LG 47LG50 No Backlight

[dick_barton]

Re: LG 47LG50 No Backlight

So you are quite correct and it is an 8A fuse.
You say you've put everything back however the backlights just flicker and don't stay on.

I don't know what else to suggest other than soldering the wires directly to the power supply board. Perhaps others can offer you their advice.

[heavymachines]

Re: LG 47LG50 No Backlight

what kind of resistance reading you get at fuse holder to ground?

No dead short?

Disconnect everything from power supply put a fuse in it and force it on with resistors. All voltages ok?


You may need a handful of fuses or you can use a lightbulb or 2 in it's place.

[heavymachines]

Re: LG 47LG50 No Backlight

if lightbulb is shines bright you have a short.


Also if something is overdrawing the power supply can and will blow fuse. Does the inverter have it's own fuse?

[neilc6]

Re: LG 47LG50 No Backlight

The fuse isnt blowing after I replaced it. I replaced it with a 10A borrowed from another board. The backlight doesnt flicker. Its more like a super quick flash as it tries to turn on.

The inverter boards have surface mount fuses that all tested ok.

The voltages are all ok from my first post. Just unsure about that one error line that is high.

[heavymachines]

Re: LG 47LG50 No Backlight

that burnt mark on connector is a big clue Perhaps let budm help with possibly doing a bypass of that bulb to see if you can get it to stay on, if it's 2 seconds to black very good possibility you have a cfl problem. just dealt with this problem on a samsung. i was able to bypass the error feedback from transformer and force the tv to stay on. Lg is a little bit different animal.

[budm]

Re: LG 47LG50 No Backlight

So did you take care of that burnt connector due to bad contact resistance?
I would turn on the TV in the dark room and see if you will see any acrcing on the connector or not.
The backlights inverter is also control by the T-CON board, you can see small flat ribbon cable between the Master inverter board and the T-CON; you can try disconnecting that small flat cable going to the T-CON and see if the backlights will come on and stay on or not, but this all depend on the condition of that burnt connector.

[budm]

Re: LG 47LG50 No Backlight

BTW, you have Master and Slave inverter boards setup, they work in push -pull mode (driven out of phase, floating drive), if one them is not working, you will see that half of the screen will not light up on side where the bad inverter board is, so look real close to see if the lamps are fully lit on the left and the right side of the screen when you see the backlights flash on, do that in the dark room.
Some explanation about Master/Slave lamp drivers, page 47 and up.

[neilc6]

Re: LG 47LG50 No Backlight

The continuity through the blackened connector measured fine. Not able to check it through to the plug side unless I pierce it with a sewing needle which may be next step.

Checking the flash in a dark room shows mainly right and left edges lighting up more than the middle but not skewed to one side or the other.

[neilc6]

Re: LG 47LG50 No Backlight

Finally some progress. Disconnected the small ribbon to master inverter and the backlights cycle for about 5 seconds on and 10 seconds off.

No arcing on the blackened connector.

Am going to inspect the Tcon.

[budm]

Re: LG 47LG50 No Backlight

Now, if the main board and the T-CON board are OK, then you should see the image on the screen since now you have the backlights source on long enough..
BTW, is the screen lit up evenly?

http://www.shopjimmy.com/philips-996...verter-kit.htm
I am having a hard time reading the labels for each pin of the inverter connectors.
Try looking to see the ERROR pin.

[neilc6]

Re: LG 47LG50 No Backlight

The screen doesn't display anything when the backlight cycles on. When it cycles off, the front red led flashes slowly.

The backlight does appear even. Will check for error pin next.

[budm]

Re: LG 47LG50 No Backlight

The key point is that when you do have the light source you should be able to see image on the screen if the T-CON and the main board are OK.

[neilc6]

Re: LG 47LG50 No Backlight

With the backlight on, the light does seem even with the naked eye but when I took a pic, it looks like there is a tube out which corresponds to the location of the blackened connector. If I take a picture from a higher angle, it is more even but think this is because of the diffuser.

Plugging the small ribbon back in and monitoring the ERROR line on the larger ribbon, it goes to 3.96V when the backlights try to turn on.

Is there a way to bypass a bulb if my diagnosis is correct?

[heavymachines]

Re: LG 47LG50 No Backlight

I would imagine yes, rather than bypassing one transformer, you would have to bypass two with jump wires one on each side, wait for confirmation on that. Might be able to find out how on youtube.



Somebody might know a better way so let them chime in.

[Andrew F. Ali]

Re: LG 47LG50 No Backlight

from what I gather here, we have a situation of a bad T-CON Bd....This will affect the backlights as well as give the no picture result. Check the fuse on the T-CON Bd, too.

[neilc6]

Re: LG 47LG50 No Backlight

The fuse on the tcon tested good and board looked ok. Looks like I can get a used one for $15 on ebay so may be worth a gamble.

Isn't the error signal generated if the transormers detect excessive current. That is what I saw on a youtube video which is why I suspect a bad ccfl tube.

[budm]

Re: LG 47LG50 No Backlight

The ERROR signal is generated when there is failure in the inverter circuit such as open lamp condition (firing Voltage applied but the lamp will not conduct), failed circuit such as overloading condition due to shorted transformer, fail due to not all lamps conduct and stay conduct, over current, etc.
Right now I do not know if the ERROR is active high or active low; High = good? HIGH = bad?
The LCD panel specification sheet should have that information.

[heavymachines]

Re: LG 47LG50 No Backlight

So we need to know the panel sticker number? post pic of it or list it. sticker is on back of panel...

[neilc6]

Re: LG 47LG50 No Backlight

Sticker was hidden behind the stand bracket and shows V470H1 - Rev C2

Googling shows it is a Chi-Mei panel with datasheet here http://www.datasheetspdf.com/PDF/V470H1-L03/842007/1

"When inverter protective function is triggered, ERR will pull low"

There are two ERROR signals - one on the flex connector from the slave and the second going to the PS board (measured 0V before=abnormal).

I re-checked the ERR signal on the PS board connector from the inverter. The TV cycles from flashing front red LED and dim blue on mode. With both small flex connected and disconnected, the ERR signal is 3.3V in the on state and 0V in the off state which seems to point to the bulbs being ok.

[heavymachines]

Re: LG 47LG50 No Backlight

if that is the case and you have all voltages within spec you should be able to bypass the main and use resistors and force the backlights on with resistors 1k ohm may need 2 or 3 of them.


Disconnect main entirely jump stby to ps_on see if it turns on.

Should hear relay click if this set is equipped with one, if no click may need another resistor to ground stby to ground with another resister at same time this should force it on. if you have inv_on/off pin that will need to be fed stby voltage as well at same time.


Will check this thread see if you have pics of all the connectors on power board from main. To better determine how to force this beast on. if you do i can draw out edit the pic what you need to do.


Ok checked thread don't have very good pics of power supply connectors. Need those. Also others can help with pics of them connectors as well. I haven't read the panel spec sheet yet. been busy.