Greg, sorry to read about the damage.
But you could check now the gate waveforms without working BS170. (Of course the gate capacity is "working".) Now there is no possible feedback from drain current to the gate circuit.
From my post before:
The voltage is >= 1V for around 70% of the period. The part of the waveform with voltage around 1 V is wrong.
It would be interesting to know if the input duty cycle of IC503 is equal to the wrong output duty cycle. But maybe without drain voltage the wrong duty cycle is not wrong anymore?
73 Ludwig
