LG 42LN5400 Works intermittently

[nrbrandt]

Hi folks, I'm a novice at TV repair and I need help diagnosing this set.

When you turn the TV on, it's hit or miss whether the back light comes on or not. When the screen does turn on, you can bring up the menu (I don't have anything hooked to it to know if the sound works).

Voltages from the power supply board to main board appear to be normal. The 24 volt pin registers 24.3. DRV ON registers 3.2 and PDIM1 and 2 are both 1.1v.

On the connector to the LED strips I get (from left to right on the connector) 112v, 28v, 74v, and 19v if the back light turns on. If not I get 56v, 0.1v, 56v, 10v.

Once you've gotten the TV to turn on, it usually won't come on again without being unplugged first.

Is this bad LED strips or else what do I need to check next?

Power supply board:
https://www.badcaps.net/forum/attach...3&d=1505188482

[dick_barton]

Re: LG 42LN5400 Works intermittently

The led voltages are given in the red box. Unfortunately it is out of focus.

[neilc6]

Re: LG 42LN5400 Works intermittently

Have replaced LEDs in about 5 TV's in the LN series. Default setting runs the backlight at 100%. Have a 42LN5700 with good backlights but not coming on and waiting for a new PS board. Usually the backlights will flash briefly and you will see the logo if there are bad backlights.

You can try turning down the backlight level when they are on.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

The led voltages are given in the red box. Unfortunately it is out of focus.

Input: 100-240V 2.0 Amp

Output:
High: 105v
Low: 70v

Should the voltage on the two positives wires match each other (same for the negatives)? Or are there a different amount of back lights hooked to each wire?

[R_J]

Re: LG 42LN5400 Works intermittently

Its a good chance the led's are giving the problem, Shopjimmy lists strips for this model and they do look like they have plugs on them which might be giving the intermitint problem.

[budm]

Re: LG 42LN5400 Works intermittently

The 56V is the feeding Voltage to both Boost converter circuits, the two are not the same because one string has more LEDs than the other string.
You should try having one LED + connected one at time to see if the screen will light up for seconds with having just one LED string connected. Right now I suspect you have bad LED string.

[Andrew F. Ali]

Re: LG 42LN5400 Works intermittently

Bad LED string in the backlight array.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

You should try having one LED + connected one at time to see if the screen will light up for seconds with having just one LED string connected.

What's the best way to do this? Take a dental pick and try to pop the wire ends out of the connector? Cut wires and solder them back together later?

[budm]

Re: LG 42LN5400 Works intermittently

What's the best way to do this? Take a dental pick and try to pop the wire ends out of the connector? Cut wires and solder them back together later?

You can lift one leg of the current sensing resistor for each LED string one at time:
I.E lift one leg of R804 then turn on the TV to see if the backlights will come on for seconds or not, the solder the lg back in place and then lift the leg of R822 and do the same test.

Is this the exact match to YOUR board?
http://www.shopjimmy.com/lg-eay62810...-led-board.htm

[nrbrandt]

Re: LG 42LN5400 Works intermittently

Is this the exact match to YOUR board?
http://www.shopjimmy.com/lg-eay62810...-led-board.htm

Yep.

It'll be a few days before I get a chance to work on this but I'll report back when I can.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

You can lift one leg of the current sensing resistor for each LED string one at time:
I.E lift one leg of R804 then turn on the TV to see if the backlights will come on for seconds or not, the solder the lg back in place and then lift the leg of R822 and do the same test.

Is this the exact match to YOUR board?
http://www.shopjimmy.com/lg-eay62810...-led-board.htm

Ok, with either resistor disconnected the TV comes on for just a second or so when you turn the power back on to it. It never comes on and stays on though.

