Chopper transistor faulty

The chopper rarely/never goes on its own, you must find the cause,
High value resistors gone higher or O/C. Look for resistors of 47k to 330k and test resistance (you may have to lift one end to test properly). The SMP control chip may have blown its probably best to change it.
I know this may sound daft, but now check the loptr for shorts (better than 20k is ok ) from loptr collector(middle leg) to chassis, for 2 reasons, first the line may have blown the power stage and make it uneconomical to repair, or secondly, at least you can disconnect it (if shorted) to prevent more psu damage.

I usually connect a 40w250v light bulb across the ht line (after disconnecting the line stage) to test the psu under load and the half brightness glow to see if it flickers or the ht is too high
Don't forget to test the diodes on the secondary side of the chopper transformer
When the chopper transistor/chip blows it always takes out the fuse or surge limiter resistor.

After the chopper is going again , leave it run for a few minutes and switch off , and see it its hot, if it is ,there is something else wrong.
If all three legs are at same level (not zero) then look for low value resistor from emitter back to mains cap -ve pin

300v ok now
now the line stage