Melzi reporting spurious temperatures

I posted this in another thread here but figured it might be better in an electronics thread...

It's a Melzi board.

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I got a blocked nozzle soon after my first test prints and nearly two weeks later still haven't printed anything else

I then realised I was putting the PTFE inner on the hotend upside down and causing continual blockages by doing so.

It was just at that time that I broke the lead to the thermistor so ordered a few from RS, part no: 484-0183 - they're smaller than the ones James sent but it seemed to be working ok at first however, at some point, whether it was when the hotend hit a temperature or if I moved something about I don't know, but the pronterface screen showed it was disconnected from the printer. I've noticed this happen before when the power resistor shorted against the brass block, normally reset the power and reconnect and it's fine but this time it wouldn't reconnect. I started disconnecting wires and then at some point I managed to reconnect.

Anyway now I'm getting a weird result - when I connect and check the temperature, the head is showing 249 before I even start the heater. The really weird thing is that the bed thermistor, which I haven't changed, is showing 194 also before applying any current. (These numbers vary +/- 2 over time).

The only thing I can think to do is to strip the head down again and recheck the resistor and thermistor connections but I'm really confused about how the bed temperature is screwy too. Just wondering if anyone had any thoughts.
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I took both thermistors out of circuit and measured them and they are both showing about 100kohm at room temperature.

The electronics are still reporting similar numbers even with them disconnected.

You have damaged the processor by allowing 12V from the heater to get to the thermistor input. Normally it only affects one pin and you can use another but if both thermistors don't read correctly it might have destroyed the ADC, in which case you needed to replace the CPU I'm afraid.

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[www.hydraraptor.blogspot.com]

oh dear. Thanks for the info nophead.