Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED failure

[Chungalin]

Grundig 40VLE6142C, 40" LED LCD HD TV
Panel: Samsung LTA400HM13 with embedded LED driver

Backlight flashes once when powering on, then remains off while there's image on LCD panel and everything else works fine.

8mm (or 5/16") hex wrench needed to open back cover, first time I need to use this tool on a TV...

24V from PSU to inverter OK. Check LED driver output and it has 4 test points: three read 11V, while one reads 0,6V. Disconnecting LED cable, all outputs to 11V, so it seems I'll have a LED problem.

I'll report progress.

[budm]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

We need good clear pictures, 24V should be the input Voltage feeding the LED Boost converter circuit so the out put Voltage for driving the LEDs should be higher than 24V.

[CapLeaker]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

probably one or more of the backlight LED's are bad.

[ReeceyBurger123]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

Deffo an Led or the branch connectors that link the Led strips I have done alot of these panels with same issue.

[Chungalin]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

11V readings are from strips return. I missed the main Vcc, that is about 100V, but can't measure with precision because it doesn't stay on, just one fraction of a second. Then drops slowly. LED driver output is a 7 pin connector:

Vcc
LED-1
LED-2
(N/C)
Vcc
LED-3
LED-4

Both Vcc pins are the same output, not different circuits.

This panel backlight is organized in 4 independent series of 30 LED, all on top side of panel. I've found 2 LED that don't light up (neither under DMM test nor during the fraction of a second that all LEDs light up). Both bad LED are in the same series. This matches the abnormal reading from one out of four testpoints (LED-1 or LED-2, can't remember now). It's not surprising that they're not open (otherwise, the whole 30 LED series would remain dark), but it's surprising that they're neither shorted!! Their reading under diode test is practicaly identical to other LEDs.

And now? Must I replace those two LED or there's a workaround? Or should I actually short them? No stacking possible in this panel.

[budm]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

OK, that makes more sense, so the string that has open LED or high resistance, the return pin (-L??) which is the Cathode will show very low Voltage compared the good LED string.

[ReeceyBurger123]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

Watched my video on how to replace them ?

[Chungalin]

Re: Grundig 40VLE6142C TV repair, no backlight, Samsung LTA400HM13, possible LED fail

I won’t replace them. I’ve shorted them with wire and now it works. I think I read a budm post some time ago with a picture of a LED surrounded with soldered wire. I thought it was a practical solution for open LEDs, but these ones are not open. Instead, it seems that they produced a larger voltage drop that driver detected and shut down.

Still haven’t measured voltage return, but it works and this is the way to go. When I can see image, I’m sure that I’ll find Backlight at 100%.

Very difficult to properly solder over these aluminium strips.

Edit:
Vcc is now 96V, under working condition. Return voltage is about 1.1~1.3V on 3 fully working strips, and 7.4V on strip with two faulty (now explicitly shorted) LEDs. Voltage drop of 3.1~3.2V per LED, so it makes sense: 1.2 + 2x3.1 = 7.4V. Let’s see how long it stands.