Polaroid p40led13 backlight issue

[steviewonder]

Hello once more,
I was wondering if anyone has any ideas on this peculiar problem.
this vestel set uses the 17ips20 power supply,and 17mb82s mainboard.
At power on,the led backlights glow dimly,for a split second,the voltage to the backlights then dissapear.
I have removed the screen for a nosey,all led's illuminate very briefly,and was suspecting one of the lighting strips to be faulty,apparently not the case.
There is 5v bl-on appearing at pin 3 on psu,and 3.3v on the dimming pin,so this seems to rule out the motherboard.A quick check on the psu revealed nothing obvious,so fitted another identical psu....and.....exactly the same...
Very puzzling...unless the other power supply could be faulty in the same way?
pics of boards to follow.
best wishes for now then,
steve

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Common issue is the led backlights themselves failing what panel does this use ?

[steviewonder]

Re: Polaroid p40led13 backlight issue

The panel is ves400unds-00,
My first thoughts were the backlamps,however all appear to work,albeit briefly.(I stripped the screen down to see).
Had a panasonic last week with a similar issue, 9 leds fried...only 18 months old.
A curious issue,not helped by a rather vague service manual for the psu...

[CapLeaker]

Re: Polaroid p40led13 backlight issue

measure the voltage outputs on the led driver with the backlights connected. Describe exactly what they are doing and if there is a difference from one string to the other.

[tom66]

Re: Polaroid p40led13 backlight issue

Yes the 17IPS20 generates 24V which is then boosted to about 60V for backlights (maybe different for 40" screen)
So check the boosted supply is present and stable. If it is low, it may cause this fault, although I would expect no light from the LEDs if it was very low.

[steviewonder]

Re: Polaroid p40led13 backlight issue

hello once more,Thanks for the suggestions so far.
it would appear that the strings are connected in series internally,as there are only two wires leaving the screen,the problem with measuring the voltage is it rises and falls so quickly that my meter (Fluke 12) hasn't the time to respond,there's 117v available to be switched,so i would imagine that's the voltage reached.It did cross my mind that even though all led's are lighting,is it possible one,or more is drawing excessive current,hence shutting the supply to the lamps down?.....mmmm

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Its very possible best bet would be to take the panel apart and see if any of the leds done light the return voltage sounds right so im suspecting an led.

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Also I have led strips for that panel I had to service a toshiba with them bulbs same issue.

[CapLeaker]

Re: Polaroid p40led13 backlight issue

try the min / max settings on the fluke

[tom66]

Re: Polaroid p40led13 backlight issue

It is also possible that one strip is open circuit, and you are seeing the LED power supply hit the over-voltage threshold with some strips lighting ok then it is shutting down because the rest are not ok.

[steviewonder]

Re: Polaroid p40led13 backlight issue

hello once more ,
well the min/max results seem inconclusive,just get greeted with OL on the fluke,possibly down to the drive waveform?.mmm.Anyway will strip the panel out once more,and this time remove the strips for a study,but as i said earlier,EVERY led on each strip illuminated briefly.
With a second psu fitted,and the same issue....so will report back soon with what i find!

[tom66]

Re: Polaroid p40led13 backlight issue

Make sure you set the RANGE on the fluke as the autoranging may take time. Lock it in the 600V range for example.
An oscilloscope can also help...

[steviewonder]

Re: Polaroid p40led13 backlight issue

Thanks Tom,
some progress..removed the panel once more,and it seems i missed the fact just one led does not illuminate,easily missed as the lamps only illuminate for a matter of milliseconds...just going to replace that and see what happens..Give me a bush tv22 or Decca bradford any day over this rubbish!.will report back,Ahh yes tom,forgot about the range settings,Long time since used on min/max and range settings!

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Knew it ! Ultra common issue failing leds do you know how to replace the led ?

[steviewonder]

Re: Polaroid p40led13 backlight issue

yes reecyburger!
not much of a challenge,it's the stupidity of me in the first place really for not 10x checking....DOH!..incidentally,these strips are manufactured by the same company that LG use in the lc420DUE panel,so expect some panasoncs failing soon! (mentioned this earlier)
a pic paints a thousand words...until the next lamp fails....I mean,really? ONE led out of 45 fails and no go at all.....gawd...but thanks for the input to tom66,capleaker and you yourself...Hopefully this challenge may help someone else viewing this thread at a later point!

[steviewonder]

Re: Polaroid p40led13 backlight issue

seems the pic has failed to upload..

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Ohh yes I have done a Panny with an Lg panel. Yes only 1 led and all out wired like christmas lights each led has to break over to power the next and complete the circuit if that fails all out trash design really and very problematic and its not just with full array sets like this the edge lit ones are just as bad. I just hate led lcd tvs in general I could rant all day about them I will always stick with plasma haha. Let us know how it goes buddy

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Any update ?

[ReeceyBurger123]

Re: Polaroid p40led13 backlight issue

Update ?

[csocsoi]

Re: Polaroid p40led13 backlight issue

Yes the 17IPS20 generates 24V which is then boosted to about 60V for backlights (maybe different for 40" screen)
So check the boosted supply is present and stable. If it is low, it may cause this fault, although I would expect no light from the LEDs if it was very low.

Hello, so this board generates about 60 V at a 40" screen? I have a 40'' Philips with this board and with no backlight, so it means if the output is about 60V that I have some failed LED bulbs?

thank you