Panasonic Viera / 17IPS20 / Backlight or Driver?

[essele]

Hi,

I’ve just got hold of a Panasonic Viera 49” screen that seems to have a backlight issue.

The screen turns on fine and runs for a few minutes and then appears to turn off, however it looks like it’s just the backlight that goes off, everything else seems ok (sound is still there if I have enough time to get iPlayer started.)

I’ve done a bit of initial investigation and I suspect the LED driver (MP3349S) on the 17IPS20 power board, as it appears to get really quite hot … pushing 60degrees when measured from the other side of the board, so it’s going to be significantly hotter than this, therefore I’m thinking it’s some kind of thermal shutdown.

The time to turn-off does also seem to be impacted by how recently it’s been on, so it does support being thermally related.

But … in this forum people with similar behaviour seem to be pointed at LED failures in the backlight.

In this TV these seem to be two separately powered strings of LED’s, I’ve run each string independently for a good half an hour each using a current source, I’ve only gone to 8mA (because I’m paranoid) and this is achieved with a fairly constant 86V, no changes with time, no flickering, no indication of any problem. (Although I do acknowledge I’m not driving it very hard.)

The other thing to mention is that the connector where the backlight cable joins the power board is quite deformed and has some small indications of charring. My guess is that there was some kind of poor connection, leading to arcing, leading to damage to the driver.

Does this all sound reasonable? Or am I missing something?

I’m really trying to avoid dismantling the panel, so I might be clutching at straws, but the LED’s do all seem to be ok, and swapping the driver chip is pretty simple in comparison.

Thanks for any help!

Lee.

[Maveriksam]

Hi essele

I have the exact same problem but all my connectors look ok.
I have striped my TV down and I have one LED that does not work. I am currently in the process to replace it.
I will watch this thread with interest and I will post back if this fixed my issue.

Do you have time to turn down the backlights to see if they last longer?

[sintv]

Ive got the same problem. All leds light up and go off after a few minutes. Ive tried a couple of 17IPS20 boards but it looks like Ill have to get the correct board for the set. Must be a stock fault

[CapLeaker]

all you can do is to turn the back lights in the menu waaaay down, like to 10 or 20 %. See if the back lights stay on longer. If they do stay on longer, my bet is a bad LED in one of the strings.

[essele]

Sorry to be so slow responding to this ... a combination of waiting for parts and being away.

Anyway, like a fool I didn't listen to everyone's advice and I changed the driver ... at first I thought it had worked, but sadly no.

So I dismantled the whole thing and discovered 2 faulty leds ... one was off all the time, the other was off when I was using my constant current source, but often came on (although sometimes with a delay) when using the TV supply.

I've just replaced both of them and initial testing seems good. The driver still gets quite warm, but seems to be around 50 degrees now rather than 60, which I'm taking as a good sign, and I've yet to be able to get it to fail.

So I'm going to see if I can remember how to put the thing back together now.

[budwich]

remember to turn down the backlight otherwise you will be doing more replacements.

[essele]

Thanks.

So it all seems to be ok from a backlight perspective!

But unfortunately the panel seems to have an issue with a small band of slightly dimmer horizontal lines, I'm pretty sure these were there before I fixed the backlight ... it was an eBay punt!

Anyway, I assume this is a panel issue so not fixable?

Picture attached.

Even if it's a write off, it was a great learning experience doing the backlight.

Cheers,

Lee.

[budwich]

not so fast on the "write off"... there are ways to potentially get rid of the "lines"... depending on the design of the panel... if it has side driver edge boards or not. IF not, a "tape cutoff" method may be used to block the signal to certain pins at the tcon that results in the drivers on the side associated with the lines to not be driven any more... clearing the picture. If there are side edge boards, then potentially there are ways of dealing with those. Read up some more on the forum about these issues and techniques and you might get a "fully working" display and learn some more things.