LG 47LN5400 Will not power up after re Cap

[Nevillet]

Hi All,

I am trying to repair the above TV belonging to a friend - who reported that the TV no longer powered up. With the exception of the front panel red LED flashing twice following power hookup and pressing the standby button on the TV or remote.

The only DC voltage I could measure were 3.5V on 2 pins of the main output connector (marked standby)

I opened the back and found a bunch of bulging caps close to the main DC outs on the power board.

After removing the board I measured high ESR in 90% of the caps which I then de-soldered and rechecked off board. The high ESRs were confirmed.

I replaced all the electrolytics on the board, checked and refitted. I now have the same symptoms as before - the TV will not power up and the only (continually on) DC voltages are the 3.5V as mentioned above.

If I measure the 12V & 24V o/p when I press the standby button there's no flicker of voltage.

I read that with LG TV's there is a software reset that has to be performed to clear the fault from the firmware - or the TV will not power up, after the hardware has been fixed.

The thread pertaining to this described clearing the fault on a different remote control and subsequently they do not work on my remote because the buttons do not match. My remote is a AKB73715603.

The thread mentioning the clearing of the fault is here:
https://www.ifixit.com/Answers/View/...+turn+on+my+tv.

Any thoughts or suggestions would be most welcome.

Thanks

[FrozenHaxor]

Re: LG 47LN5400 Will not power up after re Cap

Please post the part number of your power supply board.

[budm]

Re: LG 47LN5400 Will not power up after re Cap

Hi All,

I am trying to repair the above TV belonging to a friend - who reported that the TV no longer powered up. With the exception of the front panel red LED flashing twice following power hookup and pressing the standby button on the TV or remote.

The only DC voltage I could measure were 3.5V on 2 pins of the main output connector (marked standby)

I opened the back and found a bunch of bulging caps close to the main DC outs on the power board.

After removing the board I measured high ESR in 90% of the caps which I then de-soldered and rechecked off board. The high ESRs were confirmed.

I replaced all the electrolytics on the board, checked and refitted. I now have the same symptoms as before - the TV will not power up and the only (continually on) DC voltages are the 3.5V as mentioned above.

If I measure the 12V & 24V o/p when I press the standby button there's no flicker of voltage.

I read that with LG TV's there is a software reset that has to be performed to clear the fault from the firmware - or the TV will not power up, after the hardware has been fixed.

The thread pertaining to this described clearing the fault on a different remote control and subsequently they do not work on my remote because the buttons do not match. My remote is a AKB73715603.

The thread mentioning the clearing of the fault is here:
https://www.ifixit.com/Answers/View/...+turn+on+my+tv.

Any thoughts or suggestions would be most welcome.

Thanks

So you are 100% sure that you do not have any Voltages on other connector pins except the 3.5V standby, then that means the main board is not sending the PS-ON (PWR-ON) to turn on the rest of the power supply. You need to upload the pictures of the boards and the whole backside of the TV showing all the boards.

[neilc6]

Re: LG 47LN5400 Will not power up after re Cap

I have yet to encounter bulged capacitors on LED TVs. LN series is prone to backlight failure. Have repaired dozens. On the non-smart versions, the backlight will flash on power up or plug in and will then shut down. On smart versions, you will see the LG logo.

AFAIK, there is no fault shutdown on this model that needs to be cleared. Possibly in OLED tvs but never encountered in LED models. LGs and Samsungs don't blink diagnostic codes either.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

The numbers on the power board are:

EAX64905501 (V2.3)
TE3RA62810801382 (3.1)







Have tried adding pictures via the insert image icon. I can see the tag but not the image. Here they are via plan B:

https://www.dropbox.com/s/rknul6e7b7...-edit.jpg?dl=0
https://www.dropbox.com/s/xu26iv78su...-edit.jpg?dl=0
https://www.dropbox.com/s/tibtfgq024...-edit.jpg?dl=0

Thanks all for your help. I will be away from this for approx 10 hours. thanks again.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

Hi All,

Any further thoughts?

[diif]

Re: LG 47LN5400 Will not power up after re Cap

Here you go.

[nomoresonys]

Re: LG 47LN5400 Will not power up after re Cap

Try with power cord unplugged, unhook cable from powerboard to mainboard, plug power cord in, see if backlights come on and stay on.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

nomoresonys: I followed your instructions but the LEDs did not illuminate. I measured the outputs for the 2 back light circuits and they were at 0V.

I've purchased a 'LED light source tester' (SID LED GJ3C) from eBay and have tested the LED's which illuminate. The tester automatically increases the op voltage. On one circuit the test voltage was 120V (29mA) the other was 72V (29mA) Not sure why the variance - poss my incompetence. What is the correct voltage/current for each back light circuit?

With the main outputs con disconnected I measured the main output connector pins which were all at 0V except for the 4.5V pins (2). Is there a way of emulating the start signal from the logic board to start the PSU board a see if it op's the 12 & 24V rails?

