Lg 55ga6400 trying to power on but turn off completely

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Right now the main is not sending the required signals (BL-ON and Dim) for the backlights to come on.

...thus why I guess I should have started a new thread. It's not the power supply board at all, right? I kind of suspected that when I found all the voltage rails good. Time for a new main board, I guess. Is there any way to determine what has failed on the main board? Finding diagrams of these boards is near impossible, though I did find a diagram of the power supply board on a russian forum yesterday.

Thanks,
John

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

How likely is it that replacing the main board will fix it? Is there something else that could also be damaged that would take out a new main board if I replace it?

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

The main board may fix the missing signal, but you cannot tell until you get the known working board install.
You can test the power supply nad the backlights circuits by first disconnecting both Dim control wires and the BL-ON wire, leave everything else connected, then install 1K resistor between Standby power supply output and the BL-ON pin on the power supply board side then power up the TV and turn it on, if the backlights will not come on then you will install another 1K resistor between PWM DIM pin and the standby power supply output pin, that should make the backlights to come on when you turn on the TV, if not then you have problem with backlights section too. If the backlights do come on and other functions and boards of the TV are OK the you should have sound and pictures and all other function as well.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

That's kind of what I figured. I did the resistor jump testing you suggested at P201. I depinned the connector and put a 1K resistor from the PWR-ON to the DRV-ON (BL-ON) pin. No backlight. I then put a 1K from the PWR-ON to the P-DIM and still no backlight. So, unless I did it wrong (feel free to correct me) I think it's toast. I'm not replacing backlights and a main board.

Thanks for all the help! I do it just for the learning and I learned a fair amount from this so it's very much appreciated.

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

Not PWR-ON pin, that is the receiving pin of the power supply board, if you are going to do that then you need to verify that you do have >2V on the BL-ON pin and on the PWM-DIM pin, that is why I suggest you to use the Standby power supply output pin as the Voltage source to simulate the signals.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

OK...where do I find the standby power output pin? I don't see one labeled that way. The PWR-ON pin is the only power pin I see. There are two 3.5v pins that are hot when plugged in. Are those the standby power you're referring to? I tried the same thing using those for power but still no backlight.

I also noticed that when I depinned the wire from the main board to the DRV-On pin at P201 on the PS board that the DRV-On pin on the power supply jumped up from 88mV to 4.35V. Is that significant?

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

That means this power supply board has pull-up resistor on the power supply side instead of on the main board side.
Most of the newer Samsung TV's have built-in resistors for doing the self test so the tech does not have to fumble around doing the force on in the field, all the tech has to do is to disconnect the cable between main and power supply to test the power supply and the backlights circuit. LG may have implement this function no in newer model.
So the dim pins also go high too? If all 3 are high then you see backlights come on.
You will have to check the Voltage at the LED connector PINS against GND next to see what you get when power is first applied to the TV, you will have to leave the probes in place while doing these test for each pin when you first turn on the TV.
See attached PDF for typical PS-ON, BL-ON, and DIM control on typical TV without pull-up resistor on the power supply board, the one with pull-up resistor will just have Open Collector output on the main board.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Yes, I get 4.35v on the P-DIM and DRV-ON but only when I depin the DRV-ON wire at P201 between the main board and PS board. If I simply unplug P201 it I lose the 12 and 24v power supplies as well having nothing on the P-DIM and DRV-ON.

Edit: A-DIM stays at 0v no matter what.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

The small board that appears to have the wiring leading into the display has a connector from the PS board to it and then has a 14 pin connector from the board into the display. That 14 pin connector has a red wire on each end at the #1 and #14 pins and then black in between. I've looked at the voltages on those. The red wires go to around 40V when initially turned on and then settle in at 24V. When turned off the voltages slowly drop to zero. The black wires go to 3.3V and stay there. Again, this is with the DRV-ON, P-DIM, and A-DIM depinned at P201.

