LCD SMPS issue

Hi everyone!

This one's not audio related, but i had an issue with an old LCD screen recently.

It did shut down suddenly and then had trouble powering up again, like failing in power cycle.

I suspected a power supply issue so opened the screen, and found some burnt components on the 12V part of the PSU. (picture attached).

Seems to me that the big elctrolytic released the magic smoke and took the diode bridge with it, at least. But my concern is about the choke/transformer next to it. It looks a bit burnt too and i can't get a part number from it. By tracing the PCB, it's connected between mains AC and the diode bridge. Looks a bit small for a mains transformer though, but i don't know much about SMPS.

Is this thing a "common mode choke" a transfomer or something else?

Is there a way to measure if it did fail?

I will desolder bad components later this day and see if i can learn a bit more about this one.

Thanks for any advice / help / knowledge on this 

Thomas

It's really hard to know what other parts can have failed.

If nothing else, maybe try if you can find the entire psu pcb on e.g. aliexpress, some of them are so cheap that it dosen't make sense to repait rhe original one..

Jakob E.

That looks like mains into a common mode choke with a snubber cap and then a bridge rectifier, filter cap and then the SMPS transformer on the right edge. So the electrolytic failed short and caused the rectifier to meltdown which failed open. Try just changing the filter cap and rectifier and see what happens.

But this is all wild speculation of course. That's all you're going to get when you ask people on the Internet about something that requires a professional.

Thanks for your answers!

gyraf said:

It's really hard to know what other parts can have failed.

If nothing else, maybe try if you can find the entire psu pcb on e.g. aliexpress, some of them are so cheap that it dosen't make sense to repait rhe original one..

Jakob E.

Click to expand...

I found the exact same board on aliexpress for 12$. Not expensive but since it's an old screen with some dead areas, i was hoping to repair it for cents (second heand 19" screen can be very cheap). Otherwise, i might just salvage what i can and send it to recycle.

squarewave said:

That looks like mains into a common mode choke with a snubber cap and then a bridge rectifier, filter cap and then the SMPS transformer on the right edge. So the electrolytic failed short and caused the rectifier to meltdown which failed open. Try just changing the filter cap and rectifier and see what happens.

Click to expand...

That was exactly my guess. I desoldered components, and couldn't get a match with that common mode choke part number (SPI 8LE00081 5PG). I measured DC resistance:

0.4 ohms on each inductor and complete insulation between inductors. I can't measure inductance right now, but it seems all good to me so far.

I'll try to repare those blown parts (as well as some other electrolytics) and see what it does. That big cap is a 100uF 400V. Rectifier is a KBP206G. Total : 3€ on mouser.

I'll keep you updated

Thanks again for your help!

Thomas