Mulja tv repair portal
Equipment Repairs => LCD TV forum => Topic started by: metallicaz on October 22, 2023,10:42:58
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Hello,
My 40 inch JVC LED TV works fine, has sound, but it has no backlight. I have tested the power board and the backlight, they seems to be okay. When, I remove the ribbon cable that goes from the Main board to the T-con board (backlight turns up), and then put it back again, the backlight comes back and TV works for a few days (4-7 days). After a few days, again the backlight goes dark, but the TV works and I can hear the sound and see they picture by flashing a light.
So, I am not sure, if it is a faulty, Ribbon cable, Main board or the T-con board? Because, removing and re-attaching the ribbon cable seems to solve the backlight issue for at least a few days, apart from the backlight, everything seems to work just fine. Please help!
Many thanks
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Can't be sure the backlights are totally OK, and it is the most likely scenario. Not the T-Con because you can see a picture with a flashlight. For the backlights to work, the main board sends a signal to the power supply commanding it to activate the backlights. This signal is usually designated BL-ON, BL-EN, INV-ON or INV-EN (EN = enable). Look for this pin on the p/supply at the socket of the Main-to-P/Supply cable. It needs around 3V DC for the backlights to turn on. Check this pin when the TV fails. No 3V = no backlights = faulty main board.
Not sure if you have the knowledge to open the backlight cavity i.e. remove the screen and all the diffuser sheets behind it. This allows you to see the backlights which do operate even without the screen. You may get a chance to see one LED strip go out first, then protection operates & the micro turns all backlights off. Often too, each LED strip has a joiner near the centre i.e. each strip is made up of 2 pieces. These joiners sometimes lose contact.
If the TV uses edge or bottom LEDs, then we're in another world. But that may give you smething to go by. Bruce
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Hello Downunder,
Thank you for your reply and suggestion. It is very helpful and I really appreciate it.
Here is what I have done so far:
When the backlight went off and sound was still working and the picture could be seen using a torch.
I took the back cover off- and noticed that, when the TV is powered on using a remote, the backlights lit-up for a nanosecond, and then turns-off. First thing, I did was, I checked the Power board for the mains voltage using the multi-metre, it showed the correct voltage, Then, I checked the voltage of the backlight connector (with the backlight wire attached), the reading was 84v, 95V was printed on the board, there are total of 27 LEDs (9 x 3 strips = 81v), voltage dropped when I put the Tv on standby, and went up when I had turned it on, meaning it was working. Then, I removed the Tv's Main (logic) board cable from the power board and turned on the TV to see if the LED's will lit-up, it didn't, so, I assumed the Logic board was fine. After this, I assumed that the backlight has failed, so, I removed the screen and tested each LED's they all worked, I tested them using the LED TV backlight tester, and a multi-metre, they all worked, individually, as a strip (line) and they worked all lit up together as a whole. So I assumed a lose connection, hence, I the put the TV back together, IT WORKED FOR A FEW DAYS(less than a week), and again the backlight went off.
This time around, I followed the same procedure, BUT, WHAT I NOTICED was that, when I unplugged the Logic board's cable from the mains boards and turned on the TV, it didn't lit-up the backlight, so, I put it back, but as soon as, I DISCONNECTED THE RIBBON CABLE THAT RUNS FROM THE MAIN (LOGIC) BOARD TO THE T-CON board at the TCON board's end, and then turned-on the TV , the backlight came back (it lit-up), so, I re-attached it the ribbon cable and the TV works fine, the lights are on and everything is running, BUT, here is the problem, it only runs for few days and again the backlight goes off, REMOVING AND RE-ATTCHEING THE RIBBON CABLE SEEMS TO SOLVE THE PROBLEM, BUT ONLY FOR A FEW DAYS and AFTER WHICH, THE PROBLEM OF BACKLIGHT COMES BACK AGAIN.
Hence, I am not sure, if the fault lies on the Ribbon cable (NO DAMAGE) , mains board or a T-CON board. Because, LED's are fine, I have tested them individually. The only was to bring back the backlight seems to be REMOVING and RE-Attaching the RIBBON cable. BUT THIS SEEMS TO WORK ONLY FOR A FEW DAYS. Any suggestion would be very helpful.
Thank you
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Very strange behaviour ! Disconnecting the cable between the main board & p/supply to see if the backlights turn on, only works on Samsung led TVs, so that test is invalid. Next time it fails, instead of unplugging the main-to-T-Con lead, try disconnecting one at a time, the 2 flexie cables that run from T-Con to the display. See if either one restores the backlights. Maybe the display panel itself is returning a FAIL message to the micro,
thus disabling the backlights. I don't understand the reasoning behind this if it is the case.
On the connector between the p/supply and main there will be a pin designated BL-ERR (= error). Would be interesting to see how the DC voltage is behaving on failure. One would expect minimal volts there in normal operation, but high when t5he backlights go out. The b/light tester is a useful tool but it only applies around 30mA of current to the LEDs, whereas in normal use the LED strips draw about 300mA, thus totally different operating conditions. But we'll assume for now the LEDs are OK.
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https://quuq.org/forum/index.php?topic=10826.0
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Hello Bruce (Downunder) and SinclairTv , thank you very much for your comment and suggestions, they are helpful in attempting to resolve this issue.
The backlight went off again after 5 days, but, this time, STEP 1, I unplugged the Backlight cable from the power board and plugged it back again to see if it works, it didn't.
However, STEP 2, when I unplugged the same Backlight cable from the power board and then tried to power the backlight using an LED tester the backlight lit-up, after this I simply re-attached the backlight cable into the power board and to my surprise the backlight and TV started to work again.
Here, I am bit confused, I am not sure, if it is the backlight or power board itself which is faulty? Because, all I did in 2nd step was feed some electricity into the backlight through and LED-tester for few seconds and then it attached it back to the power board and the backlight worked, so, I am not sure, if somehow the backlight is failing or it is getting a signal after running for few hours to turn-off?
I would really appreciate your help! thank you.
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Personally, I'm now putting my money on a fault in the LED strips. Might be time to disassemble the display and inspect the backlights. Seems the
slightest tickle restores backlights. The backlights will still operate with the display and diffuser sheets removed. Have a poke around the various connectors. Bruce
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Hello Bruce,
Thank you again for your reply and advice. You have been very helpful. You are right, it may be the faulty LED backlights. I have ordered a new set of backlights and waiting for them to be delivered. In a meantime, I have gone ahead and changed the power board as well, as they got delivered the very next day and easier, plus, economical to replace. Anyways, let's see if a new set of Backlights and a power board would resolves this issues.
Thank you very much! and really appreciate! your help and inputs. :c017:
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You ordered the LED strips without checking them first? I hope they were cheap.. Most likely a bad connection, but at least you may pick it up when you strip the display panel. Bon chance. Bruce
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Hi Bruce,
I replaced the Power board, but to my luck, the replacement P. board only lasted for two days, after which, it completely went out. This time, the red stand-by light also disappeared (this wasn't an issue with the original board). So, I re-installed the original power board for time being.
I replaced the old backlights with a new set. I did carry out a test on the old backlights, they were all working, but, one of the LED, even though, it lit-up, but had a few nano-second delay (nano second flickering). Let's see, if this new set of backlights resolve this issue, at least, now I can rule out the Power-board and the backlights.
Thank you for your reply and help. You have been very helpful!
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Excellent! yes, one tardy LED is enough to induce the problem. I reckon you've got it licked.
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It would be nice to know if tv was still working after replacing the backlights