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Heatbed shows correct "ambient" temperature but won't heat up

heatbed Heated_Bed Help Octoprint thermal_runaway

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Hello There!

I think there is something fishy with my heatbed.

It shows the correct "ambient" temperature but it won't heat up / if it heats up and a pit starts at some point the Firmware toggles "thermal Runaway protection".

Octoprint shows "Heating failed, system stopped! Heater_ID: bed - Printer halted. kill() called!"

I already checked the Solderpoints on the bed itself and resoldered the blue and green wire (just to be safe), and also resoldered the Power and Groundwires.

I have checked continuity for all wires and did not find anything out of the ordinary.

does anyone have a clue what is going on?

Did you measure the voltage received by the HB? If it is 12V, then the heating element is interrupted and the HB needs to be replaced. If it does not receive the 12V, the HB is most likely OK, the problem might be in the mosfet(s).

Most likely the thermistor in the hot bed is dead, or the wires to the thermistor in the hot bed are broken.
Depending on your hot bed, you would either replace the thermistor, fix the broken wiring, or just replace the bed.

If the thermistor is bad it would not show the correct ambient temperature. It is more likely that the heating element does not have a good contact somewhere or does not receive sufficient power.

i have already ordered a new heatbed and new wires.

I don't think it is the thermistor, because it responds to temperature changes and is in the same "ballpark" as the hotend thermistor.

depending on the arrival of the wires and the heatbed I will change the wires and then, if that does not fix the issue, I will change the whole heatbed. :S

I had that too...the cables to the heatbead broke off. Had to solder them back on like 3 times until I completely changed the heatbed

Any tips on where to order a new heatbed from? I'm having similar problems, but I didn't manage to find a source for the heatbed ...

https://www.ebay.de/itm/373199539269
https://princore.de/collections/anycubic-i3-mega-1/products/hartpapier-aluminium-druckbett?variant=32099228287075

this is the combination I use on my Ai3M. The cables of the silicon heatbed are better than the anycubic ones, because they don't break so fast.
The old glass bed is changed to a cast aluminium bed and instead of the Ultrabase (foil on glass with small holes) I use a pertinax plate. Prints stick very well when the bed is hot and are easy to remove when its cold. If the adhesion diminishes, just use some rough sanding paper and you are good to go

A friend told me to get PEI with flexible magnetic printplates, but the current combo works so well that I don't want to (for now :D)

Thanks a lot! I'm actually quite happy with the Ultrabase, but if the Pertinax plate is as good or better, that sounds very promising too! :-)