LG 42LE4900 no backlight, possibly no display

[cypher007]

im trying to sort a 42LE4900, it turns on does its cylon red light thing with the power led then that goes to white but nothing on the screen. no backlight and from what i can see no display. there is a click from the driver board like a relay closing.

measured the voltages on the backlight connectors to the metal back plate and get 65v red 0v black 11.5v black this is the same for all the used connector pins apart from one that reads 9.5v.

ive tried to disconnect the main cpu board as id heard it should then just fire up the leds when power is applied, but this didnt work.

[vinceroger69]

each of the 4 backlight voltage should be 100v if i remember correctly
mainboard and psu dissconnect dont work on lgs as far as i know only samsungs
If i remember correctly these are a edge lit tv with the led driver board mounted onto the psu its been a while since ive fixed one of these

[neilc6]

LE, LV, LW and LS series suffer from BGA failures on the main board. To confirm, heat up the processor with a hair dryer before plugging in. Lots of ebay sellers offering repair service in the US. Not sure about UK.

[cypher007]

Originally Posted by neilc6

LE, LV, LW and LS series suffer from BGA failures on the main board. To confirm, heat up the processor with a hair dryer before plugging in. Lots of ebay sellers offering repair service in the US. Not sure about UK.

wouldnt it flash some sort of error on the power led if it was that? ive got a cheap rework tool i could heat it up with, though i always think im going to end up cooking the IC when using such temps. certainly back when i started electronics we were always taught not to hold the iron on the pins and work round the pins diagonally.

[cypher007]

plus when you plug it in it goes to standby then if you press the side button it tries to startup and there is a click as if the power board is trying to supply the LEDs.

[dskall]

Error codes on tv's are rudimentary at best. Heating up processor is easy way to eliminate possible bga problem.

[vinceroger69]

Originally Posted by cypher007

wouldnt it flash some sort of error on the power led if it was that? ive got a cheap rework tool i could heat it up with, though i always think im going to end up cooking the IC when using such temps. certainly back when i started electronics we were always taught not to hold the iron on the pins and work round the pins diagonally.

remember your are not trying too reflow the ic your only heating the bga chip up too expand parts and see if it tempory fixes the issue

[nomoresonys]

if no joy, maybe try to run the powerboard as stand alone, to see if you can get the backlights up, this one is unfamiliar to me so I don't know the procedure.

[vinceroger69]

psu i think used is these sets is a yp42lpba with the code EAY60803202 not the easiest to diagnose

[cypher007]

is there a way i can test the led strings without pulling the screen apart as it looks quite delicate.

ive got a bench psu that will go upto 30vdc 5a.

also have a scope, multimeter etc.

[vinceroger69]

each led string is 100v i think
This video will give you a rough idea whats involed but remember big risk of cracking the lcd so be really carefull
https://www.youtube.com/watch?v=WYHSa7kYbf4