Dan, a high probability for a broken controller. Do you have a sharp tip for your DMM to probe voltage at a pin of the controller?
Driving 300 mA you may check voltage drop from Vdd connection to the mainbord to each of the five Vdd pins of the STM32. If high current is flowing into the STM32 you should see a drop of much more than 10 mV. Last proof would be to remove the STM32 and check resistance Vdd - GND.
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73 Ludwig
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