Vizio M550SV backlight but no picture

[rockola1454]

Hi! I obtained a Vizio M550SV TV and am attempting to repair it. It powers on and the backlight comes on, but there is no picture. If I hit the menu button I can see different sections of the backlight get dimmer/brighter, so as far as I can tell, the TV thinks it is alive.

Upon pulling the main board, I noticed the previous owner attempted some repairs of his own. Two voltage regulators AMS1117 have been replaced, the two white fuses, and a few capacitors.

I noticed some sloppy soldering on the replaced parts, but probing around with my Fluke meter I am confident there are no unintended shorts. The replaced fuses are good. I checked all electrolytic caps with an ESR meter, but found no "smoking gun." I was curious about the voltage regulators, but I measure 5.0, 3.3, and 2.0 on the 3 pins for both which sounds correct according to info I've found online. Lastly, just for the heck of it I reflowed the chip under the aluminum heatsink, but that had no effect. I do notice the heatsink gets hot when the TV is on, so I assume the chip is at least getting juice :-P

I'm a bit stumped as to what to try next. Will attach pictures of my main board, if anyone has any knowledge on what I need to repair I would greatly appreciate a push in the right direction

Thanks!

[RDC55]

Re: Vizio M550SV backlight but no picture

I would start by re-working those replaced components. The solder work is so poor (particularly on the fuses) I don't think you can be sure they are electrically connected to the circuit.

[budm]

Re: Vizio M550SV backlight but no picture

Do you get 12VDC at both ends of the SMD fuse with ref. to the chassis GND on the T-CON board?
Do you get 12V on one of the two fuses on that main board?
We also need pictures of the whole back side of the TV showing all the board.

[rockola1454]

Re: Vizio M550SV backlight but no picture

@RDC55 Good point... I went ahead and resoldered the fuses and touched up the regulators just to eliminate sloppy solder as an issue.

@budm
> We also need pictures of the whole back side of the TV showing all the board.
Done!

> Do you get 12VDC at both ends of the SMD fuse with ref. to the chassis GND on the T-CON board?
I think they combined the T-CON and main boards into one in this unit... I removed the floor stand bracket and underneath I don't see a board, just ribbon cables going directly to the panel.

> Do you get 12V on one of the two fuses on that main board?
Not seeing either with 12 vdc. F1 has 5v, but F2 has 0v regardless of TV powered on or off. Both fuses test good for continuity.

Unfortunately, something I did tonight made matters worse - the backlight no longer comes on. Looking for a cause, I notice that the regulator in spot U3 is no longer working correctly - I'm getting 4.2 / 4.2 / 5.0 volts on the pins left-to-right, as if pins 1 & 2 are shorted. Regulator U9 still has 2.0 / 3.3 / 5.0, so I swapped the regulator chips thinking U3 must have gone bad, but sadly the U3 location still measures 4.2vdc on 1st and 2nd pins, while U9 still measures the expected voltages, so I am assuming both regulator chips are still good. I see 110 ohms between 1st and 2nd regulator pins on both U3 and U9, so there's not a short there causing it. No clue what I did wrong

[budm]

Re: Vizio M550SV backlight but no picture

@RDC55 Good point... I went ahead and resoldered the fuses and touched up the regulators just to eliminate sloppy solder as an issue.

@budm
> We also need pictures of the whole back side of the TV showing all the board.
Done!

> Do you get 12VDC at both ends of the SMD fuse with ref. to the chassis GND on the T-CON board?
I think they combined the T-CON and main boards into one in this unit... I removed the floor stand bracket and underneath I don't see a board, just ribbon cables going directly to the panel.

> Do you get 12V on one of the two fuses on that main board?
Not seeing either with 12 vdc. F1 has 5v, but F2 has 0v regardless of TV powered on or off. Both fuses test good for continuity.

Unfortunately, something I did tonight made matters worse - the backlight no longer comes on. Looking for a cause, I notice that the regulator in spot U3 is no longer working correctly - I'm getting 4.2 / 4.2 / 5.0 volts on the pins left-to-right, as if pins 1 & 2 are shorted. Regulator U9 still has 2.0 / 3.3 / 5.0, so I swapped the regulator chips thinking U3 must have gone bad, but sadly the U3 location still measures 4.2vdc on 1st and 2nd pins, while U9 still measures the expected voltages, so I am assuming both regulator chips are still good. I see 110 ohms between 1st and 2nd regulator pins on both U3 and U9, so there's not a short there causing it. No clue what I did wrong

No switched 12VDC then there will be no 12v to run the T-CON circuits. That switched 12V will be present when the main board sends out the PS-ON signal to the power supply board to turn on the rest of the power supply.

[rockola1454]

Re: Vizio M550SV backlight but no picture

Thanks, budm! So this story gets even more weird...

Tonight, I figured the last thing I did before the backlight quit was removing and resoldering the fuses, so I decided to try pulling them back up for the hell of it. I didn't notice any issues visually except that a couple pads are missing, so to solder the fuses back I scraped some green enamel away from traces that connected to where the pads used to be and soldered to that. Plugged it in and F2 now has 12 volts, and amazingly I'm getting the "V" logo and can see the text and menu on the screen!!! I'm guessing the original issue was something tiny in the fuse solder work after all.

Now... the regulator U3 worries me. When I first got the set I probed 2 / 3.3 / 5 volts from left-to-right on both regulators, but U3 now reads 4.2 / 4.2 / 5 volts. Swapping the regulators doesn't make a difference. I lifted U3 (and unfortunately lost the pad tied to R25) to check with a microscope if I'm missing anything, but the traces look healthy. The "Vizio" amber/white power light is dead without U3 in place, and the set will not power on without it. Could anyone help with the following:

1) What is causing the ADJ & OUT pins to both carry 4.2 volts?

2) Why with the regulator removed ADJ & OUT still both have 1.4 vdc despite not being shorted together? I would expect OUT to be lower than ADJ without the regulator there...

3) Am I correct in assuming it is bad to try and use the set as-is with the extra volt in U3 OUT? Will this cause problems, potentially kill HDMI devices plugged into it, etc.?

4) Does anyone know where to grab a schematic / service manual for this board? Perhaps that could offer some clarity...

Thanks!

[budm]

Re: Vizio M550SV backlight but no picture

You need to verify 100% that those two resistors which are used for setting the Output Voltage are connected and make good connection as shown on your picture #3.
The Voltage between the ADJ pin and the out put pin should be 1.25V which is the Vref.
The output Voltage for the adjustable regulator is = 1.25Vref + Voltage on the adjust pin.
Are you sure that R25 is really 2000 Ohms, that is way too high, it should be in 100's Ohm range.
At 200 Ohms the out out will be about 3.5V
May be that R25 is not making good connection or just bad and no way it can be 2000 Ohms resistor for that Adj pin and GND.
My calculator:
https://www.badcaps.net/forum/showpo...17&postcount=6

[rockola1454]

Re: Vizio M550SV backlight but no picture

That was it! My R25 went bad; it looked fine but resistance check kept getting between 1.5 and 2K Ohm. I used your spreadsheet to determine a 180 Ohm resistor should be there, installed one and the voltage is back down to 3.3vdc again.

Thank you so much for all the help!