LG 42LB5DF No Pic No Sound

[mmartell]

Re: LG 42LB5DF No Pic No Sound

UPDATE:

Fifteen to twenty seconds after a power-on the backlight flashes (quickly). I only noticed this because I decided to try in a dark room.

Not sure if that helps.

[budm]

Re: LG 42LB5DF No Pic No Sound

Ok. so you test the two fuses on each inverter boards with Ohm and Voltage. But there are FOUR fuses on each of the Inverter board.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Four ? I found THREE on the Master and only TWO on the Slave. Damn. Will look again. Are they all on the topside of the board ?

Can you run the backlight with only one inverter plugged in (if it's good of course) as a way of verifying a failed inverter ?

[budm]

Re: LG 42LB5DF No Pic No Sound

Sorry, you are correct, Master has 3, slave has 2, I down load the wrong pictures from SJ. The slave does not have the PWM IC, the main PWM IC is on the Master board. You can disconnect the power cable to the Slave and see if the backlights will flash.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

With the Slave power cable disconnected there is NO backlight flash at all.

Does that mean anything to you ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Bad Master inverter perhaps ??

[budm]

Re: LG 42LB5DF No Pic No Sound

That is what it sounds like since the master/slave setup work like push-pull type to get enough high voltage to drive long lamps in large TV.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

That is what it sounds like since the master/slave setup work like push-pull type to get enough high voltage to drive long lamps in large TV.

Haha that was going to be my next pursuit to find out why the master/slave technique for inverter design.

So should these be replaced as a pair or does that even matter ? I mean can one fry the other or anything of that sort.

Btw its my understanding that this set failed over time and not instantly. Would that be consistent with some inverter failure ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

So I decided to remove all connectors from the inverters except for one and I got a backlight which fired and stayed on.

So I kept plugging in until the backlight failed again to come on.

In the end I plugged in (from the top) lamps 1,2,3,4,5,7 and 8 on both Master and Slave. Plugging in 6,9 or 10 in either Master/Slave or both caused the backlight to fail.

Though the backlight stays lit there is nothing on the screen, menu, volume osd's, nothing. The set does power on and off normally with the power button. Perhaps a separate issue.

Any suggestions ?

[budm]

Re: LG 42LB5DF No Pic No Sound

If you have everything else hook up except some of the lamp, then you should see some image on the screen. So if you cannot even see the menu, then that is not right.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Well I figured it wasn't right cause I can't see anything lol.

I read somewhere here that if the inverter sees open it will shut down so I'm not sure how I even have any backlight. I would have like to verify the other lamps are working but of course the plugs are too damn short. And those connectors are damn tiny.

Might be safe to say at this point at least one, maybe both inverters have failed ? Is it possible to move forward from here without yet replacing the inverters ?

[budm]

Re: LG 42LB5DF No Pic No Sound

You should replace them in pair, but for no pictures on the screen is more problem to fix, the backlights are needed for the light source only, even if the backlights do not work you still should be able to see the pictures using the flashlight method.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

So a mainboard or tcon problem or even the panel itself ?

[budm]

Re: LG 42LB5DF No Pic No Sound

May be the T-CON has blown fuse or not getting the voltage from the main board.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Great idea! Will check it out on the morrow.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Though that reminds me... earlier I took the four screws off the TCon cover and the edge of it where it meets the top of the tv seems to disappear down to the panel and I could not get it off. Without seeing how it was attatched I was afraid I might do damage to the cables/wires/board underneath.

This was AFTER having no picture btw

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Pried up the T-Con cover to gain access (strange design!) and measured 11.84v on both sides of FP1. Nothing on the screen.

There is a flat data cable going from the tcon to the Master Inverter. There are Dim In and Dim Out pins on it. The Dim Out (to the Inverter) reads 0v while the Dim In reads about 2v.

There is a landing on the T-Con labelled TSTPGM. Is that used to generate patterns independently of the Mainboard and if so how do I activate it ?

[truep]

Re: LG 42LB5DF No Pic No Sound

Disconnect the cable between the TCON and the Inverter and see if you have backlight. I had a similar model and the TCON was the problem.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Wow that's interesting. I disconnected that cable and ALL backlights came on.

Think I will replace the T-Con and see what happens next. Thanks.

[budm]

Re: LG 42LB5DF No Pic No Sound

Ha, I did not see the cable connected ot he T-CON, it looks like the cable bundle going to the right Inverter was close to the T-CON but not connecting to the T-CON, so it does have scene dimming control, glad you saw that flat ribbon cable. Noted that in my book of knowledge.
http://assets.shopjimmy.com/media/ca...010061-top.jpg If you can live without scene dimming function, I would leaveit, it is one of the trick the manufacturers use to claim high contrast ratio.