumours of the printer not working but didn't have the time to ask around.
            To clarify, this isn't related to the bed adhesion problems, that was on the Mk3, this is the Mk2 that is in the enclosure.

            For whoever first encountered the problem: THIS IS WHAT THE TOOL BOARDS AND LOG BOOK IS FOR.
            We could have had a very fast turnaround on this if it was properly reported.

            I'll try and come up with some new kind of mount for the cable that doesn't snap off.

            For now I am taking the Mk2 printer offline while 2 new clips print on the Mk3.
            I'll replace the clips in the morning.
            Last edited by Garret H; 01-31-2019, 10:41 PM.


            • Garret H
              Garret H commented
              Editing a comment
              Clip is back in, thanks Hunter

          • #7
            To clarify, this isn't related to the bed adhesion problems, that was on the Mk3, this is the Mk2 that is in the enclosure.
            I am SO not used to the space having more than one working printer at a time. Thanks for the heads up.


            • #8
              The clip broke again. This time it broke at right at the clamping nut. The nut was completely seized up, and DarrenAndSherry and I stripped the head in trying to get it loose.
              Finally I managed to use the soldering iron to melt the PLA from the old clip away to get enough room to use sidecutters to shear the old bolt off.

              As I was mid-victory lap and trying to actually use the printer, it wouldn't come up to bed temp and we are getting a bed pre-heat error.
              I double checked and hot soldered the main power leads to the pad, so those connections at least are good.
              Its getting warm, probably warmer than it is reading. I suspect the connection on the bed temperature isn't quite right.

              Regardless my 20 minute visit to the space has become 1 hour and 20 minutes and the printer still has issues. I am unable to spend the time on the electrical work today.


              • Garret H
                Garret H commented
                Editing a comment
                Also I think the cable clips should be printed in a high temp material. If printing ABS he print bed runs pretty close to the point PLA starts to really soften up and that's no bueno.

            • #9
              I quickly glanced at it when I got in and I noticed that the LED and a resistor that sat between the pads have gone missing. They must of been broken off when the stripped bolt was removed.
              The bed preheat error is because there is an internal break in the sensor wire. The entire harness to the bed needs to be repaired as the heater wires are getting too short and with the internal break being almost anywhere it might be better to just replace the entire flexed length.


              • #10
                Ugh, keeping up on maintenance with the amount of printing is getting to be a chore.

                I agree replacing the wires would be the best and fastest route and add some slack to the whole system. I really think a better cable route should be used.

                Trying to find replacement LED's, there is a GitHub for Prusa here:

                They only list a magnetic bed, which for the 12V is this folder:
                I can find a 12V wiring schematic, see the attached PDF. I doubt much would change here anyway.
                The LED and resistor are in series on a parallel circuit, so I can't see any harm in running without them.
                Attached Files


                • Garret H
                  Garret H commented
                  Editing a comment
                  An easy check to confirm that the sensor is the issue before getting to pulling wires is simply to put a hair dryer on the bed and confirm that the temp comes up on the LED.  A hot coffee cup would probably have the same effect.

              • #11
                I dismantled the printer today to inspect and found that the wire for the thermal probe had actually been cut partway through. This is why we were getting weird thermal faults.
                I spliced the thermal control wiring back together, replaced the heater bed wire with new cable so it's the proper length again and attached a new clamp with washer so that it is less likely to break in the same place again. I also replaced the resistor and substituted in a new LED.
                Everything seems to be good now however the SD card is missing so I could not run a test print. Going forward, PLEASE be much more careful when trying to remove damaged fasteners.

                Additionally, we have a casualty. I broke the nice red handled side snips. We're going to have to buy more.
                Last edited by MIPS; 02-16-2019, 03:20 PM.


                • #12
                  John, on behalf of myself and all the other makers THANK YOU SO MUCH for your ongoing repair work that keeps the tools at the space running!

                  I apologize for my savagery in trying to get that fastener out...


                  • #13
                    Was in tonight to do something else quickly and noticed the nut and bolt had again fallen off but no damaged had occurred yet. Tightened it back together but we might wanna keep an eye on that.


                    • #14
                      Came in this morning to find someones failed print and ye olde "bed thermal runaway" error.

                      The clamp had came off and the wire was clearly broken. I soldered on an extension to said wire and it's back up and running.

                      Please check the bolt torque before printing - something I myself need to do more often.