EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: AnneRanch on September 17, 2019, 03:14:58 pm
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I am not sure here is the correct place to ask.
So if it is NOT , just ignore my post.
My LCD TV , about 5 years old , quit showing ANY video.
Audio works fine.
Followed "manufacturer troubleshooting "
" use remote and enter " menu" ... and check "source"...
DUH - I have no video....
Spent almost an hour being transferred from one "customer service representative " to another ( around the globe !) - the result was NIL.
Took the back cover off.
There are three circuit boards inside.
"RF" board - where antenna and other inputs are plugged in.
If I unplug the antenna - no audio - hence the RF input board gets at lest PART of the signal.
"POWER "board which has several wires disappearing under the cover - this is I am assuming LCD back-light source source from AND I SUSPECT THAT IS THE PROBLEM.
"video driver' - this is is a small board with two multi pin connector ( flat ribbon cables) again disappearing under the cover.
Questions to the forum
Is there a way to verify that back-light MAY be the problem - I have only VOM and do not see any test points on the "power board".
From experience - which "board" is most likely to fail?
Plan "of action "
Find "real ' schematics for the boards.
1. Replace "power board " and hopefully it can be returned if it does not fix the problem. ( No need to discuss lack of "return electrical part" policies - been there , done that. )
2. Do the same with "driver board "
3. Recycle the TV and get Edison's phonograph.
Cheers
"
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Quick check for LCD backlight failure is to shine a bright torch onto the display and check for the picture.
If there's no input you might see just a blue screen however look around and you might be able to see the OSD messages.
Often backlight gives out when the CCFL backlight tubes age and draw excessive current for the backlight HV inverters to detect when they then shut down the HV.
New tubes or convert it to LED backlight strips is the normal remedy.
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Yes, I did try the torch trick - nothing was visible anywhere.
I'll repeat with NO device attached.
I guess further disassembly will be required to identify the backlit "tubes".
I wonder if I can unplug the suspected wires and measure something.
But it is one of those tiny ribbon connectors , little hard to access.
Fun.
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Maybe able to help with some good photos once further disassembled.
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If the backlight is working you should see it even without a signal, just turn it on in a darkened room.
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As far as I can tell the backlight is not working.
So I have basically two choices:
replace the power board - this is where bunch of bunch black / white wires go to the display
replace the "video board"
All this unless I can find a way to electrically test these boards
I am assuming the "backlight" is still OK, LED's usually do not blow all at once...
I am not sure posting pictures or part numbers would be helpful for now.
I do not like to advertise - good or bad way. Sorry.
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The point in photos was to try to point to what to test.
A typical setup is:
One 5V supply for standby power.
Once TV is switched on there's another 5V supply for mainboard, 12V supply for mainboard, 24V supply for backlight. (in case of CCFL backlight)
If you can confirm the supply voltages are coming out of the power board, that's one less board that needs replacing and you can consider troubleshooting further, by looking at lower voltages (3.3V, 2.5V, 1.8V on mainboard and the possibility of mainboard not switching power to the T-con board.
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I have ordered $20 worth of "power board" , seems easier that trying to identify test points.
At least for now.
I'll post "result" after I replace it.
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Update:
Replacing "power board" did not fix the problem.
I would like to change "hit and miss" troubleshooting to more "scientific" methods.
Perhaps there is a "factory test procedure " somewhere.
Anyway - only two more PCB's left to change....
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Pics required for simple easy guidance.
You say there is no backlight so we need to identify its type, CCFL or LED.
Inverters for CCFL are notorious for shutting down if there’s any fault with the tubes.
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I'm giving up on this, without sharing more detail or having more know-how don't bother having more scientific methods.
Change all the boards one after the other, if it turned out to be CCFL's that were past it, replace the X-TV assembly that usually solves the problem for 2-3 years.
Good luck.