LG 42LB585V - Backlight issue.

[Turnip]

Hi folks,

Got one in with a quick flash of 'LG' but sound continuing. Leds fed by two wires so all in series.

Test measurements were a bit odd. Voltage initially 250v dropping to 105v but noticed on the meter there was a reverse resistance I hadn't noticed with earlier LG Led sets.

Took the screen apart finding 4 strings of 8 Leds that required 40v to light up per string. About to replace O/C Led from stock but noticed the new Leds were a little bigger and needed 5.5v to fire up.

To Ebay where China supplies 6v LG Leds at £5 for 50 - Wish I'd got some as UK ones are a pound a pop at the moment.  Nice side is that Leds can be replaced by snipping off the old one, cleaning up the PCB strip and fitting the new one with LMP solder. Taking out the plastic reflector sheet would be a real fag as there are dozens of securing plastic devices that don't seem to come off so easily - Chris.

[downunder]

Hi Chris,

8 LEDs needing 40 volts to fire should have rung alarm bells. Not knocking you, mind, cos we tend to forget there are 6V LEDs in use. But keep up
the instructive banter. I guess it's like here in the UK - once we old fogies have gone to the other side, there will be none left to do the service.
The DIYers will inherit the void.             Bruce

[bonesy]

Hi Chris,my only advise with the LG LED's,check & make sure all your strings are evenly lit,if any are just slightly dimmer,good chance within a short space of time it could bounce back to bite you,cos another string has failed.Has happened to me twice now,so any looking slightly dimmer get replaced now.Lg seem to be very poor LED wise.

[Turnip]

Hi Bruce

Thanks, this LG was my first with 6v Leds, so was naturally confused but glad I didn't slip in a 3v one and wrap it up.

Out of interest, what's with 6v Leds that seem to have a diode conducting with reverse voltage and are capable of 3 watt dissipation - For a while ?

Thought try find a service manual to see if the supply is unusual, though it appears DC as per usual. Naturally Now't available.

Thanks bonesy. In this case they all look much the same, and my duff one was burned out, maybe due to an initial bad soldering issue - Chris.

[sinclairtv]

Dont the plastic securing bits just slide out?

[zomikula]

They are suppose to, but can be a pain to get out.
Also chekc all the plastic lens, theses type are quite prone to falling off, really poor glue they have used

I owul dalos bridge the connections as have had some where this causes the backlight to go out.
Works fine with tester but the resistance seems to be too high sometimes for the tv to run them..

[Turnip]

Hi Sinclair,

There were just too many plastic bits to try bother as Leds can be replaced easily from the top with a snip of the sidecutters, a good clean up with solder wick and glass fibre brush to get down to the copper so re soldering is easier with a bit modified to solder both ends at once with LMP solder.

Hi Zomikula - Thought the larger lenses were easier to glue than the small old ones, but it's only my first attempt so won't reassemble for a day or two, see if they're well stuck. Not sure what you mean about bridging connections - Chris.

[sinclairtv]

Yeah I'm doing them like that. Saves trying to get the strings out and possibly breaking them. I also make a note of the way the plastic focus lids go on otherwise you sometimes see a spot from the front if they're slightly off kilter.

[zomikula]

They seem to be bit more of a pain
Easier to misalign
You have to make sure the black markins are centred on with the lens.
plays havoc with the back light bleed.
And for what ever reason someon these lens just seem to fall off if you just touch them on some panels.

https://www.youtube.com/watch?v=Ur-DS12NsPc&t=908s

[Turnip]

Hi Zomikula,

See what you mean from the video, as chap seemed to have half a dozen lenses fall off giving white spots and bleed through. Tend to use existing holes, but guess the new Led has to be dead central
which isn't always so easy.

Did my first one with a water based adhesive as it doesn't seem to dry out, though I normally use super glue gel - Wonder what's best ? - Chris.

[zomikula]

The ones with the black adhesive holes seem to be stonger than the beige one.
Have been using the loctite super glue gel 'flex' seems to have been hodlign up fine.
One these you use the exisiting holes but make sure its aligned perfectly,
had to take a set apart again as one on the lens hadnt sat properly and you had a bit of a bright spot..

[Turnip]

Hi Folks,

All went well, apart from on a bright screen it was obvious I hadn't centralised the Lens over the Led replaced as got a halo showing the misplaced Led to one side of the halo. Was about to take the screen off again but the cust arrived and carted it away as couldn't see the problem on a normal picture.
Suspect it'll be back so I can have fun sliding the lens about under the thick diffuser 'till I've got it right. Next time I'll know to check B4 I box it up - Chris.

[t1412]

Hi chris just done my first led strip repair quite chuffed with my self cheers
saun

 

[Turnip]

Hi Shaun,

As it happened the customer's wife said there had always been a bright patch there - Can't see why, but who am I to argue ?

Next time I'll get it right, as now know what to look for.

Congrats about yours- Chris.