If someone compared the PCB with the schematic you might have noticed that they have little in common, only the chunky PSU parts match it but I don't have any ICs on mine.
Anyway, I poked and prodded at some point it worked, so I thought it's a cold solder joint. After reflowing the solder joints on the PCB it went back to its previous behaviour.
So I poked and prodded outside of the PCB area and found out that there is some intermittent in the Amp meter movement.
I don't know what exactly it is, but when I push the plastic window it goes back to 0 and I can use the PSU. The light bulb now lights up.
Any ideas what could cause this? I'll probably take the Amp meter apart... is there anything I could substitute it with in case it breaks entirely?