Finally I’ve finished my old PSU’s renewal. I added overvoltage protection and a slim fan which is switched on by bimetal thermostats on the heatsinks when the temperature reaches 45°C. I bought both the slim fan and the thermostats on eBay. The overvoltage protection is a very simple circuit: when the voltage is above a preset threshold (determined by the voltage divider on the left), the tiristor opens and burns out the fuse.