Shorted Secondary Winding In SMPS
For your information, i came across shorted primary winding in SMPS transformer but rarely on the secondary winding. So far i came across only few SMPS transformer that have bad secondary winding. If you come across any SMPS that produce lower output on all the output lines, don’t just suspect components fault in the primary section or even in the Optoisolator circuit- check also the primary winding. A partial short in the primary winding not necessary will cause the Main fuse and power FET to blow. Sometimes it will cause all the output to have lower voltage. You have to test the primary winding with a coil/flyback tester and the reading should light up 4 LEDs and above. If it has only one or two LEDs lights up then you know the transformer have a partial short. If it has no light at all then the primary winding have a severe short circuit.
So, first thing you must do when you come across any SMPS that produce low output, check on the primary winding before checking on other components.