On 21.02.2016 14:46, Cursitor Doom wrote:
On Fri, 19 Feb 2016 22:44:08 +0100, Dimitrij Klingbeil wrote:

But once you have it out and disconnected, please make another
test: apply ca. 15V RMS to the 12.7V winding (to the one where you
measured 3V) instead of to the primary. And check if any isolation
looks like breaking down. Note that the 15V value contains some
compensation for the fact that the power supply uses inductors
after the rectifiers (and therefore the normal winding voltage is
higher than the normal output voltage). That would load the
transformer close to its normal condition and any breakdown should
become apparent.

I did just try this a moment ago, Dimitrij, but doing this just
flattens the output from the sig gen, I'm sorry to say. Hardly
surprising since it's a 600ohm unit and the 12.7V tappings are
0.52ohms 'apart'! To perform this test properly I'd have to adopt
the work-around suggested by another chap here who said use an audio
amp to get the current up. I may well have to do this if it comes to
it. The other problem is, my oscilloscope current probe is lacking a
termination unit so it's readings will be meaningless and I can't
use the true RMS current range on my DVM because it's probably going
to be out of its bandwidth at this frequency range.

Ok, there are other simpler ways to test windings under high voltage

See below for a simple test circuit that would be easily doable with a
few common parts and works like an IWT (impulse winding tester):

http://imgur.com/2qfjhaX

It needs a power supply (can be just a mains isolation transformer with
rectifier and capacitor) and it's intended to show the resonance
waveform on an oscilloscope at realistic rated voltage conditions.

The MOSFET (any 400 or 500 V type with less than 1 Ohm Rdson) is driven
with a square wave from a signal generator (frequency should be slow
enough to allow the cap to recharge, some 50 to 100 Hz) and discharges a
capacitor from 320 V (rectified isolated mains) into the inductor under
test. Under discharge conditions the capacitor and the inductor form a
resonant circuit and slowly "ring down".

The resistor heats up with prolonged operation, obviously, since it has
full supply voltage across it, so that's why it's rated 10W.

The waveform is measured (due to the high voltages involved) with a 400
V rated 10:1 oscilloscope probe. It should give a reasonably reliable
indication whether an inductor (or a transformer) is good for use at
full mains voltage or not.

The circuit works similarly to a commercially available IWT and it's
intended to be connected to the primary of a transformer. The waveform
should look like a typical IWT waveform (search for "impulse winding
tester" in Google Images to see what it looks like).

Here's a good looking waveform example:

http://meguro.com.my/wp-content/uploads/2013/05/Impulse-applied-chart.jpg

A shorted (or otherwise overloaded) coil will decay very fast or even
hardly resonate at all. A good one will resonate for many cycles.

A failing one with significant corona discharge may look like this:

http://www.ucetech.com.cn/en/App/Tpl...ds//day_150908
/201509081505159156.jpg

This test should be easy to do, and should be able to settle the
question if the transformer is "shot" with reasonable confidence.

As always, when working with high voltages, pay attention to safety!

Regards
Dimitrij