This involves deadly high voltage. Do not try at home.
If it were me, I would remove the power supply and bench test it.
I would need to connect a dummy load in place of the laser tube. I use a dozen 50Kohm 50W resistors connected in series:
Then, I need to fire up the power supply. To do that, I make some connections.
I first fire up the low voltage supplies by connecting 3.3V (pin 3,4 or 5) to pins 17 and 24.
Then I ready the high voltage supply by connecting 3.3V (pin 3,4 or 5) to pins 16 and 13.
Then I turn on the high voltage supply by connecting 3.3V (pin 3,4 or 5) to pin 12.
Using Ohm’s law. I calculate what the voltage should be at the point between the two resistors closest to HV return wire. I measure the voltage there to see if it matches what I expect.