Bestec ATX power supply failure takes out motherboards - how?

[Frusto]

eMachines, Gateway, HP, and possibly others bought cheap Bestec brand 250W
ATX power supplies a few years ago, that are prone to failing after 1-3
years. They fail in such a way as to destroy the motherboard. If you're not
familiar with the problem, try google emachines bestec power supply failure.
Anyway, has anyone investigated how exactly these power supplies fail, to
kill the motherboard, silently and without smoke? If so, do you also know
what component(s) they commonly trash on the motherboard?

I've tested my faulty power supply (it ate two motherboards before I thought
of doing the search above) and found all the outputs were right on, under a
very light bench load. I didn't check the unit under full load on all the
outputs, or for AC potential - I suppose the main switching transformer
could have shorted the primary to the secondary, or an opto isolator might
not be isolating anymore.

 

[Baron]

eMachines, Gateway, HP, and possibly others bought cheap Bestec brand
250W ATX power supplies a few years ago, that are prone to failing
after 1-3 years. They fail in such a way as to destroy the
motherboard. If you're not familiar with the problem, try google
emachines bestec power supply failure. Anyway, has anyone investigated
how exactly these power supplies fail, to kill the motherboard,
silently and without smoke? If so, do you also know what component(s)
they commonly trash on the motherboard?

I've tested my faulty power supply (it ate two motherboards before I
thought of doing the search above) and found all the outputs were
right on, under a very light bench load. I didn't check the unit under
full load on all the outputs, or for AC potential - I suppose the main
switching transformer could have shorted the primary to the secondary,
or an opto isolator might not be isolating anymore.

In my experience the CPU power supply gets taken out, often along with
the CPU and sometimes the RAM. The other failure I have seen is the
support chip goes pop and takes out the board and sometimes the PSU.
Often you can see the hole in the plastic. You may have to take off
the heatsink, if there is one, to see it.