Good evening OP10NO4, your problem is in the primary, I recommend you desolder the 4 rectifier diodes or the rectifier bridge, remember that the diodes lead in one direction, place the tester in testing diodes and place red tip at the end where the gray strip and black tip at the end where the gray strip is, if the diode is good it should mark between 500 and 600 ohms and if you put the tips upside down it should not measure anything, so you do with the 4 diodes. If they measure both ways there is your short, change them, change them all for safety. Also check the transistors that are in the heatsink that look like mosfet, they are usually NPN transistors, desoldalos and check them look for the code, the left to right pin is the base, collector center, and the third issuer, in any case check the configuration in the data sheet. As they are two internal diodes, it must measure from base to collector, and from base to emitter, placing red tip in base and black tip in collector and issuer, if do the opposite should not measure anything and the transistor is good, if you measure the transistor is bad, change them too. As for the standby mosfet also check it, check zener diodes, resistors that may be open, the capacitors.