Re: Hot end temperature variation when bed is also on

I would be looking at the heated bed and it's thermistor.

Specifically, check for any resistance at all between the heated bed power wiring and the heated bed thermistor wiring, even in the high mega-ohm ranges.

If there's some leakage resistance between them, then voltage fed into the heated bed power circuit will leak into the heated bed thermistor circuit, throwing off the readings. If the voltage gets high enough, this can actually fry your Arduino analogue inputs (the ones used for the thermistors).

I would also make sure that the wires to the thermistor are twisted together to reduce voltages being induced from the heated bed wiring. You can also do this with the heated bed power wiring as well.

Note: This can happen if the thermistor is glued to the bed, and somehow the glue is touching a PCB track and one of the pins on the thermistor. Many high temp glues conduct electricity, and it's very easy with many thermistors to get glue on at least one leg. Even a few mega-ohms is enough to cause issues with the thermistor circuit, especially if 12V is involved.

PS: The same issue is true of the hot end, and is why you need to be careful with anything even remotely conductive around the thermistor circuits.