LG 42LB5D No video or backlight, sound functions

[drumguy560]

Hi all!

As the title states, I have an LG 42LB5d that has sound, but no video or back light.

I have a workhorse Fluke multimeter to test with. I do not have access to a scope.

Here are some pictures of the boards, (all but the TCON, which is really tough to get to, but I can snap a pic of that if need be). I do have access to a macro lens if closer pictures are required.

Thanks in advance to this great forum!

[aaronwt6]

Re: LG 42LB5D No video or backlight, sound functions

Remove the heat sink on the left side of your picture on the power supply. Looks for swollen caps under it. Regardless of whether or not you can see any, I'd replace all of the Capxon caps in that area. I have 5 of these that all exhibited that symptom until I changed those caps.

Reference my listing here:

http://www.ebay.com/itm/LG-M4210NJ-M...item43c8159353

You can see 3 of them just above the plugs on the lower right of my picture that I've replaced. I believe there are 4 others under the heat sink but I don't have the boards with me right now to look at.

[aaronwt6]

Re: LG 42LB5D No video or backlight, sound functions

Oh, and also check the fuse and capacitors on the inverter boards. The caps fail on these all the time. If the caps look bad and the fuse isn't blown, you can probably just replace the caps. If the fuse is blown, inspect the small 16 leg IC on each board. I cannot remember the number on it right now, but it seems to blow out once the caps get to a certain point. Typically, it's very evident by appearance if the fuse is blown. Chip will have missing / burnt legs, or a hole in it.

Keep in mind the link I sent you to my Ebay item is a slightly different board version than yours but looks like the caps are in the same place.

[budm]

Re: LG 42LB5D No video or backlight, sound functions

http://www.shopjimmy.com/media/catal...0058-top_1.jpg
Do you have 24VDC right at the inverter connectors? All the fuses on the inverters are OK?
What DC Voltage do you have on pin 14 bl_on PIN?
Check the black fuse F1 (F 1.5A/76V) with 'OE' on top also, CMO Inverter boards are known for having this fuse going bad.

[aaronwt6]

Re: LG 42LB5D No video or backlight, sound functions

Oh, and also check the fuse and capacitors on the inverter boards. The caps fail on these all the time. If the caps look bad and the fuse isn't blown, you can probably just replace the caps. If the fuse is blown, inspect the small 16 leg IC on each board. I cannot remember the number on it right now, but it seems to blow out once the caps get to a certain point. Typically, it's very evident by appearance if the fuse is blown. Chip will have missing / burnt legs, or a hole in it.

Keep in mind the link I sent you to my Ebay item is a slightly different board version than yours but looks like the caps are in the same place.

You can disregard this post about the inverters. I thought you had a different style. I guess I should have looked at all the pictures you posted before I jumped the gun.

[drumguy560]

Re: LG 42LB5D No video or backlight, sound functions

@budm

I have 24v at the connectors, and 3.3V on BL_ON.

The black fuse tested good.

Where should I go from here?

[drumguy560]

Re: LG 42LB5D No video or backlight, sound functions

@aaronwt6 I replaced those capacitors and there was still no change

[drumguy560]

Re: LG 42LB5D No video or backlight, sound functions

Anyone have more ideas? I really need to get this TV fixed. I'm trying very hard to get my component level repair skills up to snuff.

Thanks!

[mmartell]

Re: LG 42LB5D No video or backlight, sound functions

Remove the ribbon cable going from the tcon to the Master inverter and power it up and see if you get a backlight. I had one of these that needed a new main board but for some reason the backlights wouldn't come up with that cable attatched even after replacing the tcon. Never troubleshot it further.

[drumguy560]

Re: LG 42LB5D No video or backlight, sound functions

@mmartell

Did the new main board get it to fire up? I unplugged that cable and indeed the backlight did come on.

Thanks!

[budm]

Re: LG 42LB5D No video or backlight, sound functions

So it has the dynamic contrast on this TV set (the backlights brightness is adjusted according to the scene of the pictures so they can claim the TV has high contrast ratio).
So now you do have backlights, do you see good pictures on the screen?

[aaronwt6]

Re: LG 42LB5D No video or backlight, sound functions

Disregard. Bud beat me to it.

If you don't have a picture at this point, check the voltages on both sides of the T-con fuse.

[drumguy560]

Re: LG 42LB5D No video or backlight, sound functions

@budm The only thing i did to get the backlight up was unplug the connector from the TCON. There is no picture, just a black screen with a backlight. The menu does not show up when menu button pressed.

@aaronwt6 I checked the voltage on both side and it was 1.46 VDC on both sides.

Thanks gentleman!

[budm]

Re: LG 42LB5D No video or backlight, sound functions

"@aaronwt6 I checked the voltage on both side and it was 1.46 VDC on both sides." Is that with reference to the chassis ground? If that is the case, the main board is notsupplying the Voltage (12VDC) to run the T-CON.

[drumguy560]

Re: LG 42LB5D No video or backlight, sound functions

@budm Yes that was using chassis ground as the reference.

Thanks!

[mmartell]

Re: LG 42LB5D No video or backlight, sound functions

Yes a replacement main board solved my problem. I put the set back together without reconnecting that ribbon and gave it back to my neighbour.

[lookimback]

Re: LG 42LB5D No video or backlight, sound functions

I know this is old, but I just fixed one with these same symptoms. Maybe it'll help someone else. Found the TV open on the side of the road last week. Initial finding was that I had 5v standby, but nothing else. PFC ok, 399v when powered on. Swapped the PWM controller with a known good one, still nothing. Found R127 to be cracked, replaced it, still nothing. Scoped out Q109 and Q107, saw a signal on each. Noticed Q102 only had 140v at it's source (thinking it should be 399v), replaced Q102 and Q101 just because I had them in stock. Afterwards noticed that voltage fluctuates a lot. As I'm writing this, I'm thinking maybe I should try replacing Q107 since that's what switches Q101. Anyway, after playing around for a couple hours, I decided to just order one on eBay for $17.70 including shipping. Well, it came today and after installing it I had...nothing. Well, I actually did have something. I had a working PSU with all my secondary voltages and the realization that there was a second problem. Usually when this happens I chalk it up to a previous owner creating new problems while trying to fix an existing one. Remembering reading this thread, I decided to disconnect the ribbon cable which leads from the master inverter to the Tcon. This resulted in backlight but no picture. Next, I removed the shield from the Tcon and checked the fuse. It was good. Powered it back on and checked voltage at the fuse, it was 1.5v. Should have been 12v. Ok, now I'm certain its a main board problem. Started removing the shield from the main board and like magic, it starts working. Now, as I'm removing the rest of the screws, I'm thinking maybe a pinched wire or something. Finally get it open, don't see any problem. Then I touched the wiring harness closest to the LVDS connector, it's the one which connects to P202 on the PSU. That made it shut back off. Wiggling it made it cycle on and off. I pulled the main board and found cracked solder joints on that connector. Resoldered, reinstalled, and it's fixed. Now, the only problem is some very noticeable water stains behind the screen. Hopefully, I can pull it apart and clean it without damaging it because it's perfect in every other way.