It works something like: Q1 starts to conduct and the feedback drives Q1 hard on. The current through the primary rises exponentially. This will continue until the current reaches the limit set by R5 or the transformer saturates. At this point the feedback voltage falls and Q2 turns off. The feedback now drives Q1 fully off. The cycle repeats.
So the timing for this depends on many components, and probably on the load on the supply. The L/R time constant of the primary is a big part of how long Q1 stays on.

Thanks for all your help in getting to the bottom of this.From a design perspective your explanation sounds correct. Design using 0.5 LI squared and the frequency looks after itself in terms of the inductance and power requirements

 

Its a pretty typical flyback converter, more or less a blocking oscillator.

Start with the power transistor - they often fail short all 3 ways and take out the emitter resistor. You have to test everything and search the solder side for dry joints or it'll probably just go bang again.

The reservoir electrolytic for the opto coupler supply can go bad - no regulation = saturated core and bang!

Q2 does both current and voltage control, if that circuit doesn't work; it'll go bang - if Q2 is shorted; it wont work.

Simple failure to start is probably R3 high or open. Shorted secondary side diodes can also hang it up.

Live testing is a very good chance of making it go bang - lift one end of each component and test it cold.

It's fixed. It was C7. Below from post #1:

Anyway it turns out that there was no oscillation and hence no output as C7 had reduced in value to 50u, so i removed and fitted a new one and bingo we are in biz. C7 is fitted very close to the heatsink of Q1.....too close and it dries up.

 

if the snubber is just a snubber i wonder what governs the oscillation frequency of the circuit.......maybe my old friend C7.

C7 is simply the feedback capacitor - a whole range of different things cause the operating frequency to land where it does.

That part is often a lower capacitance foil type and has a series resistor in the PSUs I've encountered.