LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter scenes

[Stimpers]

Hello, good to be here. Thanks for having me!

I'm a TV repair novice with IT support skills, and some experience in building PCs. Unfortunately I don't own any specific TV testing equipment, but I can probably do some constructive damage with the right assistance.

It is urgent that I get my TV working again asap, so I'm resigned to throwing some money at the problem. I have to spend Christmas alone this year, I can't afford the TV to pack up, mid Bond film!!

The set is two years old, but I acquired it in full working order 2 months ago. A few days ago it developed the following intermittent fault:

The output turns black (backlight stays on) for a split second, before returning to normal picture.
-The blink only occurs during certain bright or 'pixel busy' scenes.
(Interestingly, when I pause on a frame which is causing a blink, the duration between blackouts are precisely the same. On one occasion, every 13.9 seconds. Another time, every 3.9s. Another, every 8.0s.)
-Turning the backlight/brightness down will stop the blink
-Occurs on all sources - HDMI, USB, RF
-No break in audio, or loss of any other functionality

I carefully removed the rear panel and had a look.
-No bulging/leaking capacitors on power board
-No visible damage on any boards
-No loose connections. (I carefully dis/reconnected all)

Photos provided were taken before I registered, for my own reference. I don't have a pic of the entire chassis, but at the time I took pics of all four visible boards. If you would like higher quality pics I will happily dismantle and take some more.

Is the AV board the most likely culprit, as someone else advised?
If so, I would really appreciate knowing where I could find the rare replacement. EMOS are checking, can't find any on eBay.

If not, which part should I try to replace first?
Can't tell you how helpful your input is, guys. Thank you in advance.

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

I wouldn't immediately suspect the main/AV board. Those two primary (400V) capacitors can cause this problem. Though they usually cause backlight blinking...

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Thanks Tom. So you think I'd be wiser to replace the power board first?

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

You can try to check it by testing voltages.
A question -- if the audio is very loud does it also cut out with the picture?

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

OK, maxed the volume on a loud file - no audio cut out when blink occurs.

I don't have voltage testing equipment, but could potentially pick some up.

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

You can get a half-decent DMM from Maplins for about £5.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Ok thanks, Ill pick some up tomorrow. Let me know if I should get anything else, cheers.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Ok, I am equipped with DMM - I will read up on how to test, more advice welcome! Cheers.

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Can you list the pin-out on the connector going to the main board from the PSU, it is labelled next to it.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Sure thing:

2/1 :: Power-on :: NC
4/3 :: GND
6/5 :: GND
8/7 :: 5.2V
10/9 :: 5.2V
12/11 :: GND
14/13 :: 12V
16/15 :: GND
18/17 :: 20V
20/19 :: ON/OFF :: NC
22/21 :: ERROR :: ADIM
24/23 :: PDIM1 :: PDIM2

TV prepped for further poking around.

Haven't used a DMM before, so advice welcome on how to test voltages etc- I'll research in the meantime.

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

OK, voltages of interest are 5.2V, 12V and 20V.

First turn off hold on your meter by pressing the hold button until the "H" goes away.
Set meter to "V=" 200. Put the black probe in COM and red probe in V/ohm/mA/->|-.

Place the black probe in the TV chassis -- exposed screw hole is a good place.

Place the red probe on each pin of interest, only need to do one for each voltage as they should be identical. To measure the pin you need to get the metal edge around the pin, with the connector still plugged into the main board and PSU.

If you find the voltages to be dropping out as the screen goes black then the PSU is suspect.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Thank you Tom.

I am clear on all points, except how to connect the metal edge of the pin whilst connectors are plugged in.

I'm YouTubing guides while we go. Close up pics of PSU/Main connectors/boards:

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

It's tricky but just around the wire going into the connector you want to touch the probe against that. The metal should be exposed very slightly.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Sorry for the delay. Typically, once I had dismantled and set up, the problem took an hour to reproduce! Ugh.

Results:

8/7 - 5v = fluctuates between 10.7 to 10.8 (no change during blink)

10/9 - 5v = fluctuates between 10.7 to 10.8 (no change during blink)

14/13 - 12v = fluctuates between 26.3 to 26.5 (no change during blink)

18/17 - 20v = 45.8 to 45.9 (jumps to 46.2 during blink)

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

You meter must be on the "V=" not "V~" setting.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Noted!
(On my meter, closest symbol to '=' is a line with 3 dots underneath. I assume this is the one)

Results:
8/7 - 5v = 5.2, no change during blink
10/9 - 5v = 5.2, no change during blink
14/13 - 12v = 12.3, no change during blink
18/17 - 20v = 21.1 to 21.2, jumps to 21.3 during blink

On a separate note, maybe irrelevant:
I can hear the distinctive, very faint buzz of the screen during operation. (Same buzz I've heard from every healthy LCD).
During blink, the buzz - which is almost definitely coming from this board - cuts out.
As previously stated, backlight is definitely operational 100% of the time. The display transmits 'black'.

[selldoor]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Instructions for your meter are here
http://uni-trend.com/manual2/ut30BCD.exe


"During blink, the buzz - which is almost definitely coming from this board - cuts out."

Very carefully remove, clean and replace the flat ribbons going to that board

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Thanks selldoor - I have some isopropyl alcohol, I'll power down and try as you suggest.

[tom66]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

If selldoor's suggestion fails, test the voltage at both ends of the fuse F1. If it is dropping out, then the problem could be on what is powering that board. Right now, the PSU appears to be OK.

[Stimpers]

Re: LG 47LD920 Display (not backlight) blinks off every 8 seconds during brighter sce

Thank you Tom,

Cleaned contacts as selldoor suggested, just waiting as precaution for alcohol to evaporate.

I'll assume I should test as before, V=200, and report on fuse F1 shortly.