So to set the backdrop of this post, I was cleaning the hotend of a printer (Powerspec i3 Mini v2, which is a Wanhao i3 Mini rebadged and heated bed added) I picked up for cheap. There was some stubborn residue so I decided to warm the end. However, while it was still plugged in and warming I saw a spark while using the brass brush on the nozzle. Not sure where it actually sparked (thermistor or heater cartridge wires) but now the board constantly reads 193 °C for the hotend whether or not it is hot (or even plugged in for that matter.)
Looking around this seems to indicate either a bad thermistor or a messed up mainboard. The thermistor will read ~120k resistance at room temp and fluctuate based on temp changes (ice will cause it to go down, heat will cause it to rise), so can I take that to mean the thermistor is most likely still okay?
Doing some probing on the board, it seems the resistor that makes up the voltage divider on the thermistor circuit is out of spec (when probing it with a multimeter it reads only 470 ohms instead of 4.7k). Is this reading while in-circuit valid? Could this have been caused by a short in the thermistor? Can I build a small adapter to splice in a new 4.7k resistor? or will this just throw everything off more? If I were to build a new version of Marlin could I adjust something in there to get the temps back to something recognizable?
I figure since I got the printer for 40\$ I may as well use it as a learning experience.
Also, if anyone has the original firmware from this printer I would appreciate a copy.