Need Some Repair Advice LG 47LG50

[69sixpackbee]

Hiya all!

First of all, let me preface this post that I am fairly knowledgeable with a good deal of electronic repair and it has been geared at primarily vintage audio repair and I have a great deal of test equipment to my avail but, I am limited to the repair of LCD televisions aside from the "basic" concept and operation contained within.

With that out of the way...

I have a question regarding the repair of my LG 47" LCD that maybe you can help with. I get no pic and no sound. From the get-go, the standby LED is on as it should be then it flashes several times (approx 8 seconds) when switched "ON" and the blue (on) LED is illuminated but it is very, very faint. When it transitions from standby to on, the backlighting will "flash" and that's it. There is no backlight visible in a dark environment. I have read numerous ramblings about the usual maladies associated with the power supplies on these sets so that is what I first tackled. All of the voltages outputting from the power supply is good. The 24VDC, 16VDC, 12 & 5VDC are all good when being measured at the respective pinouts on the power supply board with all cabling attached. The green SMD status LED on the back of the main board flashes for a few seconds when starting up and remains on when it boots up. The illumination inside of the Opt-Vid port is visible as well. I still can power up the set with the remote so the comm. is still working. I checked for an image with the "flashlight" test but to no avail.

Before I start to throw parts at this dinosaur I am hoping that you more knowledgeable folks can give me a few pointers.

This TV is an LG 47LG50-UA

Thanx in advance for the help!

Bud

[Shinju]

Re: Need Some Repair Advice LG 47LG50

Welcome,

A couple of things here.

Please post photos as attachments of the back of your unit exposed, try to get clear photos of each board for the members here to help you identify bad parts easier.

Part numbers are nice but not required unless someone asks

Pictures are very much needed here

Good luck on the repair!

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Will do!
I have pics so far, of the board numbers. I did remove every board and looked at them under my microscope for any cold solder joints, blown pico fuses and blown/leaking caps but to no avail. All boards (P/S, T-con, Inverter/slave and main) "looked" good.
I will post some shots of the back of the set with the shields still removed from the boards.

Thanx!

[Locutor13]

Re: Need Some Repair Advice LG 47LG50

Hi,

I have a similar problem with a LG LCD 42LG50. Check the following video to see if it is the inverter board, it was the problem with mine. You have to do it at night or in a very dark room.

https://www.youtube.com/watch?v=TG3i_G8GQF0

Note: If the blue light is on your Tv is working without the backlight.

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Hi,

I have a similar problem with a LG LCD 42LG50. Check the following video to see if it is the inverter board, it was the problem with mine. You have to do it at night or in a very dark room.

https://www.youtube.com/watch?v=TG3i_G8GQF0

Note: If the blue light is on your Tv is working without the backlight.

...even if the blue light is so dim it is virtually undetectable?
I can only tell it switched to blue from viewing it from the backside of the board through the empty through holes!

[budm]

Re: Need Some Repair Advice LG 47LG50

Can we see the whole backside of the TV so we can see how all the boards are connected?

[Locutor13]

Re: Need Some Repair Advice LG 47LG50

Mine was dim too. Try it.

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Here are some updated images:

When it is on I cannot hear any HV raster that I would be familiar with (think CRT sets) and I'm sure by the twin XFRMR's on the P/S board would lead me to believe that this should be the case to fire the bulbs. Not sure what the arraingement is on this set (I am kinda in the dark still) but I would think it is some sort of EEFL. The service manual is pretty vague and the lack of any and all schematics makes it even worse!
I do not have the "proper" equipment to diagnose these LCD's

The one image shows the blue "ON" LED as viewed from the back. It is so dim you cannot see it from the front through the bezel surround even in a very dark environment. The "Stand By" and "Powering On" indicators are pretty noticeable, however.

BTW, those suspect looking traces on the ribbon cables running up from the T-con are okay. I rang them out for continuity and adjacent shorting and they are okay.

Thanx!

Bud

[budm]

Re: Need Some Repair Advice LG 47LG50

OK, TV unplugged, then disconnect the white flat ribbon cable between the T-CON and the MASTER INVERTER board (left board), then power up the TV to see if the backlights will come on.

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Okay, I will do that when I get back home this evening in about 3 hours or so and report back. In the interim, I have printed up you're AWESOME troubleshooting guides and I will be scrutinizing them intensely!!

...to be continued!

Thanx!

