the doides fail all on their own, especially in samsung power supplies. the -60 is a 600V transistor and the-50 is a 500V transistor, there shouldn't be any problem replacing it with the higher voltage device. the diodes should be replaced with a faster diode. if the originals were 200nS diodes, replace them with a faster diode. diodes in switching supplies usually fail from internal heat. the heat is caused by the diode having too long a reverse recovery time. the heating can be worsened by dried out electrolytic caps.