Samsung LE40B750 flickering

[aki]

Hi All,

Hopefully you can help me solve an issue with my TV, which is only 1.5 years old. The screen has started to flash as if the backlight suddenly brightens.

This does not happen all of the time and sometimes the TV is stable for hours before this starts. Also, once after first turning on the TV it took several minutes for a picture to slowly appear but this has never happened again.

After having read some of the posts on these forums I decided to take off the back and check the capacitors. Although I was hoping to see bursting capacitors - all of them appear to be OK.

Question is now - could this be caused by something else or is it an early indication that the capacitors are on their way out?

For info the capacitors (13 in total) are all Samwha and rated at 105degC:

3x WB W7E 1000uF 25V
2x WB W7A 1000uF 10V
4x WL W7C 4.7uF 50V
1x WL WNC 3.3uF 50V
1x WL WOC 2.2uF 50V
2x WL W9A 100uF 450V

Thanks

Paul

[digge]

Re: Samsung LE40B750 flickering

Samwha aint the best there is, however i wouldnt say they are the worst either. Hard to say if there is something going on with the caps without measuring them. Measuring ESR in circuit might give a clue as if they could be bad or not, but if there is no visual signs its hard to guess whats wrong.

For datasheets on the caps you can go to their productpage
http://www.samwha.com/electric/templ...product23.html

If in doubt you could take a picture and post and have all of us have a look and see if we see something suspicious.

[aki]

Re: Samsung LE40B750 flickering

Thanks for replying.

I will try and get some pictures if I get a chance to take the TV apart again soon. I have a multi-meter which can measure capacitance - so I might get some readings for more information.

[aki]

Samsung LE40B750 flickering UPDATE

OK, it's been a while since my original post but I've had a chance to get some photos of the IP board of my TV.

To update - the TV appears to work very well from cold, but after 10-30 minutes the backlight begins to flicker quite badly and eventually goes out completely.

When starting from Standby mode the backlight does not light up at all??


I do not know if it is significant, but the heatsink marked HS806 (at the top centre of the board) is very hot to the touch - a lot hotter than any of the others.



In order to answer a few other questions I have measured the output voltages on the connector at the top left hand side of the board and these seem to be ok.

5.1V reads 5.22V
13V reads 12.78V

Output resistances on the 4 transformers (starting with the top one):
1:
0.988 kOhm
0.987 kOhm

2:
0.975 kOhm
0.978 kOhm

3:
1.013 kOhm
1.011 kOhm

4: (this is a different model number)
0.651 kOhm
0.650 kOhm

Hopefully someone can spot a potential problem, because I'm lost on what it could be. Anything else I should be testing?

Thanks

Paul

[aki]

Re: Samsung LE40B750 flickering

Also, a quick search in the internet has shown that there is a Revision 1.2 of this board. Anyone have any indication as to what may have changed? Just thought this might point towards what is likely the weak link in the board?

http://www.chsinteractive.co.uk/elec...ch-samsung.htm

[tom66]

Re: Samsung LE40B750 flickering

So the backlight actually gets brighter during the flickering? That's an odd symptom.

[aki]

Re: Samsung LE40B750 flickering

Originally I thought that was what was happening.
However, it is just the backlight switching from the current brightness to off.

[tom66]

Re: Samsung LE40B750 flickering

Originally I thought that was what was happening.
However, it is just the backlight switching from the current brightness to off.

In which case the problem is more difficult.

BTW it is normal for that particular heatsink to be hot as those two diodes carry the full current for the inverter and probably dissipate about 10W each.

I am unfortunately not very good at repairing inverters. With one monitor I gave up on the inverter, pulled it out and the bulbs, and replaced them with LEDs.

The £136 price for that power supply is extortionate (most go for about £20-50), so I will try to help where I can.

First check is to try unplugging some bulbs. Unplug all but the top connector. The TV will work but only the top quarter or so will light. See if it has the same problem. If it does, it's likely in the main input board, or the inverter control. Also try with the bottom connector, to reduce the chance of a bad bulb changing the results.

[aki]

Re: Samsung LE40B750 flickering

Thanks for the suggestion

Managed to get the bulbs disconnected, but it did not behave as you described:

Even with only one connector ALL the bulbs light up when the TV is turned on, but then all go out after about 2 seconds.
I've tried many combinations of bulb connectors and it seems that as soon as one connector is removed all bulbs will light up but only for 2 seconds.

This seems very strange - as soon as all connectors are replaced the backlights work fine until the flickering starts.


Paul

[jsog]

Re: Samsung LE40B750 flickering

Those are good pictures! Please take one of the bottom of the board. The failure after it heats up could be cracked solder joints on the inverter transformers. Or possibly somewhere else...

The inverter is smart, and that's why you get the "two seconds to black" when not all the bulbs are connected. It sees that something isn't right, and shuts down to protect itself. Don't connect or disconnect bulbs with the power on. If you have another LCD panel with the right CCFL connectors, the bulbs in it can be used to see if an individual bulb in the Samsung panel is bad.

[aki]

Re: Samsung LE40B750 flickering

Thanks for the reply jsog

As requested, I've attached some pictures of the bottom of the board which hopefully are good enough. There are some close ups of the worst looking solder joints, but to my naked eye they still appear to be ok. Hopefully someone can spot something.

[digge]

Re: Samsung LE40B750 flickering

Possible suspects imo (there might be many more of those)

Could be a cold solder joint somewhere. Try to get some kind of magnification and a bright light and go over the board. Even though your pictures are very good we dont get the "3d" of looking at it so it might be hard to spot if its not heating up and leave a trace behind (blackened board or similar).

Check the caps closely as well, some brands are known to bulge downward, dont remember if Samwha is known to do this but better make sure and check.

Someone else could fill in with more ideas.

[jsog]

Re: Samsung LE40B750 flickering

Those grommets/inserts make it harder to see cracks. A unit that stops working after it warms up suggests cracked solder joints, but capacitor characteristics change with temperature, too.

Replacing all the electrolytic caps except the two large 100uf 450V ones is cheap and would eliminate them as a possible problem.

Something I hadn't noticed before: the TV is 1.5 years old, but the power supply board has a 2008 manufacture date! That makes the chances of a recap fixing the problem even better.

PS: please use good, low-ESR caps. Anything sold by the badcaps.net store is good. Many people like Panasonic FM/FR/FC series, too. If you post your location, people can advise specific sources.

[aki]

Re: Samsung LE40B750 flickering

Thanks again for the suggestions.

I think I will get some new caps to replace these. I'm in the UK and I take it that http://uk.farnell.com is an ok source?

I take it that the newer FR series are the best option - or are the tried and tested FM/FC series the safer option?

As for the possible cracked solder joints. Is it simly a case of re-melting the existing solder to ensure the cracks are gone or is it best to completely remove the solder and replace?

Paul

[jsog]

Re: Samsung LE40B750 flickering

Smaller capacitance values may only be available in FC, which is fine. I like FR if they're available in the size needed, otherwise FM.

[blib]

Re: Samsung LE40B750 flickering

got the same problem. Did the caps fix it?