LG L196WTQ Monitor strange problem

[hgg]

Hi guys,

I have another LG monitor that stopped working.  It displays the LG logo, then switches off,
then displays the logo again  and so on.   Its has been in use for 4 years approximately.

I took it apart, had a look at the mains power board and while there was a lot of black dust,
it looks ok.  I have cleaned everything.  No bulging capacitors.  I measured all of them and they
have a very low ESR of 0.017ohms.  All the 1000uF ones have exactly the same ESR value.
They are SAMXON and SUNYOUNG branded.  The big one in the first input stage had a lot of
the black dust on its positive terminal.

When I put everything back together it worked again... but I don't know for how long.

Is it possible that these capacitors have such a low ESR values?   
I cannot find any new Panasonic ones with such low values! 

Do you think that I should replace them anyway with new ones?
The ones I found have an ESR of 0.025ohms. 
Will they be ok, or will they get hot with their higher impedance?

My final question is, could this kind of problem be just a *stuck power on button* ?...
I have removed all the capacitors now but if someone has a similar LG model, can you please
do the following test:  keep pressing the Power On button continuously and see if the monitor
does the same.  e.g. blinks the LG logo on and off until you release the button.

That, would help a lot!

Thanks again.
George.


Power Board


Capacitors

[SeanB]

Did you unsolder them to get the ESR? I would change the small electrolytics on the primary side in any case, and put new capacitors to replace the Samyoung  and Samxon capacitors anyway. Check as well on the main board for other electrolytics, and check/change them if needed.

[hgg]

Hello SeanB,

Yes, I did unsolder them and I got the same ESR results with both the MESR-100 & DE-5000.
If the values are real then it looks like they are high quality & spec capacitors.

I have just resolder them back again to check if the problem was from a stuck power on button,
but its not.  When you keep pressing the power on button it does not cycle the power.

I don't know why but it works again.  I will leave it on all the time to see if it will do the same.

Is there any definite way to really test a capacitor ?
It is such a simple component with such a difficult diagnosis.

[SeanB]

Could have been the contaminant on the boards doing weird things to the microcontroller, and cleaning fixed it. If capacitance and ESR are within spec then it is fine. Did you do those small caps on the primary side as well, those are known to fail by going high esr. Typically they should be well under 1R at 100kHz. If they start failing the unit either does not start or starts slow.

[hgg]

I hope it was the black dust.  It was like carbon dust and there was quite a lot on the board.
Not sure how it got there in the first place!



Capacitance and ESR are fine.

I did check all the electrolytics, although I did not remove the ones you mentioned.

I have left it on with full brightness.  Lets hope it will not explode... 

[SeanB]

You live downwind of a coal fired factory? That is some serious dust, or the ships in the harbour are burning heavy bunker oil inside the harbour always.

[hgg]

    no, not really.

Its from a friend who lives in a very clean house and suburbs.
Very strange.  Maybe one of the components is slowly disintegrating ...

[menushadesanayaka255]

i heard their is a safety feature built in to some monitor to off when the high voltage transformers go bad....

[hgg]

The transformers in the inverter section look fine.  The monitor has been working all night with no
problems yet.

The strange thing is that there is no black dust in other boards like the controller board.  They are clean.
And if you have a closer look at the photo below, the dust is radially spaced around the upper voltage
regulator that is attached to the heat sink.  Could it be that this is beginning to deteriorate from
extreme heat?



 

[SeanB]

Those are the hottest running parts and have the highest voltage gradients on them, so will attract any charged dust and keep it around.

[hgg]

Indeed, the hottest parts were the two voltage regulators and the yellow transformer at the right.
The high voltage transformers were not hot at all.  I checked the two regulators with a flashlight
and they did now show any sign of external damage.

Anyway, I think I will reassemble it and return it.  Nothing else to do...  It works now.   
(The black dust remains a mystery since it was only in the power supply board!)

Thank you for your help!

[hgg]

I returned the monitor and guess what.   In the same room they have a candle burning 24/7,
because of death in family.  When they clean the room they see a lot of black dust.
Mystery solved I think.  Maybe that dust was the problem after all.

Case closed.   I hope...