LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

[Dannyx]

Good day folks. Got this LG 42ld750 that wouldn't fire up: the led at the front would just stay red and fade in and out. Taking some measurements on the power supply quickly revealed it to be (apparently) fully functional: got 3.5Vsb on the "standby" pins, which I then applied to "pwr ON" and then I got the rest of the voltages coming up OK (including the usual 390v on the large filter caps). To my mind, it's the inverter that doesn't function properly: I've got around 3.5v showing on the ERROR pin of the connector going to the main board and applying 3.5v to the "inv ON" pin doesn't turn on the backlight either. This supply is kinda "smart" and signals the main board not to turn on with a faulty backlight, so I can't even perform the "flashlight trick" to know whether the main board works or not. I think I can cut the ERROR wire and that would get the set to turn on, but there still wouldn't be any backlight. I'm not too sure where to go from here: is it a bad HV transformer, a bad lamp, or something else entirely that's causing an "ERROR" voltage to show up ? The first thing I'll try is measure the two transformers with an ohm meter and see if either of them has a lower resistance than the other - that would indicate where the fault lies. If that doesn't yield any definitive results, looking on the back of the board reveals a large IC with many pins which is most likely the backlight controller. Don't know what chip it is though, since I haven't looked into it yet, but I'll try finding and bypassing its protection/shutdown function to see if that gets the set to power on (or at least the backlight to come on when I "jump-start" the thing). What do you guys think ? I've attached pictures of the power supply front and back (not very clear this time, I admit). A new power board would be much too expensive, not to mention risky, if the fault turns out to be one of the CCFL lamps !

[CapLeaker]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

do the backlights come on at all (2 seconds to black) or nothing at all. Can you see the menu etc shining a flashlight into the screen?

Resolder the CCFL transformers, they look like they are having bad solder joints. Matter effect it looks like there isn't any shortage of cold solder joints on the back of that board.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

do the backlights come on at all (2 seconds to black) or nothing at all. Can you see the menu etc shining a flashlight into the screen?

Resolder the CCFL transformers, they look like they are having bad solder joints. Matter effect it looks like there isn't any shortage of cold solder joints on the back of that board.

No, they don't seem to come on at all and I explained further down in my post that the main board refuses to power on with the ERROR voltage present, so no, I can't see anything on screen either (yeah, I know I tend to write a lot and people may not have the patience to read the whole block). How can you tell it's got bad joints ? The picture's pretty sh!t

[CapLeaker]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

You have to find out if the main is sending PSon and BLon. If you don't have both, or either one of them, force the PSU on with the backlights connected. Bad solder joints have usually rings. There can be solder joints that can look good and still be bad too. That happened to me before.

BTW get yourself a better camera then.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

You have to find out if the main is sending PSon and BLon. If you don't have both, or either one of them, force the PSU on with the backlights connected. Bad solder joints have usually rings. There can be solder joints that can look good and still be bad too. That happened to me before.

BTW get yourself a better camera then.

The camera is the one on my Galaxy S3 and takes pretty damn-good photos (I often prefer it over my DSLR at work). These ones turned out mediocre because they were taken in a hurry
Anyway, the power supply DOES turn on when I connect 3.5v to the "on/off" pin, but the backlight doesn't when I do the same with the INV ON pin. You're saying I should connect both pins at the same time to 3.5v ? Might work, considering that the ON/OFF pin is needed to get the supply to generate the voltage required the run the transformers, which in turn have to be controlled on/of.

[CapLeaker]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Yes you have to do PSon and BLon at that same time. That should power up the whole PSU and the backlights.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Yes you have to do PSon and BLon at that same time. That should power up the whole PSU and the backlights.

Or at least that's what it should NORMALLY do - we'll see....

[CapLeaker]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Yes you have to do PSon and BLon at that same time. That should power up the whole PSU and the backlights.

I did what you told me and the backlight came on, so the power supply is definitely OK. I think the main board is busted, as it's not sending any of the two control signals (INV ON and PS_on).

[CapLeaker]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

When you forced the PSU on, did you check that all other voltage rails are up and running as well? If it does, undo the "PSU force on" and put the main board back. Turn on the TV and check PSon and BLon pins. If one is missing, your main board has a problem.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

When you forced the PSU on, did you check that all other voltage rails are up and running as well? If it does, undo the "PSU force on" and put the main board back. Turn on the TV and check PSon and BLon pins. If one is missing, your main board has a problem.

BLon is the only one missing (hence no backlight). PSon is present and so are all other voltages when the main board is connected. No dice though: the red LED still fades in and out indefinitely and no picture would come on screen....bad main board for sure. Probably too difficult to repair though :|

[CapLeaker]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Slap a new main board in and save yourself a big headache.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Slap a new main board in and save yourself a big headache.

Of course (provided the chap is willing to pay the price)

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I have a similar issue, LG 42LD750 stand-by LED is slowly flashing, cannot turn the TV on, no audio.

Voltage on power source pins is:
3.4V Error pin
3.34V P-DIM
3.33V P-DIM2
0.34V A-DIM
0.037V INV-On/Off
3.41V standby
3.24V Power On
20V 20.5V
12V 12.26

386V on mains 82uF capacitors

Looks like a failed main board, right? Is anybody able to determine based on these nubmer what I should check on the mainboard?
Of course I could replace the main board but it doesn't worth, the TV is quite old, time for replacement. But if there is any chance to fix it, I'd like to do so.

[dick_barton]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

With the mainboard removed from the power supply you could carry out the same test as in post #6 and check that the backlights and supplies come up.

[budm]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Since you do not have BL-ON signal form the main board, you should also check and see if you do have switched 12VDC feeding the T-con board or not because you may have more than just BL-ON problem.

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Thank you guys and sorry for my late reply.
All supply 12V voltages connectors have approximately 12V on the PSU. I checked the wires connection leading from PSU on the mainboard as well.
Not sure if I can force 3.5V on inv on/off pin (potential damage to the mainboard?)
Attaching some images of the TV plus voltages on the connector from PSU to mainboard.
I haven't inspected the T-Con yet since it's inside the TV frame, I'll try to remove it out later. Looks like there are 2 separate boards for T-Con.
There is no audio as well but this is said by the low A-Dim voltage.

[budm]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Your main board has integrated T-CON.
I need good clear closed up pictures around the two connectors of the main board that go to the LCD panel.
To simulate BL-ON signal you will have to disconnect just the BL-ON wire going back to the main board first before forcing on the backlights using 330 ~ 1K resistor between BL-ON pin and 3.3V standby of the power supply board.

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Here are some detailed close-ups in original non resized photos:
https://photos.app.goo.gl/v8vkNmFQmjCD4JY8A
I'm going to try the test.

Zero voltage on the contacts marked in red when the main board is powered on: https://photos.app.goo.gl/8A85oQty4MAHEJQr5
Checked all 12V pins still have correct voltage on the 24 pin connector on the mainboard (from PSU).

[nomoresonys]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Read post #18, do test, report results.