Transformer Fault in a PSU?

Holm Tiffe holm at
Mon Jan 13 02:01:08 CST 2020

Rob Jarratt via cctalk wrote:

> The PSU in my VAXmate failed recently and I think I have found a problem
> with one of the windings on the transformer not ringing when I test it with
> a ringing tester. The technical description of the VAXmate says that one
> winding is operated in flyback mode and I wonder if that has any bearing on
> the ringing test. Also, one of the pins appears not to be connected to
> anything else, so possibly a broken (melted?) wire.
> Can anyone help with the following questions:
> Does the fact that one winding is operated in flyback mode have any bearing
> on the ringing test?
> Can a transformer fail with a bad smell with no outward sign of damage?
> Can this transformer be replaced (see picture in blog)? - I suspect not
> unless I find a donor PSU.
> Where should I look for a possible cause of the transformer to fail (if
> indeed it has failed?).
> I have posted more details here:
> u-failure/
> Thanks
> Rob

The startup of the PSU is working as this:

after switching on the mains, the voltage over C12 rises, thre is a
startup Pulse generated with C14. After the Voltage reaches 8-12V
the UC3842 is starting up, switching the Transistor.
At this point the Voltage generated in P1, rectified with D6,D7
has to take over the supply for the UC3842 for the normal run.
If that doesn't happen, the UC3842 gets out of Supply and stops.

I don't think that your schematics are correct, specially around
C12 and C14.

A Ring test for an Transformer will pass or not, there are no windings
where it would pass, when other not pass, since the windings are
magetically coupled, so losses from shorts are coupled too.

Connect the PSU with a light bulb of approx 60Watts in series to the
mains (for 110V) 100-200W for 220V. This is functioning as an cold
conductor (PTC) so that the PSU doesn't explodes when something goes

Supply the UC3842 with 12Volts from an external source trough an diode
(1N400x) (danger, life circuit!) and see what happens. (first switch
on the mains, later the 12V) Mostly the fault is on the secondary side,
bad rectifier diode or a bad cap.


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