Help with a broken inverter

[analog_sa]

Let me start by admitting i am really at a dead end

This inverter used to provide high voltage for my 19" lcd while still working. I should have scrapped it straight away but now i have an entire Sunday wasted reading non-audio datasheets and generally suffering without a diagram, so there's no turning back.

It is a full bridge based on a BIT3193 driving two mosfet complementary pairs, which in turn drive two step-up transformers, primaries connected in series. The mosfets overheat and eventually die. And i can't figure why.

It looks as if the 3193 works. It has internally 1.25v on the positive input of the error amp and receives scaled down feedback signal on the negative input. The negative input is at 1.28v, which i assume means that the PWM controller works. I tried varying the feedback divider and the feedback works over a large range of values. Even if the output is several times smaller than it should be, the mosfets still overheat. I only get about 5-10s for measurements before a pair blows, so i haven't examined the driving PWM signal well but the feedback signal seems fine.

The only idea i have left is that one of the transformers has developed a partial short. Any brighter suggestions?

[guitar_joe]

sometimes you can tell by looking at the transformer. See how it's burnt.



But I've learned to stay away from these power supplies because most of the parts are proprietary one off components and you'll never find a schematic.

As far as I know once a qualified technician finds that a power supply is faulty in an LCD he simply replaces the board.

[Bootstrapper]

Originally posted by guitar_joe


As far as I know once a qualified technician finds that a power supply is faulty in an LCD he simply replaces the board.

This seems to be true, and applies to all kinds of repairing nowadays. I happen to have several measuring instruments waiting for repairing but that might never happen. I checked one company offering repairing services and according to their website it costs 100 €/hour to repair an oscilloscope! That means nearly $150 for one hour - and who said it would be fixed in just an hour?

This is insanse, I could by a better one with less cost. After all this I should be thinking of recycling and repairing stuff in order to reduce waste and such things, but it's damn difficult sometimes

[analog_sa]

Originally posted by guitar_joe



As far as I know once a qualified technician finds that a power supply is faulty in an LCD he simply replaces the board.

In this particular case the board also contains the smps for the entire monitor. It's probably unobtanium anyway. It is entirely possible to use two generic inverters instead but the brightness controll will be less elegant. On second thought it's much easier to buy a new screen, after all my hobby is not repairing screens

[guitar_joe]

Yes all the LCDs I've looked at have the whole power supply on one board.

I've got lucky a few times and got them working again.

There's nothing wrong about having a go at fixing these things I do it for a hobby. Things with microprocessors and unobtainable schematics dishearten me I must admit.