LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

[DXseekerMO]

Hello everyone!

Here's the run-down on this TV. Pictures posted below.

* LG TV model# 50UH5530
* Symptoms: Turned off by itself and appeared to be unresponsive.
* After power reset TV tries to turn on with backlight flash
* Occasionally will turn on, boot and show picture/sound.
* TV only turns on when 13V pin reads 10.2V stable in stand-by, with main board connector attached or detached
* When power button is pressed and stable 10.2V is present 13V pin comes up to 13.2V stable
* When it shuts off by itself 13V pin shows unstable voltage from 8-12V
* The problem occurs on 3 separate power supplies and 2 separate main boards
* Backlights are less than a year old, ShopJimmy.com replacements
* Backlight settings have been at 66 (on a scale of 100) since new backlights installed.

Ordinarily I see a backlight flash on these and expect the backlights to be bad....but they're not even a year old. The TV actually worked for a day-and-a-half for me, came on, picture, sound, but eventually faulted and went back to the symptoms above.

So the band is stumped here.....does anyone have any suggestions or ideas?

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Further observation:

With the original Power Supply installed voltage at the 13v pin in stand-by is a stable 10.2v. When the TV is powered on voltage raises to a stable 13.2v until after the backlight flash, then voltage at 13V pin drops to 7v stable briefly then becomes unstable from 8-12v.

I have to be missing something here. I'm now wondering if what I'm seeing is this TV's way of going in to failsafe mode with a backlight fault?

I'll be surprised, real surprised if those SJ backlights failed. If they have it's a first for me with SJ LED strips!

[CapLeaker]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Turn the backlights all the way down and retest.

[nomoresonys]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

3 power and 2 main, thinking it must be in the backlight circuit, maybe a connection, could be an led too??

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Turn the backlights all the way down and retest.

Hi CapLeaker, thanks for the reply. I tried that but couldn't see the menu enough to make it happen.

3 power and 2 main, thinking it must be in the backlight circuit, maybe a connection, could be an led too??

Hey nomoresonys, how's it going? Yes I'm sure as well it's in the backlight circuit. After my friend dropped it off it worked for about a day and a half then faulted again. Just like so many of these I've worked on it has one weak LED. Connections look good.

What threw me was the unstable voltage on the Power Supply. I've heard of problems on the 13v rail on these, but for low voltage not unstable. Bottom line I got this one wrong.

I'd still love to know why the 13v goes unstable after the fault?

[CapLeaker]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Try something different like forcing the psu to turn on without anything connected to it, besides power mains.

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Try something different like forcing the psu to turn on without anything connected to it, besides power mains.

I did that as part of my diagnostic. With AC power applied there was 10.2v on the 13v line (expected) and was initially stable. Then I hooked the main board/backlights up and monitored the 13v pin while turning the TV on. The voltage came up to 13.2v as expected until the backlight flash, right after it dropped to 7v and shortly after became unstable. I then unhooked the main and backlight connectors, retested the 13v pin and it was still unstable.

I performed a "power reset" on the PS (unplug for an hour) and re-tested with the same findings/results.

I'm wondering/thinking when the LED Driver IC gets the wrong return current and shuts off the backlights if there is some feedback from somewhere causing the unstable voltage. I traced the 13v pin to the hot side of the PS and found unstable voltage all over the place, reference main cap ground.

This is the mystery to me. Why the unstable voltage with a backlight fault? Inquiring minds want to know! lol

[R_J]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

You might have a bad, C5 or C14, these caps filter the vcc supply to the primary ic's, if they are weak, the VCC voltage can drop and the ic goes into standby/burst mode. This vcc voltage is also supplied to the optocoupler (photo cell side) so it can make the power supply think it was turned off

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

You might have a bad, C5 or C14, these caps filter the vcc supply to the primary ic's, if they are weak, the VCC voltage can drop and the ic goes into standby/burst mode. This vcc voltage is also supplied to the optocoupler (photo cell side) so it can make the power supply think it was turned off

Thank you R_J! If this is the case and those caps are weak would there be a measurable change on one side of the opto, say maybe a voltage change/fluctuation I might be able to see with a meter?

[R_J]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

There likely is a change in the vcc voltage, check it across C5, it is the easiest to get at from the top. I was looking at a simlar schematic [168P-L5L01F-00] plus the trace side of this board, the vcc voltage may not connect directly to the optocoupler, It is hard to trace past R65 on the board.

[CapLeaker]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

You might have a bad, C5 or C14, these caps filter the vcc supply to the primary ic's, if they are weak, the VCC voltage can drop and the ic goes into standby/burst mode. This vcc voltage is also supplied to the optocoupler (photo cell side) so it can make the power supply think it was turned off

He already used another PSU. Don't think 2 different PSU's should have the same problem.

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

There likely is a change in the vcc voltage, check it across C5, it is the easiest to get at from the top. I was looking at a simlar schematic [168P-L5L01F-00] plus the trace side of this board, the vcc voltage may not connect directly to the optocoupler, It is hard to trace past R65 on the board.

Should I check C5 using the DC scale?

He already used another PSU. Don't think 2 different PSU's should have the same problem.

I thought the same....but one never knows. I actually tried 3 of them. If it is a PS problem it would be nice to trace it down.

[budwich]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

does an LG have some "force on" sequence equivalent to sharp / vizio panels?

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

does an LG have some "force on" sequence equivalent to sharp / vizio panels?

If it does I don't know the procedure, but would try it if explained to me.

[R_J]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

If you tried 3 different power supplies with the same result, then it is very likely that an led on the strips is bad. and just because the came from shopjimmy does'nt mean they can't fail. Bet SJ gets them from ..... China

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

If you tried 3 different power supplies with the same result, then it is very likely that an led on the strips is bad. and just because the came from shopjimmy does'nt mean they can't fail. Bet SJ gets them from ..... China

Yes I bet they do! Besides this model TV seems to be hard on LEDs anyhow.

It's going to be "backlight mania" around here next week. I'm ordering a total of 5 sets for different TVs.

I'll update when LEDs installed!

Thanks everyone for your input!

[DXseekerMO]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Ok so here's how it turned out:

The backlights arrived today....exactly 48 hours after ordering. ShopJimmy is awesome!!!

I stripped this TV down to its skivvies and powered the TV on with the LCD/diffusers removed....the TV turned on....and remained on for 3 hours until it faulted. I tried to fire it back up and the bottom 2 rows of backlights did not flash. I confirmed it 5 times.

After that I detached the electrical connectors from the bottom 2 rows of LEDs, tested them with the backlight tester on low and high current....and they all lit up. It's like deja vu.....I had a different TV of this model that did the exact same thing. I had to ask myself...."what am I, a sucker?" lol

Since the bottom 2 rows failed to light I replaced them first with the new strips. The TV fired right up 5 times in a row. I installed the remaining new strips and it's now in testing.

So I guess the 13v rail falls all to hell when the backlights fail. Lesson learned!

[nomoresonys]

Re: LG 50UH5530 intermittent unstable 13V on 3 separate power supplies, band stumped

Nice, good to know about that 13v rail, I'll be watching out for that failure symptom.