Check the secondary transformer outputs with a simple ohm meter (at first), getting nothing is normal on an output with a shorted diode. But not a OPEN diode, just a short/closed one.
If that doesn’t work, you need an instrument that measures/shows basic induction (like a curve tracer, or a ring tester, or an LCR meter). To retest. Also I’d you find an odd reading coil, remove its immediately connect component (often a diode leg) to disconnect it from the circuit to retest. The diodes do a partial disconnect in the correct polarity, so using that direction with your meter you can get something. A short doesn’t have that disconnect so to check the winding, you’ll need to remove the short, then retest.
Also make sure you drain the large caps before all this.
Another two great tests are, with PSU unplugged, shove the ohm meter into the dc side of the connector (using the proper dc ground pins) and see what you see at each voltage pin group does. If you see very low and 0 ohms, that dc rail is shorted on the output stage.
second trick, if you have a nice DC bench current-regulated PSU, set a ideal dc voltage for each rail (in turn, one at a time) and limit your current to say 300mA to start, and shove that back into the correct DC section (3v for 3.3, 12v for 12v, etc) out (using correct polarity) and see what the draw is. Don’t do this for too long. If nothing is taken, that dc rail isn’t shorted. If it takes most of or all the power, quickly remove it. That rail is shorted. Now you’d know where you start, great trick as it detect failed secondary diodes too.
But the more I think about your description, the more I think you may have a failed, main transformer, due to shorted secondary winding. I hope I’m wrong, as you’re not be finding that part easily, but at least it’s not an IMPACT PSU, you’d have zero chance of finding that transformer.