My PC and printer PSU are plugged into the same extension cord, they certainly both have a ground connection. And this is actually the issue, because the resistance of the ground connections is low enough to have a significant fraction of the heater current coming back into the PSU negative terminal. And you can't really prevent that, it's just Ohm's law. Better ground (lower resistance) will make my problems worse, as more of the returning current will take the ground path instead of the direct path back to the PSU.
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I'll try the resistor, thanks for the suggestion.
I'll try the resistor, thanks for the suggestion.