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Okay, I popped the ribbon cable from the header on the T-con and the screen is still dead.
.....Bud

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

I dove into this a bit more and I am sure it is not getting the signal to fire the bulbs. The power supply voltages are all good coming from the PSU.
I tried the CFL test with a good curly-que bulb soldered onto one of the headers and it was silent. I took a closer look at the ribbon cables that ran from the T-con up to the board on the top and on each one there were some very warm traces at one time as evidenced by the melted insulation on the ribbon cables. I did measure 12VDC B+ on two separate conductors going up to the T-con so it looks like the main board is switching the power up to it. Just a thought. I checked the 4 SMD fuses on each of the inverter boards and they are fine. The +24 is there but whatever circuit is "pulling it down" is not working to fire the XFRMR's from what I can tell. I checked the +24 right at the cables going to them with my "automotive" series-connected test lamps and the voltage is rock steady as is the +12 measured at the cable going to the main board.

Kind of at a quandry at this point.

[budm]

Re: Need Some Repair Advice LG 47LG50

What DC Voltage do you get at that ON/OFF (Inverter ON/OFF same as BL_ON) pin? It should be > 2VDC for backlights ON, and <1VDC for backlights OFF.

[selldoor]

Re: Need Some Repair Advice LG 47LG50

So is the 24v on each inverter board - can we have pictures of them?

It is also a worry that you say there is NO sound? Have you got a sound source
dvd playing or something?

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Okay, I made some more DMM measurements at lunch.
I took these readings from both of the cables that go to each inverter board.
BR1 = 1.1V
ON/OFF = fluctuates between 1.4V & 2.1V
PWM-DIM = Full 5.04V
ERROR = 3.2V

I also checked the 12V power again coming from the PSU. I snipped the one lead marked "12V" that goes between the PSU and the main board and attached the auto lamp with the bright filament only. I got a reading of 10.4V. The lamp would pulsate a bit and the reading would vary between 10.1-10.4V. There was voltage coming back up through the same lead from the main board so I must assume that there is possibly more than one lead supplying 12V to the main from the PSU ?
This portion of the PSU is supposed to be able to supply 2.25A and it seems pretty paltry in as that it is starting to puke powering a bulb and whatever else is was. Maybe this part of the PSU is weak?

Thanx!

Bud

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

So is the 24v on each inverter board - can we have pictures of them?

It is also a worry that you say there is NO sound? Have you got a sound source
dvd playing or something?

I had a DVD player attached to the input in the back of the set with a movie playing and was pressing the "INPUT" buttons slowly and repeatedly in hopes of finding the correct input but to no avail.

The attached image is on the connector that runs down to the inverter board.
There are four 24V leads and they were all identical readings.

Thanx,
Bud

[selldoor]

Re: Need Some Repair Advice LG 47LG50

I have seen that - you need to test all the transformers on each inverter board
8 or 9? I am signing off now but to test them you need NO power to the board and set your meter on ohms 2000. There are 8 pins you need to test

1------2------3------4


5------6-------7------8


1 to 5 and 4 to 8 on each.

Also there may be fuses and mosfets but I cant help without the pictures.


Did you test BOTH inverter boards with the voltage

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Will do!
Yes, I checked both boards with identical voltage readings. I will check the XFRMR's tonite.

I will take some closeups of the inverters as well.

Thanx again for the help!!

Bud

[69sixpackbee]

Re: Need Some Repair Advice LG 47LG50

Okay, here's an update.
I checked all 20 of the transformers and they all ring out at 1.06K Ohms between legs 1-5 & 4-8. Couldn't see anything awry on either board. Measured the voltages at the headers on the inverter boards and I am getting identical readings as stated before but, here's the kicker. After about 10 minutes of warm up the control voltages change! I then get the following readings:
BR1 = 2.05V
ON/OFF = 3.83V and steady
PWM-DIM = still 5.05V
ERROR = still 3.3V

When I first start it up the voltage at ON/OFF remains near 0V and soon after the CPU is done booting(around 5 sec.)it starts to fluctuate from 1.4-2v thereabouts. The voltage on the ERROR terminal remains at 0V until approx 10sec after boot up and then it locks into 3.3V.
Is it possible that the ON/OFF control voltage is not being input to the inverter within the given timeframe and therefore it gets locked into error mode?
Then, regardless if the correct start voltage is met later on, because it is in Error, it prevents the bulbs from being lit?

I still have the issue on no sound and no visible movement in the LCD.
I will post pics of the inverter boards when I am not using my phone.

Thank again for the help!

Bud

[budm]

Re: Need Some Repair Advice LG 47LG50

Do you have the LCD panel model number? I can look up the spec and it should give info on the T-CON and the Inverter board signals info.