LG 42LB5DF No Pic No Sound

[mmartell]

Re: LG 42LB5DF No Pic No Sound

You mean like local dimming ? This is not an LED, I wasn't aware they could implement that type of function with CCFL's ?

Besides, though the backlight is on I still have no pic, I should still suspect the T-Con, no ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Ordered another T-Con, will update when it arrives.

[budm]

Re: LG 42LB5DF No Pic No Sound

It is not local dimming, it is for the whole screen, not like the backlit LED.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Ok received and replaced the T-Con board.

Result - Same symptoms.

If I unplug ribbon from tcon to master inverter backlights fire up no problem. If I slide that cable back in then as soon as it makes contact inside the socket the backlights go out.

Any ideas ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

I've attatched two pics of either end of the flat cable connecting the T-Con to the Master Inverter.

If the cable is unplugged the backlight works and the voltages are as follows on the inverter

Dim_In ~ 2v
Dim_Out ~ 2v
Sen_CB 5v

Whenever the cable is plugged in, the backlight goes out and the voltage on the Sen(sor?) Pin goes to 0v.

I checked that pin on the tcon board and it appears to be grounded which can't be right ?

Did I get a bum board or should I be looking elsewhere ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

And the pics...

[mmartell]

Re: LG 42LB5DF No Pic No Sound

T-Con Pin 5 of Con1 (Pic one above - Labelled Sen_CB on matching Inverter pin) shorts to ground on both my original and the used one as shipped to me.

However the inverter shows 5v on that pin so when I plug that cable into the tcon that 5v goes straight to ground and shuts off the backlight. I can't see where that pin goes on the board to even trace it but I'm guessing it shouldn't be grounded.

Help Please !!

[mmartell]

Re: LG 42LB5DF No Pic No Sound

I realize not everybody wants to read through three pages to get involved so I will summarize as if this were the first post

TV was found to have no backlight - all voltages from PS seemed to check out as good.

Unplugging the ribbon cable from the Master Inverter to the T-Con allowed the backlight to operate as normal but still no pic or osd.

Replacing T-Con with used yielded same result.

5v was found at Inverter (see above post) which grounds out on the T-Con for both the original and replacement.

Did I get a bad board or do I have a problem somewhere else ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

I sent off several emails to ebay members who are selling this tcon asking if they'd test the pin in question for a short to ground.

I have received several replies, inconclusive as I can't even be sure they are testing the proper pin etc.

They overwhelmingly report that pin shorted.

This only confuses me further. The implication is that there shouldn't be 5v on the matching inverter pin but my gut is telling me it SHOULD be there - even without a schematic.

I've d/l the SM from Elektrotanya but it doesn't appear to have a standard schematic so is useless to me.

HELP !

[dhallet]

Re: LG 42LB5DF No Pic No Sound

Any resolution with this unit?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

No.

I tried two replacement T-Con's with same results.

Am going to order a mainboard at this point as I can't imagine what else it could be.

[dhallet]

Re: LG 42LB5DF No Pic No Sound

I am working on one with the same symptom. I just got off the phone with LG tech assist and he said to change the T-Con if it's not that then try the main board and if that don't do it try the inverter. Geesh, I said, so LG tech assist is saying just throw boards at it till you find which one fixes it. That's great. The tech assist guy just laughed and said "Yup". I then asked why are the back lights coming on if I disconnect the ribbon cable coming from the inverter to the T-con and he said "I have no idea why it is doing that and I cannot find out why it is doing that". Keep us informed on the result with the main board. I am thinking, like you, it has to be the main board. I don't have a scope at the moment to see if there is in fact video being outputted by the main. Good luck Keep us informed on what the main board does.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Strange isn't it ? Did you measure 5v at the SEN_CB pin too ? And of the three T-Cons I have laid hands on the matching pin is grounded so how does that make sense ? Which is why I could see that circuit on the inverter could have malfunctioned...

But since there is no video I think a mainboard should be first.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

And btw I came across a Russian site that claims if you put 1.5v on the TST_PGM pin of the T-Con it will output a test pattern onscreen. Have not tried it myself - the mainboard is currently out of the unit as I was going to check for high esr caps. I think it's possible that other than the BL_ON voltage the mainboard may be outputting incorrect signals to the inverter.

[dhallet]

Re: LG 42LB5DF No Pic No Sound

Any word on the fix for this set?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

I put it to the side as I received another unit that took my attention for a bit.

Since then I have acquired an oscilloscope (which I haven't used before) and will be using it soon to test for the presence of video signal or lack thereof coming from the mainboard.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Hey dhallet it looks like in my case the chip PI3HDMI341 (hdmi controller ?) is shorted out. I also have another short in one of the supply lines to the BCM chip so I'd have to assume at minimum the mainboard is faulty and a replacement is necessary.

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Update: Installed replacement main board. Set fired right up, osd visible as was tuner noise and cycled through inputs.

Reassembled the unit and remounted to neighbour's wall only to find all inputs dead. Fu@k! That's a rookie mistake I won't make again.

However, I did NOT plug back the ribbon cable from the inverter to the t-con for fear of blowing something again on the main board. (I measured the voltage on the suspect pin and it was still there so I decided to be safe since I had backlight without it)

Does anyone know if there is any feedback in that circuit that would prevent the inputs from working or did I get a bum replacement? ?

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Hmmm... there were at least three size variations of this set. Are those mainboards interchangeable ???

[mmartell]

Re: LG 42LB5DF No Pic No Sound

Main board replacement working fine. TV back to life. Thanks to all.