LG 65UH5500 Momentary Flash Of BackLights

[Michale32086]

This is the 3rd TV today that has this issue..

Momentary flash of the backlights.. Display is fine, just not backlit..

My first instinct is that it's a bad backlight strip... But that hasn't worked out for me so well recently, so I decided to do some more checking before I break....er... break open another display panel..

Glad I did..

With my LED Tester, I was able to determine that the LEDs are good... I did some measurements on the Power PCB and the middle Inverter Board...

Let me see if I can map this out coherently..

On the Power PCB, the top 4 pins show 90V and the fifth one is marked NC... When the Power PCB is plugged into the Inverter Board, I get 55V on all 5 pins.. The 4 marked 90V and the 5th pin marked NC...

When I check the pins WITHOUT the Inverter Board plugged in, I get the 90V on the 4 pins marked 90V and 0V on the 5th NC pin..

I should note that the Inverter Board connector is marked 80V on the first 5 pins and shows 55V when the TV is powered up..

Does all of this indicate a bad or shorted Inverter board??

[Michale32086]

Re: LG 65UH5500 Momentary Flash Of BackLights

I should also note that on the Power PCB, pin 1 (STB) has 3.5V and pins 2, 3, 4 that SHOULD be 13.2V is 7.5V.....

[Andrew F. Ali]

Re: LG 65UH5500 Momentary Flash Of BackLights

First of all, If this TV is a LED TV that is an LCD TV with LED backlights then that thing you are calling an INVERTER is really called a LED DRIVER. Inverters were found in the older LCD TVs that had CCFLs as backlights.
So if you disconnect the connector from the PSU to the Main Bd. does the backlights come ON and stay on?

[Michale32086]

Re: LG 65UH5500 Momentary Flash Of BackLights

First of all, If this TV is a LED TV that is an LCD TV with LED backlights then that thing you are calling an INVERTER is really called a LED DRIVER. Inverters were found in the older LCD TVs that had CCFLs as backlights.

DOH!! Yea, my bust... It definitely is an LED...

So if you disconnect the connector from the PSU to the Main Bd. does the backlights come ON and stay on?

The Main Board is not directly connected to the PSU... The LED Driver board is in between... But when I disconnect the Main Board from the Driver board, I get nothing. No flash, no nuttin..

[Andrew F. Ali]

Re: LG 65UH5500 Momentary Flash Of BackLights

OK. so disconnect the connector from the Driver to Main Bd. and see if backlights come on.

[budm]

Re: LG 65UH5500 Momentary Flash Of BackLights

OK, so I am a little confused.
Have everything connected
1) When TV is OFF, What Voltage do you have on 80V pins of the power supply board.
2) When TV is turned on what do you have on 80V pins?
3) When TV is turned on, what do you have on PS-ON pin on the power supply board?
4) When TV is turned on, what do you have on DRV_ON on the LED power supply board?
5) What Vf readings do you get on all six LED strings?

[Michale32086]

Re: LG 65UH5500 Momentary Flash Of BackLights

OK. so disconnect the connector from the Driver to Main Bd. and see if backlights come on.

Nothin...

[Michale32086]

Re: LG 65UH5500 Momentary Flash Of BackLights

OK, so I am a little confused.
Have everything connected
1) When TV is OFF, What Voltage do you have on 80V pins of the power supply board.

Funny thing.. I can't turn the TV off... Either by remote or by holding the power button.. The TV goes off then comes back on...

2) When TV is turned on what do you have on 80V pins?

The 80V pins (on the DRIVER board) are at 55V...

3) When TV is turned on, what do you have on PS-ON pin on the power supply board?

On the Power PCB, it's marked STB (marked PS-ON on the DRIVER board) which is 3.3v

4) When TV is turned on, what do you have on DRV_ON on the LED power supply board?

The only DRV-ON is on the DRIVER board and it's at .9V

5) What Vf readings do you get on all six LED strings?

.6v

[budm]

Re: LG 65UH5500 Momentary Flash Of BackLights

Funny thing.. I can't turn the TV off... Either by remote or by holding the power button.. The TV goes off then comes back on...




The 80V pins (on the DRIVER board) are at 55V...



On the Power PCB, it's marked STB (marked PS-ON on the DRIVER board) which is 3.3v



The only DRV-ON is on the DRIVER board and it's at .9V



.6v

That must be an error with Vf reading of the LED string, they cannot be 0.6V if each of the LED string lighted up using the LED tester/ What kind of LED tester is it?
BTW, if you cannot turn the TV off, then at least you do have main board problem, plus DRV_ON signal also looks too low to turn on LED power supply.

[Michale32086]

Re: LG 65UH5500 Momentary Flash Of BackLights

That must be an error with Vf reading of the LED string, they cannot be 0.6V if each of the LED string lighted up using the LED tester/ What kind of LED tester is it?

See attached pic

BTW, if you cannot turn the TV off, then at least you do have main board problem, plus DRV_ON signal also looks too low to turn on LED power supply.

Yea, it's beginning to look like a Main Board issue??

[fcisolutions]

Re: LG 65UH5500 Momentary Flash Of BackLights

Do you think it is the mainboard or the driver board. I am getting the same issue. Can you get any sound .

[budm]

Re: LG 65UH5500 Momentary Flash Of BackLights

See attached pic




Yea, it's beginning to look like a Main Board issue??

Then the Vf of the LED string cannot be showing 0.6V and light up.
That built-in Volt meter will show what the Vf of the LED/LED string are.
I.E. if the LED string has 10 3V LED then the Vf showing on the meter should be 30V, but since that LED puts out about 30mA, then the Vf will be a little leas than 30V.

[budm]

Re: LG 65UH5500 Momentary Flash Of BackLights

BTW, since you do have PS-ON, what DC Voltage do you have between the two legs of the main filter caps?

[Alastair E]

Re: LG 65UH5500 Momentary Flash Of BackLights

Classic symptom of an O/C led in the panel. I did a 47" LG today exactly the same issue.

It only takes 1 to be dead to cause 1 sec to black on these LG's....

[nomoresonys]

Re: LG 65UH5500 Momentary Flash Of BackLights

Hmm, try unhook leds, then see if tv turns on and off with power button.

[nomoresonys]

Re: LG 65UH5500 Momentary Flash Of BackLights

You say all leds lit up with proper voltage with your tester or you couldn't tell if they did?

[dick_barton]

Re: LG 65UH5500 Momentary Flash Of BackLights

I should also note that on the Power PCB, pin 1 (STB) has 3.5V and pins 2, 3, 4 that SHOULD be 13.2V is 7.5V.....

I think I would be looking to see why there's only 7.5V and not 13.2V before doing anything else.

[budm]

Re: LG 65UH5500 Momentary Flash Of BackLights

The 7.5vDC for the 13V STBY may be normal in standby mode, since there is no separated standby power supply so they may keep the Voltage low just enough to run the main board, and when PS-ON is present and then the PFC Booster circuit kicks in then the 13V and the 80 will be normal.
That is why I ask OP to see what DC V he has on the main filter cap since the PS-ON is present.

[nomoresonys]

Re: LG 65UH5500 Momentary Flash Of BackLights

I think I would be looking to see why there's only 7.5V and not 13.2V before doing anything else.

.

[Michale32086]

Re: LG 65UH5500 Momentary Flash Of BackLights

Thanx to all the replies.. I had grandpa duty that took me away from the shop early..

I'll try all the suggestions when I get to the shop first thing in the AM...