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There are two wires coming from the 300V supply. It really feels like the black one is not connected. One reasons why we ask for pictures.what do you mean by return supply?
There is a loop from +300 to resistor, to MOSFET, back to 300V return. (negative) It feels like that loop is broken.
Or there is 150 volts of ripple on the supply.
That is why we want to see the scope picture for the +300V. Or for the Source.