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

The board in the middle of the back is the LED driver board, it has two boost converter circuits 9L1 , L2, and 4 MOSFET's on it, you can see two output filter green lytics caps (100uf 100v) on the right side of the board, one cap per each boost circuit.
The panel has 12 LED strings, 6 string per one booster.
I expect the backlights to at least come on for seconds if the boost converters are good, you have 12 strings so n way all 12LED strings will be bad, and you have two circuits so not likely both to be bad at the same time.
Looking at your pictures I see a small connector from the T-CON to the LED driver board, that connector is for zone dimming control. so try disconnecting that connector to see if the backlights will even flash on.
24V is the input Voltage to boost converter circuits. 40V output seems low but I cannot tell what that Voltage should be since I cannot see the output Voltage spec printed on that LED driver board, but at least the boosters are working otherwise you you will not have higher output Voltage than the 24V feeding Voltage.
There are two fuses on that LED driver board also.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Got it. So I still have the DRV-ON and P-DIM depinned at P201 (A-DIM is plugged back in but I think you mentioned it's probably not used and I get no voltage on it at any time.) As suggested, I unplugged the small cable from the TCON to the driver board and turned it on. I am now getting a backlight that comes on and stays on. Progress! Is this indicating just the failed main board? It's probably worth a new main board but I just don't want to take a chance of damaging a replacement main board. If I plug in either the DRV-ON or the P-DIM then the backlights will not come on.

I saw those fuses on that board and actually checked them yesterday. They're good and the filter caps have low ESR (tested with my Blue ESR meter while in circuit only).

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

Once you have light source then if all other functions of the main board, T-CON, panel are OK then you should see pictures and hear the sound, if not then you have more than just bad DRV-ON signal problem.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

All I have is the backlight. No picture and no sound.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

I'm reading through your TV troubleshooting guide again (it always helps to read again after you learn more about the systems) and I'm getting the the very dark screen with no pics and no sound and almost like there is no backlight but the backlights are working.

Is that indicating a problem in the TCON board as well? (per #4 in the guide) I guess I better pull the cover on that board.

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

I expect the sound to be present if the main board is good. If you have scope then you can look at the LVDS signals from main board to T-CON board, see if they are present or not. There is also SMD fuse on the T-CON for the 12V, check that. Then there are many Voltage test points on the T-CON too that needs checking.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Attached is a pic of the TCON board. I see lots of very small test points on it and I could certainly probe them with a scope but without some service info/diagrams how do I know what I'm looking for? How fast are these signals?

Also, I'm a little confused at the progress so far. Am I correct in assuming that my power supply is good, my display driver board is good, and my backlights are good, and that I am now just trying to determine if either the main board is bad or the TCON is bad, or both? With no DRV-ON or P-DIM signal does that already condemn the main board and now I need to determine the TCON board status?

I really appreciate the help. Even if I ultimately don't fix it I've learned a lot already. Thanks!

John

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

I've looked all over the main board and can't find which wire should be the 12v output from the main board to the TCON board. It has to be on one of the ribbon cables but the terminals are so tiny I don't want go probing every one and possibly short something out.

I have probed all over the TCON board, mostly on each side of the larger surface mount caps and can find no voltage anywhere on the board. I cannot find a fuse anywhere on the TCON board, either. Would it be a good assumption that power is not making it from the main board to the TCON board? I've got good 12v supplies going into the main board and I found a couple of caps on the main board that have 12v on them so there is 12v on the main board but I'd like to try and figure out which pin coming off the main board to the TCON board should have 12v on it. Is this the right direction to be heading? If there's no power coming from the main board to the TCON board then the main board is highly suspect as the problem.

Without power on the TCON board I can't know if it's any good or not. I did check for shorts across the caps on the TCON board and didn't find anything obvious.

[vinceroger69]

Re: Lg 55ga6400 trying to power on but turn off completely

between the bottom two connectors theres a diode d111 just above theres a device f101 i think thats the fuse what voltage do you get on both sides of it

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

Right now you just want to verify if the T-CON is getting the switched 12VDC from the main board for it to run or not by verifying that you do have 12VDC at both end of the SMD fuse on the T-CON board with ref to GND.
The test points we need to check are DC Volatage test points, I.E. V1D2 which will be 1.2V power supply, V3D3 which will be 3.3V power supply, ect.
Right now we know that the LED driver board and the power supply board are OK, but we do not know if the T-CON or the main board is OK or not, the failure of T-CON could have triggered the error to tell the main board not to send control signals for backlghts but I do not know 100% for sure. We can only do so much without having details info about this TV model.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

between the bottom two connectors theres a diode d111 just above theres a device f101 i think thats the fuse what voltage do you get on both sides of it

Thank you! Sadly, eyesight is the first to go when you start to get older! I looked that board over more times than I can count and did not see the F101 plainly (though very small) written there. There is no power on either side of that fuse so as I suspected there is no voltage going to the TCON board. The fuse appears to be good and there is no short to ground on either side of the fuse.