LG 55UB820V 4K Powers up but No Backlight

[london-freddie]

Hi, needing some help again please, LG 55UB820V seems to power up but no backlight coming on, torch test shows LG logo in the dark background
Suspecting the usual led failure on the backlight strip, took the screen apart and checked every led with a 3v supply looking for a faulty led. Every Led lit up ok, now wishing I had not stripped the screen and thinking that it has a power board problem, BUT what and where
Power board is a EAY63149401
Will get some pics on asap

[budm]

Re: LG 55UB820V 4K Powers up but No Backlight

You need to test the whole LED string from the LED connector that goes to the LED driver board since you have open circuit in the connection in the whole series of the string

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Hi Thanks Budm, trying now to get some pics uploaded, just checked voltage at Led connections on power board and only seem to have got 49.4V on both + connections I was expecting somewhere around 90v or more
This is an edge lit panel

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Just noticed there is an orange led on the power board which is flashing 5 times when the tv is powered on, I assume that is some form of a fault code

[budm]

Re: LG 55UB820V 4K Powers up but No Backlight

You need to leave black probe on the chassis, red probe on + then turn the TV and watch the meter to see how high it jumps up to and then settle down to 50V (which is the feeding Voltage to the Boost converter circuit).

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Yes Black probe on ground red probe on Led +, power tv on both connectors show 49.4 steady but no backlights on

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Are you thinking led strip fault or power board fault, not wanting to reassemble the screen panel only to have to take it apart again to get to the led strip, every led did light ok when 3v applied to each end of the led

[budm]

Re: LG 55UB820V 4K Powers up but No Backlight

If it never jumps up to 90 then you need to verify that you do have have BL-ON (DRV_ON) signal (>2VDC), no BL-ON = No boosted Voltage = no backlights.

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

OK still checking budm, no never jumps to 90v, 49v is best
just checked DRV_ON signal at powerboard connection 0v

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

just jumped 3v to DRV_ON pin, backlights come on

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

So I assume that I have NO DRV_ON signal for some reason

[budwich]

Re: LG 55UB820V 4K Powers up but No Backlight

as a quick test, if you disconnect, one at a time, the tcon to panel cables, does that change the result any?

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Ok disconnecting tcon to panel ribbons makes no difference
with DRV_ON jumpered tv powers up to the LG splash screen but does not go any further, remote buttons ie settings etc do nothing tv just remains on the splash screen

[budm]

Re: LG 55UB820V 4K Powers up but No Backlight

You have main board problem, but I don't know exactly where.

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Yes Pretty sure you are right budm, trying to source a replacement board now

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Update, sourced and fitted a replacement mainboard, same result ie: tv powers up to LG Logo showing on the screen for two or three seconds and then backlights switch off, back to a black screen
Front LED is not lit so I assume tv is still powered up
Orange LED on the power board starts flashing four flashes when backlights go off
I have run out of ideas only power board or backlight leds that I can think of now, bearing in mind when I checked the backlight leds they were all lit

[vinceroger69]

Re: LG 55UB820V 4K Powers up but No Backlight

if you disconnect the lvds lead too the tcon does the tv stay on?

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Just tried disconnecting lvds to tcon board, same result backlight lights for two or three seconds and then goes out

[vinceroger69]

Re: LG 55UB820V 4K Powers up but No Backlight

does the dr on signal still drop to 0 volts? seems odd same fault with replacement mainboard

[london-freddie]

Re: LG 55UB820V 4K Powers up but No Backlight

Idea, is it possible to disconnect mainboard from powerboard, and then force the backlights to turn on by bridging drv on bl on pin on powerboard