[petehall347]

Re: LG 47LN5400 Will not power up after re Cap

Here you go.

c257 appears in backwards and looks bloated on top

[R_J]

Re: LG 47LN5400 Will not power up after re Cap

petehall347 is correct, C257 is backwards, and no you can't just turn it around, you need to replace it again.
The fact that C247 is bulged like that does suggest the power supply DID turn on as that is the +76 volt line that supplies the backlight circuit.

And disconnecting the cable between the main and the power supply WILL NOT turn on the power supply or the backlights.

[petehall347]

Re: LG 47LN5400 Will not power up after re Cap

lets hope it didnt get powered up again after the pic . op might be sore from it exploding .

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

OMG! Thank you all so much!! TV Fixed!

I replaced C257 and we're in business!!.
BTW the bulged cap [47uF/160V]read 49.08uF ESR 0.50

BIG lesson learnt - thanks to you all.

I'll be back! Very best wishes.

[nomoresonys]

Re: LG 47LN5400 Will not power up after re Cap

Try with power cord unplugged, unhook cable from powerboard to mainboard, plug power cord in, see if backlights come on and stay on.

THIS is a simple initial test, used by "Karl Childers" and thousands of techs and hobbyists WORLDWIDE EVERY DAY, if the powersupply has pull-up resistors and nothing is wrong with the powersupply or backlight circuit the backlights WILL come on and stay on, THE END.

[nomoresonys]

Re: LG 47LN5400 Will not power up after re Cap

In this case it was a backwards cap so nothing good would happen til it was sorted no matter what resistors were in place, anyway nice job glad to hear its ok. By the way for those who don't know and it must be very few because it's so VERY OBVIOUS, hear is when you listen to something and here means right here in this place, so it's NEVER "glad to here it's fixed"

[neilc6]

Re: LG 47LN5400 Will not power up after re Cap

Turn down your backlight level on this model to around 60 on every input. And turn off Energy Saving which will crank up the backlight in a bright room.

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

Houston - We still have a problem....

I've been soak testing the set today since 'the fix' this morning. I thought that the set had turned it self off - but that was not the case.

The back lights has stopped working.

After taking the back off, there was a hot smell from the screen (through the holes).

The PSU board also seemed pretty hot, especially the large transformer.

I measured the voltages on the main connector - all correct: 12V, 12V, 24V,

The screen connector has 4 wires marked LED- & LED+ (A) and again LED+ & LED- (B)

I measured the voltages and current of these pins (with the back light connected)

Pin 1 LED- 21.8V
Pin 2 LED+ 78.6V
Pin 3 LED- 23.8V
Pin 4 LED+ 78.6V

Current between 1 & 2 = 9mA
Current between 3 & 4 = 13mA

I re-checked all of the electrolytics for polarity, ESR & bulging and all were good.
Also tested all of the big diodes, all were good.

Help please.

[budm]

Re: LG 47LN5400 Will not power up after re Cap

"Current between 1 & 2 = 9mA
Current between 3 & 4 = 13mA"
How did you go about measuring the Current between pins?
I hope you DID NOT put the amp meter probes on pin 1 and pin 2 (or pin 3 and pin 4).

[R_J]

Re: LG 47LN5400 Will not power up after re Cap

How did you check the current between 1&2 and 3&4? 9~13ma is not even close.
+78 volts suggest the led boost circuit is not on. Monitor pin1 LED+ when the tv is first turned on, it should read around 100v If it does start at 100v and drop to 78, it could be that you have a few bad led's in the panel and the circuit is going int protection.
I would think the LED+ would be around 100v and the LED- voltage would be closer to 1~2 volts when the led's are working

[Nevillet]

Re: LG 47LN5400 Will not power up after re Cap

The current measurements were with a DC clamp meter (UNI-T UT210E)

I can put a Fluke 85 series in series with the circuits to get more accurate measurements if required.

Using the automatic LED tester mentioned above the voltages for 1&2 = 94V (dimly lit) and for 3&4 80V (normal brightness)

n.b. Brightness being assessed via the screens rear holes.

Using the Fluke in record mode and in series with 1&2 the current max measures 28mA

Using the Fluke in record mode and in series with 3&4 the current max measures 256mA

This was measured with the TV off - then turned on using the hard wired buttons.

...............
I measured the following screen voltages (Fluke 85, manual range, record mode) using the same switch on method above.

On Pin 2, LED+, with the LED's connected the max was 175.2V and reduced to a steady 78.4V after 20 or so seconds
On Pin 2, LED+, with the LED's DISconnected the max was 173.0V and reduced to a steady 78.0V after 20 or so seconds

On Pin 4, LED+, with the LED's connected the max was 102.2V and reduced to a steady 37.2V after 20 or so seconds
On Pin 4, LED+, with the LED's DISconnected the max was 174.8V and reduced to a steady 78.0V after 20 or so seconds