Re: When should a power supply be declared Perma-Dead?

["Matt J. McCullar" <mccullar@flash.net>]

I agree with Don -- it's definitely a problem with the power supply itself,
though the motherboard might still have some sort of a short (say that three
times fast) on it.

The fact that the fuse blows instantly tells me that it's probably a shorted
full-wave bridge rectifier.  This is one of the first things that the 120
VAC coming in sees once it gets past the fuse.  This gizmo is what changes
the AC into DC and it's nothing but four diodes connected together.  If even
one of these diodes shorts (and they do), POW!

There are protection diodes on the DC outputs, but if one of those shorts,
you may not get a blown fuse right away; you'll usually get a series of very
high-pitched squeaks from the power supply as it keeps trying to supply
current to the output but is continuously shutting itself down.

There are probably some services out there than can do an Apple power supply
exchange with you, but if you're out of options, I think I could fix it for
you.  I'm in Texas.  (I repaired several hundred of those things a long time
ago.)

If you've got a multimeter and know how to use it (or find a friend who
does), I recommend checking the resistance of the individual power supply
rails on the motherboard itself, with the p/s disconnected, to make sure
there isn't a short somewhere.  Perhaps two contacts inside a connector may
be touching.  If you do find a short, start pulling out peripheral cards
until the short disappears.  This will help isolate the problem.

HTH,

Matt J. McCullar