LG TV 47LB650V flickering
jeannets Posted messages 28331 Registration date Status Contributeur Last intervention -
Hello,
My LG TV is blinking continuously. The image and sound are present, but nothing is visible (backlighting is flickering).
After disassembly, I realized that one of the LED strips was defective. I ordered and replaced all four strips. Unfortunately, after the repair, the same symptoms occurred.
When I measure at the output of the power board (whether connected or not), my multimeter reading flickers, and the voltage is not stable. Is this normal?
Best regards,
Best regards,
géniolo
9 réponses
Hello,
No, that's not normal.
Either there is a short circuit in the LEDs, or there is a power overconsumption.
Or the switching power supply that powers these LEDs is faulty... it is often located on the power supply board of the television...
Are the replacement strips identical..? same voltage, same current...?
You can try, but I assume there's some regulation behind it... probably done by the "Transistors" that are under the aluminum plates...?
I believe there are no risks, unless collapsing this 90v if it draws too much current.
Hello, first of all thank you for your quick response.
They seem identical to me (the only difference I noticed is that the original strips required 97V and the new ones require 95V) measured with a LED tester.
When the strips are unplugged, should we have a fixed voltage on the connector?
Best regards,
géniolo
These backlights operate on current... so with a break in the line, we find no voltage at the terminals...
You can connect a resistor at the terminals (around 100 ohms at first, then 50, then 25) to try to find it) and measure the voltage, whether stable or not...
We need to know how many LEDs there are...?? and to know the reference...? to know the nominal current and do the calculation.
Be careful: there may be a fault... a break or a short circuit on the ribbons that power the strips...
---- Is the photo of this power board...?? the one from your TV...? or one from an advertisement, because it looks to be in good condition... But the markings... Not readable.
There are fuses, we should check all of them...
-- Measure the voltage at the terminals of the capacitor near the inductors... it should be around 300 Volts without variation.
By supplying (with a LED tester) directly on the connector that plugs into the power board, I am able to light up all the LEDs with a voltage of 95V. (The LEDs are divided into two groups of two strips). Before replacing them, I was able to light up only one group. (I conclude that the strips are functioning)
Regarding the measurements across the capacitors
LED strip connected
RED 400V - YELLOW 90V - GREEN 90V - BLUE oscillates between 190V and 215V - BLACK unstable voltage
LED strip disconnected
RED 400V - YELLOW 90V - GREEN 140V - BLUE 170V - BLACK unstable voltage
Regarding the fuses, all OK
Best regards,
géniolo
"Before replacing these, I was able to power on only one group. (I deduce that the strips are working)"
Did you try it with just one group... then the same with the other group? Once one, then the other... and does it flicker? Or not? When powered by the TV, and not tested...
Yes, we can say that the strips are functioning if they are properly powered... which could indicate that it's the power supply that's at fault, especially concerning the part that powers the strips.
The global power supply, the 400V, should be fine; it's towards the output that things get tricky.
A clearer photo would be better; if we zoom in on this one, it's not legible. With a higher resolution, upload it to cjoint.com and share the link here.
We would need the brand and model of this board, likely written on it.
You could also disassemble this board to show the state of the other side of the circuit; there might be other components or burn marks on the tracks, weak/dry solder.
Under the aluminum bar, there should be a power transistor and two fast diodes; that's probably where the issue lies...
Depending on your skills and tools, are you able to solder and repair? It might be necessary to change this board entirely; it could perhaps be quicker or feasible, depending on the price...!!! We should also consider that.
Yes, I have tested the strips one after the other. Whether the strip is functional or not, connected to the board or not, the power supply does not stabilize.
On the other side, there are no visual signs of a defect.
Yes, I am equipped to repair; I am an electronics technician...
What I would like to know is that when I do not connect the LED strips to the board, should the power supply stabilize, or does it need a load? Because the LEDs are new, but do they have the same current consumption?
I will try to replace the filtering capacitors on the LED output first.
Sincerely,
géniolo
OK,
The consumption of the LEDs can be determined by measuring the currents... but it's not easy with a dimming... it's a series of pulses, around 300 Hz I believe, otherwise by reading the LED specifications and with the datasheet.
One solution is also to lower the consumption of the strips, to ensure we are not over-consuming which triggers the safety feature... to do this, you need to insert a resistor (in series) in one of the power lines of the strip... let's say 80 ohms to start with and decrease gradually.
I assume that a MosFet or a fast diode, under the aluminum bar, may be weak, perhaps we even need to replace them all at once..??
This board is available in Agen ::https://www.ebay.fr/itm/395488992246?_skw=Carte+alimentation+EAX+65423801+%282.1%29+Rev+2.0+pour+TV+LG&itmmeta=01J7Y9R2NCB0HRPE7VWNQGQ74Q&hash=item5c14fb1bf6%3Ag%3AiW0AAOSwR4ZmeCQ%7E
Unlike many where it is unavailable..!!
I assume that the LEDs are powered in packs of about 10.
I will check all that.
In case these components are okay, my idea may be silly, but what do you think?
On the two capacitors highlighted in yellow in my photo, I have a constant 90V. After making sure, of course, that the voltages cut off when the TV is turned off, do you think I can connect my LEDs to them?
Best regards,
géniolo