Is this CX600W SMPS transformer toast or whats a SMPS supposed to read like?

[lordvader88]

I've worked on this broken PSU before, and this time I pulled the main transformer. The windings all read OL or 0.05 ohm on the 2ndary (I just created that). The 2 primaries are only 0.10 and 0.20ohm. I'm reading about 0.03 ohm with my Brymen869 shorted. So this is nothing like bigger transformers I've checked where the it's like +2ohm pri, +50 across sec. This is a step down trans, and pretty small, but IDK.

Using a 120VAC to 12VAC wall transformer, I'm feeding that into the pri side of the SMPS Trans , and I did a short circuit test and it drew 1.6A-ACrms , I also just used a 1k resistor into the prim, and put it on the scope across the resistor and I'm getting 40Vpp

I get nothing off the other side, looks the same on the scope without power.

How would I even know that the pri. is not fried and just acting a pure wire. The windings have me confused more than they should.


I'm avoiding the conclusion here, but am I missing something or is is dead...which means taking it apart and another big roadblock

pictures of the smaller low quality 1, but about the same
http://www.realhardtechx.com/index_archivos/Page17610.htm

the 120V to 12AC (I'm so used to AC-DC wall packs...) is 230 ohm : 2.5 ohm, and it's a bit smaller than the SMPS trans

[james_s]

SMPS transformers typically have very few turns, so the windings will read almost short circuit if you're just measuring DC Ohms. If you measure the impedance at several kHz then you will get more sensible readings. I'm not surprised a transformer designed for probably 25-50kHz is not working at all when you drive it with only 60Hz, I wouldn't expect it to.

For what it's worth, in decades of repairing stuff I don't recall ever seeing a bad SMPS transformer in anything, ever.

[lordvader88]

I'm also forgetting it's some kind of flyback trans, and I should see what my MS5803 LCR meter thinks of it

[james_s]

Flyback refers to the topology of the power supply design, not the transformer itself. It's a high frequency SMPS transformer with a ferrite core, it will need to be driven at several kHz before it starts to work properly.

[lordvader88]

Well it reads as inductors on the LCR meter, all about 440nH to 68uH, with Q factors of 20-82, and +100pF between windings.

Ok, I'm going to check more things while it's out of the way, and continuing mapping the schematic. I've checked a lot of it and replaced the CM6800 chip. I tried it with just a DVD player and I never saw a thing from the PFCout or PWMout, and not a thing on the secondaries at all. But I know the chips and layout better in the last 2 days for when I solder it back.

I haven't properly checked the output diodes out of circuit yet, I hate this super high-temp. solder, and I figured it was easier to get the trans out 1st anyways.

[vk6zgo]

Well it reads as inductors on the LCR meter, all about 440nH to 68uH, with Q factors of 20-82, and +100pF between windings.

Ok, I'm going to check more things while it's out of the way, and continuing mapping the schematic. I've checked a lot of it and replaced the CM6800 chip. I tried it with just a DVD player and I never saw a thing from the PFCout or PWMout, and not a thing on the secondaries at all. But I know the chips and layout better in the last 2 days for when I solder it back.

I haven't properly checked the output diodes out of circuit yet, I hate this super high-temp. solder, and I figured it was easier to get the trans out 1st anyways.

The LCR meter is the best thing for testing  SMPS transformers, but you really need a "good" sample to compare with.
Shorted turns are pretty much impossible to see with a DMM, as the difference in DC Resistance between a good winding & one with a shorted turn is minimal.

Years ago, I had a transformer in a Sony SMPS, part of a KX27 Profeel picture monitor.
It had been left running for weeks with a black input, & sometime in that period, the BJT transistor went short, but nobody noticed the monitor had died.

The transformer was well "cooked" but, as they are covered with black varnish, it was not obvious.
I killed a few components before another KX27 came in with an unrelated fault, so I could compare the two transformers on the LCR meter.

[IanMacdonald]

A resonance test will often identify a bad transformer. Feed in a low voltage, low frequency square wave via a capacitor, say 1nF, and 'scope the voltage across the same winding.  There should be a decaying sinewave at each transition. A shorted turn will result in very fast decay of the sinewave, or none. Obviously helps if you have a similar example to compare with, but in general a transformer that will show no resonance at all is a bad one.

[james_s]

I have a ring tester for checking TV flyback transformers that works similarly, doesn't get much use these days but comes in handy occasionally. I found it to be a fairly reliable indication but so far those HV transformers are the only high frequency transformers I've seen fail.