Good day folks. Pretty strange issue with this TV here: the backlight won't come on despite the LEDs being OK. I lit them up all individually with a power adapter and they all come on really well, however when I try it with the power supply, nothing happens - not even a glow - NOTHING ! One interesting thing I noticed is that if I use my finger to touch the points where I scraped the track to reveal the copper trace for testing the LEDs, the LEDs light up faintly. The same goes for my meter: when I put my positive probe on the "return" (minus) end of each of the strips (the negative probe being on chassis ground) that strip would glow slightly. This "method" also allowed me to confirm that the LEDs appear to be OK, since if I keep my finger in the top right, the whole TV is lit - very dimly, but lit nonetheless. This doesn't mean that the LEDs are OK though: it could be one is failing under load, but it's not even making a flash or something - nothing turns on at all...
There are 4 LED strips in series and the connector going to the PSU (CNL802) has 4 wires, but it's really just two wires, since the colors are in parallel to increase the current capacity, so we've got blue "send" or OUT let's call it, and black "return". The blue wire is connected to the + of the bottom strip. The - of that strip goes to the + of the one above and so on until the top one which has the - "output" connected to black return. I get around 170v on the blue wire of the PSU and this corresponds to the + pin of the bottom strip, so there IS something going there (not a dodgy connection). After the whole series, I get around 70v on the black wires....is this a PSU issue or is one of the LEDs failing ? I read another post where the chap indeed replaced the PSU....wouldn't really want to do that, so perhaps someone has had this issue and managed to fix it. No shorts on the PSU btw. The resistors are also OK (low value). I posted some pictures of the driver area. We're assuming the rest of the PSU is of no interest, since the TV DOES come on. Just a quick thought: every Samsung PSU I've worked on so far turns on the backlight by default when it's not connected to the main board....could this one be the opposite like many inverter-based TVs and actually require an ON signal applied to the BLU_ON pin to turn it on ? I highly doubt it, since there is voltage present on the strips...VERY strange.
There are 4 LED strips in series and the connector going to the PSU (CNL802) has 4 wires, but it's really just two wires, since the colors are in parallel to increase the current capacity, so we've got blue "send" or OUT let's call it, and black "return". The blue wire is connected to the + of the bottom strip. The - of that strip goes to the + of the one above and so on until the top one which has the - "output" connected to black return. I get around 170v on the blue wire of the PSU and this corresponds to the + pin of the bottom strip, so there IS something going there (not a dodgy connection). After the whole series, I get around 70v on the black wires....is this a PSU issue or is one of the LEDs failing ? I read another post where the chap indeed replaced the PSU....wouldn't really want to do that, so perhaps someone has had this issue and managed to fix it. No shorts on the PSU btw. The resistors are also OK (low value). I posted some pictures of the driver area. We're assuming the rest of the PSU is of no interest, since the TV DOES come on. Just a quick thought: every Samsung PSU I've worked on so far turns on the backlight by default when it's not connected to the main board....could this one be the opposite like many inverter-based TVs and actually require an ON signal applied to the BLU_ON pin to turn it on ? I highly doubt it, since there is voltage present on the strips...VERY strange.
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