I just put in a new chpper transistor and bridge rectifier. The PSU powered on, whining and buzzing for a couple of seconds before the fuse blew again. The new chopper has shorted.
The three associated transistors on the primary side are very important, two, acting as a latch limit the chopper's base current and the other the emitter current, so if they are not working the chopper transistor can be destroyed. Also, any substitute part for the chopper must have suitable collector voltage ratings or it will fail immediately, which is often why for smps supplies it pays to find the exact transistor or spend time selecting a replacement with better ratings, but sometimes that can be tricky due the the many other properties of the transistor.
One solution, is to replace all the primary side semiconductors in one go with exact parts (check all R's and C's there too. If it still fails after that, you could be confident the fault is on the secondary side, though that is less likely.
When you powered it was it loaded ? If not its possible that voltage overshoot might have caused the SCR crowbar circuit, on the secondary, to deploy.
I would suggest, order the exact three small transistors and the chopper transistor. Fit them at the same time (you have checked the diodes).And order a few extras.
Load the 5V supply output with something like a 6V 21W auto lamp. Or you could buy a power resistor to load it, but the lamp gives a visual indication and has a low initial (cold) resistance which will be helpful at start up.
And try the supply again. Make sure to fit the new small transistors with the correct orientation.
If it still fails then its onto the secondary side for fault finding.