Buck converter controlled by a BJT based astable multi-vibrator with zener-based feedback?
It's a poor circuit design. Q3 is an emitter follower and therefore the maximum voltage that can be fed to the output inductor is somewhat less that the DC voltage from the bridge smoothing capacitor. R1 and R2 set this voltage and maybe it's 1 or 2 volts below the Vcap voltage or maybe it's more.

Even with R1 shorted out and R2 omitted, the rise time of the multivibrator will not be that sharp and this will create significant switching losses in Q3 - these losses add to the conduction losses due to Q3 being an emitter follower.

This would mean to me that the design should be scrapped - there are too many things to put right to get an efficient switching regulator from the bones of the current circuit. 

If you decide to try and fix things up by going thru a series of modifications with the inevitable series of further questions, count me out. I've told you what is wrong with it and you should see the light.