Philips 47" TV 47PFH4109/88 repair

[soldar]

1.5 hrs in normal people's time is 6 hrs in my time. I would still give it a try if it were smaller but the thing is huge and I cannot disassemble it and have in the way. The way I like to do my projects is one day I carefully open the thing up, inspect it, plan a course and leave it there. Two hours right there. Another day I change the LEDS, another two hours. Another day I reassemble everything. All very relaxed. I do not like working under pressure. And, as I say, I have no use for it myself.

And as for selling it, if there is something I hate is trying to sell used things. People are a pain. They stand you up, they will try to bring the price down, they will argue, etc. I hate it. And if I try to sell it as-is I do not think it would attract any buyers even though I am in Madrid. I think I would rather give it away or trash it.

For now I think I will just put it aside and give it some thought. Diogenes is my middle name.

[soldar]

I just had an idea but I do not know if it makes sense. To recap: the led backlight stops working and we are assuming it is driven by a constant current PSU and some LEDS shorted and the voltage does not rise enough so the PSU shuts down. Right?

So would it be possible to insert in series a resistor that would raise the voltage enough for the PSU to work?

I mean, the LED backlight turns on briefly so it does work. Rather than replace it maybe I could do a shoddy repair by just inserting a resistor?

I am even thinking I could try testing the LED backlight with an external PSU.  IIRC it was something in the order of 35V and I could gradually raise the current until it looked good.

What are the chances of that working?

[DavidAlfa]

That won't work. It blinks briefly because the voltage goes too high.
Adding a resistor will keep the voltage under control, but at the same time the damaged led strip has a much higher threshold voltage than normal, so it won't light up.

Think it like a 9V zener that drifted to 12V, causing PSU error.
After adding load resistors to keep the voltage at 10V, no current will pass through the zener.

[soldar]

Wait. Let me think about this. The backlight power supply is a (switching) constant current source with upper and lower limits.

I am assuming the LED strip has one or more LEDS that failed shorted ... because if they failed open the LEDs would not even blink. So the result is a strip with some shorted LEDs which will work at a lower voltage than the PSU will tolerate and that is why the PSU shuts down.

If I put in series a resistor, or even some regular diodes, to bring the total voltage up again then the PSU will not complain and will work. Right?


Aside: I am working on installing some shelves in my workroom and the room and bench are unusable for now but once it is all finished and usable again I am thinking I could just open the back of the tv set and connect the LED backlight connector to an external PSU limited in current and raise the voltage gradually and see what happens. I think somewhere before I get to 50 V the backlight will light. Then I could even provide an external PSU for the backlight totally separate from the TV set.

In summary: If the backlight lights up for a moment it should be possible to keep it lit.  Yes?

[DavidAlfa]

The problem is not the shorted leds, but then open ones.
Every TV I fixed had several shorted leds, but often only one was the culprit, completely burned off, opening the circuit.
It's not a perfect "cut", so the higher voltage might be able to partially conduct over the burned stuff.

The backlight does basically like this:

[soldar]

Well, I guess that dashes my hopes. Still, because the backlight is just a simple two wire, +/-, circuit, I might try pushing some current through it and see how it behaves. I am not quite ready yet to disassemble the whole thing and replace the backlights but it looks like it may be the only real solution..