42LE4500 Backlight Turns Off Fault Confirmed !

[ReeceyBurger123]

Right got this crap ass wank Lg set here, original fault Led backlights cut off after a few mins Ive had this before on this model and never been able to solve it so just swapped Psus. So I ordered another Psu but its for the other 42" verison that uses an Lg panel and not Auo like the set here so it wouldnt work.

So I decided to steal the main Led driver Ics off the big green driver card as thats what people diagnosed to be the issue.......But its not after replacing both Ics the set still failed meaning someone misdiagnosed the fault here but shit happens.

Anyway so I then swapped the whole driver card over from the Psu for the wrong screen and installed the driver for the one for that screen onto the other Psu and the set works sweet

So the issue is on the main supply not the driver card, will dig into it further tomorrow and have a good play about.

[CapLeaker]

Re: 42LE4500 Backlight Turns Off Fault Confirmed !

interesting indeed.

[Bennjam]

Re: 42LE4500 Backlight Turns Off Fault Confirmed !

Please forgive me if I'm breaking protocol by replying here. This is my first post to this VERY useful forum.
I've had one of these sets hugging the bench for far too long now but now I think I might have a runner. As reported by many others the set may provide reliable operation for a couple of days(or more?) then the backlights shut down, usually after start up, or shortly after. I initially went over the PSU and green sub pcb and carried out some reflowing. No change, so I consulted this Site and found a testimonial regarding the AS3693B driver being the IC responsible for the problem.
Although, curiously, I never found a follow up to confirm that replacing this device had cured the problem.
Oh well, worth a go. I sourced the chip and replaced it - and- you've guessed it. No change, fault still there. Next move was to open up the display and have a look at the LEDs. I could find no visible discrepancies, the illumination appeared to be of even intensity. The backlight consists of eight strips of 26 LED series strings and an oscilloscope check of the driven end of the strips revealed a markedly different waveform on one of the aforementioned strips. I had a squarewave of around 30V on seven of the strips and on the remaining strip 40-45V. An LED strip from a scrap display was pressed into service to take the place of the suspect strip. The TV had five days of soak test with no backlight shut down. As an experiment I reconnected the current-hungry original strip and waited for the back lights to go out again. Which they duly did - at that point I reconnected the scrap display set of LEDs and was very pleased to note that the backlights stayed on. Hopefully, might be a positive result. BTW this is an AUO display.

[ReeceyBurger123]

Re: 42LE4500 Backlight Turns Off Fault Confirmed !

Its one of the controllers or Optos on the psu is my bet but Ive not got to the bottom of it yet sadly.

[Bennjam]

Re: 42LE4500 Backlight Turns Off Fault Confirmed !

I'm quite confident that I've nailed the reason for random backlight shutdown on my 42LE4500 (was a customer's set but they decided to buy a new set after a discussion on the possible cost of repair).
One of the eight LED strips was responsible,despite there being no distinguishable difference in brightness intensity to the other seven strips. The set's been running fault free long enough now to convince me that all's well. At least until another LED strip starts playing up!
Hope you crack yours soon (not literally!). If it's of any help I could supply some reference voltages? For my part dusting off the ol' 'scope and pressing it back into service provided the big clue to the problem in this telly.