29 May
2008
29 May
'08
12:56 p.m.
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.
All three transformers are fed mains from the same place: T3 has an output
voltage, Tunnamed has an output voltage, T1 has no output at all - not even
noise. The output of T's 1 and 3 feed into the 12v board; one side is OK
(from T3), the T2 side has nothing.
Most of the time, when a SMPSU blows its fuse
violently, one
of the power semiconductors, often on the mains side, has
gone dead shoirt. If you're lucky, a rectifier diode. If
you're unlucky, the chopper transistor, which has possibly
been damaged by a failure in some other component (that's why
I said 'unlucky -- if you just replace the transistor, the
new one will most likely fail again at switch-on).
In this case, a replacement fuse held, which makes me suspect
that some other component in the mains circuit has filead
open-circuit. Maybe a current sense resistor or a
surge-limiter thermistor, or...
Although I would normally agree with you, in this case I'm pretty sure the
electronics prior to the transformer are OK - witness the power coming
through T3. I can only assume that the primary winding of T1 has failed; but
I've not yet removed it from the board to test it in isolation.
Cheers,
Ade.
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