Is It So Hard To Answer This Simple Question?

[TVRE]

Hi
I have put 2 posts on this forums and get no answer. I hope someone can help me out this time.

I have a Maganavox RM8341 projection TV with no standby power. I have removed the standby transformer and I need some help.

The transformer has power on the primary (5 PINS) but no power on the secondary (3 PINS).

I would to know exactly how to test this transformer.

Where do I place the probes?

If possible ,can someone give me more specs on this transformer?

[Frank]

You only can check for continuity out of the circuit, you have to get a sckematic to get pin volts,

[TVRE]

Hi
I get continuity on the primary ,BUT only with the pairs. (ON THE PRIMARY THE PAIR ON ONE SIDE OF THE GROUND GIVES CONTINUITY AND THE OTHER PAIR ON THE OTHER SIDE OF THE GROUND ALSO CONTINUITY.)
Thanks

[ross]

I'm not sure about the specifics of your transformer, but from what you say and what your pictures show, it seems that this transformer has two different primary power sources (inputs), each of which are converted to the secondary (output) power, I assume not simultaneously. Whether I'm right about that, transformers pretty much all function in the same way, so these thoughts should still apply.

I assume you're just trying to confirm that the xfmr is toast; unless the problem is in the connection to the pins themselves, the transformer windings usually can't be repaired.

You don't mention continuity on the secondary, you should also read continuity in the secondary winding. If no continuity, you have an open in the secondary and will have to replace the xfmr unless the open just happens to be at the pin; you could repair it there if you're a decent hand at soldering.

Also check to make sure you don't have continuity from either primary to the secondary, and probably you shouldn't have continuity between the two primaries (I think you may have indicated that they don't have continuity between each other). If you have continuity between either primary and the secondary, you have windings shorted together and would need to replace the xfmr. Note, because transformer primaries and secondaries are inductively coupled, connecting an ohm meter between primary and secondary windings might possibly indicate some degree of continuity, but only momentarily and it should decay to zero continuity in a couple seconds or so.

Hope this helps you out.

[TVRE]

Thank you soo much for your help.
I have tested the transformer for continuity and I get continuity on the secondary windings. I also get continuity on both primaries. I don't continuity across both separate primaries nor from the primaries to the secondaries. I tested the transformer while of the board and I get power going into the primary but get nothing at the secondary.
I guess from the test the transformer is good.
Thanks again for you help.

[torbjorn]

I think the standby power supply is a simple type of SMPS with a few watts of output?

If you have access to a proper LCR meter, then measure the inductance and loss factor of the primary winding (the one connected from the positive output of the mains rectifier bridge to the collector or drain of the switching transistor.
Compare the results with a similar transformer, for example from the standby part of a computer power supply or a SMPS mobile phone charger.

If the inductance is very low and losses high (low Q factor or high D value), then the transformer probably has some shorted turns.
Another clue of shorted turns is that you get very difficult inductance values when measuring at different frequencies (usually, it is good to test at 1 kHz and 10 kHz).

If you have no LCR meter, you could try a measurement using a tone generator and oscilloscope. The theory of such measurements can be found in any school textbook on physics or electronics.

[TVRE]

Thanks again for the help.
I will try to see what I can do. I will try to mount the transformer back and see what I get.

[Guest]

You wouldn't expect to get continuity from the primary to the secondary!