LG 47LA620S no power no LEDs

[dicky96]

Hi guys, I've not posted for a while (not had any difficult faults to fix lol) but anyway...

I have this LG 47LA620 that is pretty much dead

There is a definitely problem with the backlight LEDs - I see different voltage on the two chains with my backlight tester (about 68V on one chain and 74V on the other) and with the LCD panel removed I know I have a few LEDs out.

However both chains do light up with the backlight tester and stay on.

But I have another problem I think, the set does not even try to power up. When powering up I get nothing when pressing the power on/off button on the TV, no backlight Flash on/off, now power LED on or flashing.

I have 3.5V STBY but no other voltage from the PSU

I don't get any voltage on PWR-ON pin on PSU connector

Even though I have no PWR-ON, when I power the set up and press the on/off button, after about 5-10 seconds sometimes I get a single tick sound - it sounds like a relay switching or spark discharging, but I see no relay... or sparks for that matter. I cant reproduce this every time I apply mains power though

Any suggestions? Does the mainboard need some other signal before generating PWR-ON signal?

Cheers
Rich

[dicky96]

Re: LG 47LA620S no power no LEDs

Forgot to add /- disconnecting the LED backlight connector makes no difference, and I currently have the LCD panel removed.

Rich

[attainteddragon]

Re: LG 47LA620S no power no LEDs

i would start by jumping pwr-on and drv-on to 3.5v with a 470 to 1k resistor to start with. main board disconnected of course. see if you get your boosted pfc voltage and see what the backlights do. also, it looks like the area around the backlight boost circuit has been getting rather warm, so check your solder joints while you are at it.

[dicky96]

Re: LG 47LA620S no power no LEDs

Was a bad connection on the power ribbon to the main board. Just the back light problem to fix now, but I already knew about that

Cheers

[attainteddragon]

Re: LG 47LA620S no power no LEDs

that makes things lots easier. happy led replacing.

[budm]

Re: LG 47LA620S no power no LEDs

"I see different voltage on the two chains with my backlight tester (about 68V on one chain and 74V on the other)" That is because the two LED strings do not have the same amount of LEDs, you can see the Voltage chart for the two LED power supply outputs are not the same.
Morel likely you do have shorted out LEDs problem.