On Sunday, April 5, 2015 at 5:18:23 AM UTC-7, Cursitor Doom wrote:
Hi all,

Well, having established that the mains filter had fused and having cut
it out of circuit, I was advised to reconnect the supply ...I did this through the
variac ...and now
something else has gone phut. No smoke, but a distinct sizzling noise
that lasted about 5 seconds before petering out.

Undervoltage on a SMPS causes high currents, so don't try to run it from
a Variac at low voltage.

It's easy to see (black soot and melted metal) or sniff out (acrid smoky aroma)
a failed component in this scenario. Pay attention to the pass transistor and
the bridge rectifier, those frequently fail short (and a mains filter rarely fails at all).
Also look for fusible resistors (they're marked like resistors, but act as fuses).
If the pass transistor has failed (open or short), also check its base drive for
damaged components.