Mulja tv repair portal
Equipment Repairs => LCD TV forum => Topic started by: pan on December 23, 2011,10:44:51
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Hi,
I’ve got a prob with a LG 42LF66.
The picture & sound are fine when 1st switched on, but after the set gets warm about 10 / 20 mins the picture starts to distort break up across the screen horizontal lines which eventually course the backlight to turn off.
I’ve checked the 24v on the inverter panel reeds ok a fraction under 24v.
There was a cap on the p/s C202 680uf which was bulging and I replaced but still no joy.
I’m not sure but I think it could be a T-con fault.
I’ve sprayed freezer on the T-con but the fault still persists.
Has anyone has any thoughts on the possible prob
Kind Regards
Pan
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hi
before you go any further check caps on psu first
some will not show if bulging check with cap meter
dave
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Hi Dave,
Yes I've taken out a number of caps and metered them but still no joy.
Pan
(http://i41.tinypic.com/263u5va.jpg)
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Can't zoom in on the picture you posted ..but looks suspiciously like the row rather than column driver on the Tcon board ...or worse...the internal matrix of the screen itself..
rgds
VS
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Hi Pan ...
yep ..yook a look at the bigger photo you sent ..
Still looks like a faulty T con board or the lcd panel itself ..
Just a possibility that bad electrolytic smoothing caps are responsible ..but since you've already explored this option ...I suspect it Tcon or panel
rgds
Evan
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Hi,
..... which eventually course the backlight to turn off.
I think that bit is significant.
If it was JUST a panel or tcon fault, why would the backlights go off?
I would be looking at the inverter.
I have had 3 sets now where the inverter causes interference. On one set it just made a buzzing noise on the sound, on the second it interfered with the IR receiver, and on the third, it caused picture break up on freeview.
So if my hunch is correct, interference from the inverter could be causing picture interference.
What I found an the 3 sets (all different) that had this problem, was backlight intensity was important. In all cases, setting the backlights to full brightness stopped the interference.
So if you have backlight intensity in any of the menu's, set it to full and see if it makes any difference.
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I would also take the screen apart and check for any arcing around the backlight tube holders
good luck
Daz
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From memory I think the inverter brightness is pwm controlled and this has been known to cause problems, ill dig my notes out over the next day or so.
Tony Walker
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Hi Guys,
I was doing some further tests when I noticed something.
As you can see from the photo (I know its miniature) I’ve got the set connected to a signal generator, connected to AV1 by scart.
As the fault started to occur I pressed the input button on the TV remote, and by mistake selected DVB, there is no aerial connected to the TV the DVB symbol was clear to see top right hand corner of the screen no distortion at all, when on DVB I selected the main menu, the main menu came onto the screen as clear as a bell no distortion/breakup I left the set on test for some time and it was fine.
When I selected AV1 the colour bars came back on distorted and after a matter of minutes the back light turned off.
Guys I’m I missing something?
Regards
Pan
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Hi
I guess I’m a little naive/ inexperienced when it comes to picture faults on LCD TVs.
But I thought when there is a picture problem; picture break up it would affect the menus on the other inputs.
When I place my meter probe on any one of the 2 wires of the 10 plugs on the master or slave inverter boards the backlight turns off.
I’m thinking the picture interference after a time is the backlight assembly going intermitted with this TV.
I’ve not got a clean room workshop that I understand is required to repair screens/replace CCFL tubes.
Daz did say it could be the tube holders; I’m inclined to believe that could the prob.
If anyone has any thoughts please can you share them with me
Ta
Pan
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:051bye:
for me its eprom problem or main board.
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Hi, I have similiar problem with 42LF66 :
the screen turns off after 2 minutes of being first turned on but the sound remains, and remote control not work,
then :
1) I have to leave it like this about 20-30 mins, unplug power, plug in and it starts normally.
2) If I unplug right after screen disappears and plug back - the green light flashes 4 times and nothing happens, then I have to leave it to cool down and start from beginning.
I changed caps, except big ones (450v) hard to find them, no solution.
I will be lucky if fix this problem before and can give me solution. :c017:
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i know you said you changed it but i just fixed the same fault by changing the 680uf 35v to a decent panasonic cap, i know poor quality caps can be problematc
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Hi All
I've had to call it a day on this one.
Regards
Pan
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Hellow Pan,
You have check for Dry solder -Resolder inverter all transformer pin [leg] at solder side ? check lvds cable for Pit at end of cable contect.
wbr
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ok i have the same problem again as pan, tv will stay on all day with the back off.
when i put the back on after about 20 mins a kind of interference starts then the picture goes off.
the back light stays on all the time, while there is sound just no picture.
Ive checked caps on psu and changed a few.
after lettng it cool down for a few mins I put a hot source on the main, t-con and psu boards one by one and cant get it to go off.
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If the ldvs is disconnected does the back light switch off?
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ive narrowed the fault down to the top of the main board, where it looks like it has been getting hot ive put a cooling fan over that area with back on, been on for 7 hours
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http://www.ohmsupplies.co.uk/epages/62027733.mobile/en_GB/?ObjectPath=/Shops/62027733/Products/KIT153
Click on the link and check all the parts shown, this is a very common problem with this board, I think the cap in the PSU threw you off a little. From memory of you unplug the ldvs the backlight goes off so that lead to inverter. This model seemed to be very popular for a 37" but.the same fault happends on the 42 with it being the same board (again, from memory, please check)
I can't be 100% certain of a full cure but it would be the 1st thing I did to the set.
Regrads
Andy
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well all those caps ar in the area, where the board has discoloured with heat.
Thanks for that Andy ill change those and post results
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Fixed !!!
changed those caps and hey presto the tv has stayed on over night
Cheers Andy
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Like I said, very common but its so easy to blame everything else however once you've seen were the parts are then it should make sence as to why the picture is distorted. On the backer for the kit it also mentions an ic to solder for increased reliability & the 5v line on the set should be more stable (if you don't change it the set can bounce) its also not uncommon for one of the caps to pop while you unsolder it.
Did that happen with you?
Regards
Andy
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no caps popped, but found them very hard to unsolder.
where was the info on the ic andy ?
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On the backer inside the kit, later versions of the board have a heatsink on it, or perhaps early versions had one & they took it off
Regards
Andyo