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LeChuck71
uha bad news ;(
but how could I shorten the thermistor input to12V?
I don't know without seeing your bed and its wiring, but it is the only reason I know of that causes analogue inputs to get stuck at a mid range value.
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LeChuck71
I have a the E3D fan connected to the 12V power supply. The additional cooling fan for the PLA was added after the symptoms occured.
How can I "repair" this? Replace the arduino board or the ramps? can i replace any part on the boards?
If you have the skills to replace surface mount chips then you need to replace the ATMega on the Arduino board, otherwise the whole Arduino.
Alternatively if you have a spare thermistor input pin you can reassign that in the firmware.
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LeChuck71
Sorry for the many questions, I just want my bed heating up again
ah and probably the most important one, how can I prevent this from happening again?
Hard to say without knowing what happened. Connecting 12V to any of the 5V I/O pins will cause damage. Normally the problem happens at the hot end due to poor insulation or broken wires due to motion.