Transformer repairs

[Ade Vickers]

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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