Does the mini bed temp control has ‘upscale burnout’? I am getting a thermal run-away alarm from OctoPrint, so I’m guessing that either the thermistor or wires to it are open.
The bed heater and bed thermistor wires commonly break on the Mini. I would start there.
I checked the continuity between the connector on the circuit board and the thermistor connector and could to get it to go open and I also measured the resistance of the thermistor which was around 137 ohms and did not move even when I moved the bed around. I’m not sure what the resistance should be at around 70 deg (20 C).
Today I cleaned all of the connections and reassembled everything and the minin is happily printing as normal. I can only assume that one of the thermistor wiring connections was momentarily going open due to vibration. The base temperature is now drawing a perfectly straight line where just recently it had begun to overshoot its set point slightly.
While troubleshooting my bed heating problems I notice a cable on the electronics board which was not properly connected.
I have tried to identify where it connects in the attached picture. I see no reference to it in any of the pictures on the website. Anyone know what it’s for?
I’m guessing those are endstop wires, blue and black by chance? You can see where to plug those in on Step 5 here, but IIRC they don’t actually do anything. A vestige of earlier prototypes, I believe.
I can only assume that one of the thermistor wiring connections was momentarily going open due to vibration.
Ehhh I’d be careful - thermal runaway on the heat bed is a moderate concern, and as @nopick mentioned, Mini 1.0-1.03 experienced this with higher frequency. At the least, make sure you have the latest firmware installed, as that includes better thermal safety features. It should shut down the print if it detects thermal runaway instead of heating the bed ad infinitum.
When I open the wires to the thermistor right away I get a bed temperature control alarm and the heater shuts down so I believe that the software has ‘upscale burnout’ implemented.
It’s interesting that wires which where poorly terminated would remain once they had become redundant, but as you say, those are the ones I was referring to.
Thanks for the input.