DIY Bench Power Supply (Really Easy and Cheap)

I have that similar issue with my converted computer power supply. I replace the fan Thinking by upgrading it, it will perform better but instead I got the same result the only thing I really did instead i have upgraded it by adding a plug-in connector, It might as well not even have a fan in there at all. So what I have discovered by doing a experiment with the fan it was running off of a 5 volt line making it perform like that. So what I done is by hooking it up that same fan up to a 12 volt line and after that I got the results that I want it. You should try hooking up your fan up to a 12 volt line in your power supply.

how can i add a main switch? like the one at the back?

I made it myself thanks to INPLIX website

And from what I saw you also failed to set your meter on 20 V.