SMPS Transformer Failure

[b2theory]

Any thoughts on the mechanism that would cause the primary inductance of a ferrite core (E core) transformer to drop from 1mH to roughly 50uH?

[Zero999]

A shorted turn, either on the primary or secondary winding.

[b2theory]

A shorted turn, either on the primary or secondary winding.

Both the primary and secondary have lost equivalent inductance.

[drussell]

Both the primary and secondary have lost equivalent inductance.

Well, obviously... 

It is a transformer...  The primary and the secondary can "see" each other...

[Zero999]

Unfortunately, there's no way of telling whether the shorted turn is on the primary or secondary winding. If you know the DC resistance of the windings of a good transformer, of the same type, then you'll stand a chance: the resistance will drop due to a shorted turn, but otherwise you're out of luck.

[xavier60]

The primary winding can be damaged by high fault current caused by switching transistor failure in mains powered SMPSs. I suspect that high temperature melts the enamel coating on the wire.