A customer brought this PSU (Power Supply Unit) from his PC. The PSU was made in China, cheap, no name model and the PSU took out from the Pentium 4 PC. Customer said he heard loud noise from the PC when he was working on it then PC shut down itself.
After removing the PSU cover it became clear where the noise came from. Two chopper transistors were blown up and one of the high voltage capacitors was bulged and leaked.
First I removed the radiator with power transistors remaining because they were split into separate pieces but stand-by circuit MOSFET transistor 2N60 was whole and OK.
Then I started looking for any other damaged components like burnt resistors or diodes, but surprisingly found nothing, even the fuse was not blown.
Two blown transistors were D304X, I replaced them with E13007. I also replaced both high voltage capacitors even though one was visually normal. Chances are high that if one goes bad maybe the other is also get affected.
Before testing it I replaced the fuse with 100Wt light bulb for safety measures and plugged the PSU into AC outlet. The light glowed and dimmed instantly as it indicates there is no short in primary side of PSU. I measured VSB and PS_ON pins (violet and green wires on ATX socket) and they both were normal +5V. Then plugged old heavy hard disk drive as a load and shorted PS_ON with GND. (green wire with one of black wires).
PSU started and worked normally for an hour. I turned off the unit and checked for excessive heat on PCB but it was normal. I re-soldered the fuse, assembled it back, turned on with load, and measured output voltages for noise ripples by probe. All voltages within range and no noise ripples. Plugged into old P4 test motherboard with separate sound and display cards, kept it turned on for a day, everything worked fine. That was an easy case of ATX power supply repair.