[Solved] VESTEL 17IPS71 Power Supply: Lack of Voltage at LED Output Resulting in No Backlight

Hello
What could be the reason for the lack of voltage at the LED output in this power supply, so there is no backlight on the TV? The rest works, sound, channel switching, even the image is like I light up with a flashlight.
Please help.

Analyze the diagram:



there are few items to check.

Check the backlight first, the LEDs are usually damaged.

When I repair the power supply at the board output, I will check the diodes, because I do not know what voltage to check.

Lack of power to the LED may be caused by damage to the LED itself in the matrix. As you may know or not, the LEDs are connected in series, failure of one diode will open the circuit and the power supply may fail to supply voltage.
I would start by checking the LED in the matrix - you have to disassemble the matrix and check the LED strips.

You were right, one LED in the strip is burnt, can I omit it (bypass), or replace it, or maybe use another? I will add that there are 2 strips of 11 LEDs, and the voltage that I found on the marking is 94V-0.

You can't get around it, you have to replace it with a new one.
Check the other diodes, sometimes it happens that not only one should be replaced.

Sorry to dig up an old topic, but it seems important to me that a colleague may have mistakenly concluded that the supply voltage of the LED strips is 94 V. I found an explanation on the Internet that the symbol 94v-0 refers to the fire resistance standard for PCBs and does not mean voltage.

https://www.wellpcb.com/special/what-you-need-know-about-94v-0-circuit.html

Of course there is no 94 V voltage in this system. All voltages at key power points are shown on the schematic diagram and should always be consulted.

Down aaanteka

aaanteka wrote:

Examine the diagram: there are few elements to check.

Where on the diagram is the LED power circuit, because I can't see it?

You can't see it because you don't have the power supply first. Secondly, you are not familiar with the description of the connectors and the entire diagram. Be patient and get involved in the repair.

After the repair is done, make a modification, or in two years you will be repairing the same. These are 340 mA LEDs. For me, this power supply gave them 440 mA. Reduce the LED current. This is done by changing one smd resistor on the mp3394s chip. In this diagram, it is not actually visible, it is incomplete. To find it, use the catalog note and the desire.

If you repair the LED and it still won't work (there will be a blink when you turn it on), you have a damaged chip mentioned above. A few such flashes and one of the LEDs will burn out.

I have a similar problem, except that the TV started to turn off after a few / several seconds from turning on. When I set the ECO mode, I managed to turn it on for a few minutes. When I started measuring the current, of course in the ECO mode, the meter showed between 500 and 400 mA depending on the displayed image on both branches of the LED power supply, because I connected the meter to their common +. As I noticed, the LED branches are controlled by minuses (two) that go directly to the above-mentioned chip. During the measurements, in addition, one of the LED strips stopped shining and as I measured, there is basically no power on it. The meter shows that this minus is missing because there is 11 V in relation to the common plus and about 60 V on the other branch. If the LED strip is damaged, I assume that the power supply should supply voltage. One more thing, in the place of the chip, the plate is fried, on the other hand, it is practically dark brown, which indicates a high temperature.

Hello again, a few things have become clear. Replacing the system did not help, the TV turned on for a few seconds, so it remained and then I decided to take up the lead raised in the topic. I took care of the current control in the LED circuit. I increased the resistance in the control circuit of the IC by 4.7 K? and, surprisingly, the current in Eco mode decreased to about 230 mA, and in normal mode it shows about 380/390 mA, so when dividing it into two branches of control, the current is normal and the TV is already does not turn off. Nevertheless, I have a question, what caused this state of affairs, is it the fault of the burned-out LEDs, or something with the electronics? A few more things that I have noticed are worth adding. I have 2 such TVs, although I did not take the latter apart, the power consumption measurements for both are currently completely different. Efficient in normal and eco mode shows the consumption of 65 W and 42 W, while the one after the repair now shows 38 W and 27 W. It is not understandable to me, because with the correct operating values of the system, which I have now set in the repaired TV, the power consumption should be similar. When I increased the LED current a bit, the TV turned off and a good TV takes more power from the mains, which means that the LED current is actually higher, and this TV does not turn off. Could anyone explain it somehow?

Hello
I have the same problem as the one mentioned above, the TV turns on normally, works for about 15-20 minutes and the LEDs turn off. I turn off the remote control, turn it on and work 10 minutes. I have a sooty MDD1901 transistor from the bottom in the power supply, I temporarily cool it down with a fan and it works without any problems. Tomorrow I will replace the transistor and let you know. TV is Technica 42 "from Tesco.

klapouszek4, can you please tell me which resistor did you modify? what's the name and position of that resistor ? thank you

With the 17IPS71 power supply it is R113, the SMD resistor on the underside of the laminate. According to the 6.8k diagram, it was really 1.5k, I replaced it with an 8.2k resistor, after the replacement it works fine.
A diagram in the attachment.

@ robertdaniel1000
Did you solve your problem

I have the same problem.
Everything works, but the backlight goes out after 10-20 minutes.
After switching on, it goes back for a while and then goes off again.

I couldn't see a failed LED,
at least not visually in the blue screen,
I didn't open the part.

I have the mainboard 17mb95 210114R3a and the 190814R4 17IPS71 as a power supply.
..R4 = revision 4 for the power supply unit?

As for the other things discussed here, the R113 has a value of 2.7k for me (not measured)
and the MP3994 has an S on my power supply board so MP9334S,
I hope that's true, I screwed it back on too quickly and forgot to take a photo.

I have a 17IPS71 board, LED power comes on for 30 seconds and then LED backlight turns off.

Checked various resistors, caps and diodes - nothing obvious.

Seems heat related as it takes longer if cold and gets quicker to turn off after it's been on, so got an IR thermometer out and scanned the board. Whilst the backlight was on. Look like Q17. The MDD1130 MOSFET warms up to 27C then the backlight stops and the MOSFET starts cooling again.

Have ordered a replacement MOSFET, will see if it fixes it.

27 degrees doesn't sound dramatic, does it?

In the meantime I have found a defective LED in the backlight.
But I haven't exchanged anything yet.

Am thrilled. The tip with the R113 is worth gold. I scratched up a conductor track and then soldered a 10kΩ trimming potentiometer in series with the 1.5kΩ using small wires. The success is resounding. The device works again. Thank you for the post.

You don't have a defective LED in the backlight?

Which problem was with you exactly that you solved with it?

And if you change the value, the picture also gets darker, doesn't it?
That basically changes the maximum adjustable brightness of the backlight, doesn't it?

No, the LEDs are ok. I measured the current on the common line. The current was 440 mA with the trim potentiometer fully left. Trimming potentiometer set to the middle, as 5kΩ plus 1.5k, approx. 6.5k current now 330 mA. The picture looks a little darker, but the backlight does not switch itself off and continues to run for several hours. A complete success for me.