Does the power supply work ok when connected to the hot-end?
Also, try disconnecting the heat bed wiring to D8 and check for any shorting between the wires (which will include the pcb heat-bed tracks) and ground- not between the 2 wires themselves.
Also, try disconnecting the heat bed wiring to D8 and check for any shorting between the wires (which will include the pcb heat-bed tracks) and ground- not between the 2 wires themselves.