Newest Member to the Failure 20 Club

Hi All
Been through all the board cannot see why this is occuring.
Using IRF9Z24N from Farnell
Transformer is in correctly (Pri to Output Jacks, or “Backwards”)
R35 & R36 are 200k
using LTC1661 DAC

Can provide scope shots.
How can I measure Vth of these IRF9Z24N devices ( My Analog theory is not what it used to be )

Regards
SF