[ortek_service]

Could the audible frequency oscillation be a clue that it wasn't running at the right frequency, and that led to the demise of the chopper transistor - presumably due to excess voltage / current and it didn't overheat (or internal peak power dissipation was such that it failed so quickly that it didn't have time to get hot)

It may also be worth checking any diode / capacitor / resistor snubber components (assuming transistor doesn't have built in 'efficiency' diode clamp on output).

Otherwise it's probably a good idea to check all primary-side capacitors for value / ESR (on electrolytics) and any low resistance-value e,itter current sensing resistors. And also that (Opto-coupler?) feedback is all OK (Maybe putting a temporary resistor across primary-side, to limit power / output voltage to start with)

I suppose the other suspect is unfortunately the Chopper transformer has got shorted turns / is arcing-over.

Another thought is that the audible-frequency could be due to an overload (Although may usually expect it to cycle at rather low 'ticking' frequency in that case, with repeated re-tries)