E480-B2 no backlight

[Nick's Tvs]

E480-B2
Double check screen with flashlight and image comes in perfectly. Plug in dvd player as well to double check sound, and sound is good.
24 volt going from PSB to LED driver board.
I order EEPROM from shop jimmy and replace it.
Tv now turns on no problem backlight is on customer picks up set.
2 weeks go by and customer says tv is doing the same thing again.
Shopjimmy says they will send another EEPROM complimentary just in case the chip actually failed.
I replace it again, but this time it doesn't fix the issue.
PSB is still giving off 24 volts to the LED driver board, but driver board sends out 0 volts to the backlight.
Backlight never comes on, not even a quick half second flash. Is it possible the main board could have an issue somewhere else?
Has anyone come into this issue before?

Main board part number: 55.76R01.A01
LED driver part number: 55.76N04.B01

http://www.shopjimmy.com/vizio-55-76...or-e480-b2.htm

http://www.shopjimmy.com/vizio-55-76...iver-board.htm

No pics but this set is one of those where half of the back doesn't come off kind of like the westinghouse, Emerson, and so on.
PS on the left LED driver in the middle connected to the PS with the metal pins. Then cables going to Main on the right side.
Sorry no pics. If needed ill post.

[budm]

Re: E480-B2 no backlight

Too bad the pins are not labeled, it may not be getting the BL-ON, DIMMING control signals. It looks to me you have main board problem.

[Alastair E]

Re: E480-B2 no backlight

Could be one of the LED's gone open.

Had same thing with a 40" samsung panel in a vestel pos!

[budm]

Re: E480-B2 no backlight

Q1, L1, D1, Caps TC2~5 are for the Boost converter circuit to supply the Boosted B+ (connector CN3 PIN 1~6) the six LED strings ANODES (there six PWM controlled power MOSFETs).
So put the DCV meter between CN3 PIN 1 (or Test Point TP16) to se the DCV jumps from 24VDC to much higher Voltage when the TV power switch is activated.
Pictures provided by shopjimmy.
BTW, please tell the rating of the caps TC2~5.

[Nick's Tvs]

Re: E480-B2 no backlight

Ill give that a try tomorrow morn and let you know.
As for the led driver board connector that goes to the led strips cn3 I get 0v no matter if the leds are connected in or not.

[budm]

Re: E480-B2 no backlight

You should have at least 24V on cn3 pin 1, if not you have open inductor or open circuit in the boost converter if you do have 24VDC feeding it.
As you can see in the example circuit, when the MOSFET is not switching, the output Voltage will be just about the same as input Voltage, in this case, 24VDC.
http://www.learnabout-electronics.org/PSU/psu32.php

So you must have 24VDC if you do have 24V at the input of the board.

[Nick's Tvs]

Re: E480-B2 no backlight

I double checked all pins of connector 3 and I have 0 volts on all pins. That IC U3 has 24v going into it but only one leg has voltage coming out of it and its 5 volts that goes to a surface mount resistor and the other side of the resistor has 0v.

[Nick's Tvs]

Re: E480-B2 no backlight

Test point 16 goes from 0v to 0.059 volts when tv first turns on and then stays at that. So pretty much stays at 0.

[budm]

Re: E480-B2 no backlight

Check L1, D1, L3 for open circuit.
That L3 looks really small for what it does.

[Nick's Tvs]

Re: E480-B2 no backlight

I couldn't find L1 but D1 tested ok and L3 on diode mode showed open at 0.00 ohms

[budm]

Re: E480-B2 no backlight

L1 i s the big black box that has Inductor symbol I draw on top next to 24DC input connector. The main key is do you get 24V at those points I marked in the picture.

[Nick's Tvs]

Re: E480-B2 no backlight

Wait I didn't realize L1 was the big square thing and turns out that one also is shorted. I took them both out of circuit as well to make sure.

[Nick's Tvs]

Re: E480-B2 no backlight

So I just double checked and no I dont get 24 volts at those points except for the one side of L1 facing the connector.

[Nick's Tvs]

Re: E480-B2 no backlight

Weird thing just happened. After soldering the l1 and l3 back in circuit and double checking my last post about the 24 volts not being at those points, the leds turned on. However I am getting 37 volts on those points. Not 24. So L1 has 24 volts on both sides d1 has 24 volts on l1 side but then changes it to 37 volts on l3 side and then pin 1 if connector 3 is 37 volts as well.

[budm]

Re: E480-B2 no backlight

Wait I didn't realize L1 was the big square thing and turns out that one also is shorted. I took them both out of circuit as well to make sure.

L1 and L3 will have VERY low resistance readings.

[budm]

Re: E480-B2 no backlight

Weird thing just happened. After soldering the l1 and l3 back in circuit and double checking my last post about the 24 volts not being at those points, the leds turned on. However I am getting 37 volts on those points. Not 24. So L1 has 24 volts on both sides d1 has 24 volts on l1 side but then changes it to 37 volts on l3 side and then pin 1 if connector 3 is 37 volts as well.

As I explain before, the circuit we are testing is Voltage boost converter that converts 24V to higher Voltage to drive the LED strings, if the circuit is not boosting, the output will be the same as the input Voltage as I have shown in the diagram and on the link that explain how the circuit functions.
So do you get backklights now? I have a feeling you have bad solder joints on L1 inductor.

[Nick's Tvs]

Re: E480-B2 no backlight

Yeah tv seems to work fine again. Must have been a bad solder joint. But that's really weird to see those on an LED driver board.

[budm]

Re: E480-B2 no backlight

Yeah tv seems to work fine again. Must have been a bad solder joint. But that's really weird to see those on an LED driver board.

Great, glad to hear that it is working now, the Boosting circuit was not working due to bad connection on the Boost inductor L1.
So now you know what to expect at each path of the circuits next time when you see boost converter circuits.

[Nick's Tvs]

Re: E480-B2 no backlight

Thanks budm without you I would have probably bought a new main board and a new LED driver board which would have wasted time and money.
Customer will be happy to hear all is back up and running.
Thanks again