[budm]

Re: LG 42LN5400 Works intermittently

Are you saying withe one resistor lift up one a time you do not see the backlights come on for seconds with one resistor connected one at a time?
We are looking to see which one of the two LED string will not come on test, we are NOT fixing the TV to make it stay on.
Post #1: 'If not I get 56v, 0.1v, 56v, 10v." that already indicating that one of the string has open circuit due to 0.1V reading. We are just verifying it.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

I was watching the front of the TV last night but I tested it again today looking for backlights through the screw holes on the back of the tv.

If both resistors are disconnected, the backlights flash for a split second.
When one of them is disconnected, the backlights come on for about a second. (behaves the same for either resistor)
When both are connected the backlight stays on, but doesn't turn on every time.

What does that tell us?

[budm]

Re: LG 42LN5400 Works intermittently

HMM, something is not right, when both resistors are lifted then the backlights should not come on at all.
But since you are having intermittent problem with the backlights then it is hard to be 100% sure to know what is really going on.
I also re-read your post #1: "Once you've gotten the TV to turn on, it usually won't come on again without being unplugged first." Unplugging first to be able to turn on the TV again may indicate problem on the main board. This is a tough one.
"If not I get 56v, 0.1v, 56v, 10v" This reading to me indicated backlight problem, i just hope that you are not having two problems.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

Yeah, I suppose I should have specifically stated at the beginning that sometimes have to powercycle the TV a few times to get the red LED to come on. That problem would be before the backlights can cause an issue, right?

Also sometimes when you plug the TV in turns the screen on without having to hit the power button.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

When I was troubleshooting this on my own I found this vid and wondered if it may be the problem. I'm not setup to cook a chip though: https://youtu.be/kbjNq1AyF00

[budm]

Re: LG 42LN5400 Works intermittently

One thing I see right now is this:
'Voltages from the power supply board to main board appear to be normal. The 24 volt pin registers 24.3. DRV ON registers 3.2 and PDIM1 and 2 are both 1.1v.'
The power supply board is getting the correct command signals (DRV-ON, DIM) from the main board to turn on the backlights so the backlights should have come on and stay on.
The power on signal is also present otherwise you will not have the switched 12V and 24v supplies.
That is why I am thinking that you may have power supply and main board problem due to the fact that sometime you do have to unplug the TV from the outlet.
BTW, these LN5000 series are know for bad main board which to fix it correctly the CPU needs to be re-balled. The LEDs are also known for going bad.
There are many thread for this model:
https://www.badcaps.net/forum/search...rchid=11545527

[sidds]

Re: LG 42LN5400 Works intermittently

I worked on 2 LN5400's last week. 1st one had a catastrophic failure in power-supply board, taking out the PFC IC, MOSFET, Fusible resistor and a few passives on the SMD component side. Got those parts from alibaba and replaced and the TV is good since then.
The other one had a bad T-CON board, caused by bursting of SMD Cap near a tiny FET or the other way round. Replaced the T-CON board that time as I could not identify the FET nor the SMD Cap.

[nrbrandt]

Re: LG 42LN5400 Works intermittently

Was that damage visible? I haven't looked at the Tcon board but the other power supply and main boards don't show any charred spots or bad looking caps.

I'm not really ambitious/knowledgeable enough to start popping off caps or transistors to start testing all of them.

[sidds]

Re: LG 42LN5400 Works intermittently

For the power supply problem, the fault was very obvious in that the fuse was fused ( ) i.e. open. Could not identify any shorts in the rest of the board or any other components but I think the PFC was already on its way out. Powered it up using a 60W incandescent bulb in place of the fuse and then the magic smoke and popping sounds appeared. Thankfully, those components which self destructed were readily and cheaply available as compared to the power supply board ( AUD 3 V/S AUD 30 + shipping for power supply board) and turned out to be a rather simple fix.
For the T-CON board issue, the components could be clearly seen as exploded. So I did not try to fix it but kept it as a spare as I am in need of the gamma IC for another TCON I am trying to repair which is having solarisation issue. Cost of TCON plus shipping AUD12.