>
> Tony Duell wrote:
>
> > Hang on a second...
> >
> > Why do yuo think the transformer is the problem?
> >
> > This sounds like a switch-mode PSU. And while
transformers
> > can and do fail in those, it's not at all easy to
diagnose
> > the fact. They normally have windings witha very
low DC
> > resistance and will test as a dead short on any
nornaml multimeter.
>
> OK, the reason I suspect a transformer; there's power
getting as far as the
> primary windings of the transformer (can be seen on an
oscilloscope); and it
> seems to conform to the "chopped" DC that I
now know a switcher expects.
And how exactly are you measuring that? Remember a voltmeter
(or in this
case a 'scope) has 2 leads, and measures the difference in
votlage
between them, What are you conencing the ground side of the
'scope to?
My guess is the system 0V rail. But the chopper circuit is
directly
connected to the maisn, its 'common' is the -ve side of the
mains
smoothing capacitors. But of course if you connect the
ground lead of the
'scope there, you'll flow fuses if you're lucky, nad do a
lot of damage
if you're not, becaue the 'scope is earthed, as is one side
of the
incoming mains (effectively). You can't probe around the
primary side of
a SMPSU unless either you have a 'scope with differential
inputs and
enough common-mode rejection to handle full mains voltage or
you run the
PSU off an isolating transformer.
What frequency is this pulsing DC that you're seeing? If
it's really the
chopper waveform, it'll be 10's of kHz. My guess is it's
nearer 50Hz, and
whay you're seeing is effecively rectified mains due to the
fact you've
got the 'scope common connected to ground.
-tony
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