I accidentally let the print head of my Anet A8 crash into one of 2 clips that are holding a glass sheet onto the hot bed, and suddenly the screen went blank and the printer rebooted, then it started reading 739°C from the extruder thermistor. I tried changing the thermistor, but that didn't have any effect. Also tried swapping the extruder thermistor with the hot bed thermistor, but there was no effect on both. I checked both the connector and the socket of the thermistor for shorts but found none. Any ideas about the reason this is happening?
-Using Skynet3D 2.3.2 on the stock Anet board.
I tried burning the bootloader and flashing the firmware onto the Anet V1.0 board using an Arduino UNO as an ISP, but that had absolutely no effect.
R41 located next to the hotend thermistor header and found out it has a resistance of 1.5kΩ, while it should have a resistance of 4.7kΩ, so I suspect this is the main reason behind this high reading. Now the only thing left to figure out is how the resistance of this resistor changed.
I was able to figure out which resistor to measure with the help of this schematic: https://github.com/ralf-e/ANET-3D-Board-V1.0/blob/master/ANET3D_Board_Schematic.png
EDIT 3 I tested
T56 (located near the headers) and
T55 (located near the ATMEGA1284P) for continuity, and found out there's no connection between those, while they should be connected according to the schematics. I also checked the hotbed's terminals
T53 and found continuity between them, which means the problem might be in the trace between the thermistor header and input pin of the ATMEGA chip (this trace is VERY thin, so any overcurrent might cut it), or any component in this trace.
and found out it has a resistance of 1.5kΩ, while it should have a resistance of 4.7kΩ, so I suspect this is the main reason behind this high reading. Now the only thing left to figure out is how the resistance of this resistor changed.
You can't measure the resistance of a resistor in circuit - the resistance probably appears to be lower to your multimeter because of some other circuit elements. There's also no reasonable explanation for how a 4.7k resistor could suddenly turn into a 1.5k one. It's highly unlikely this resistor is the cause of your issues.
It is more likely something else is damaged, likely the AtMega1284p microcontroller itself. When your extruder touched the bed clip, perhaps the 12V from the bed shorted through the clip and to the extruder? I would guess that the 12V shorted itself to the thermistor input, which subsequently blew the ESD protection diode on that input. This might explain the high reading and the low apparent resistance of R41.