Vizio m557-g0 no response

[triplefour]

yes im still fixing tvs...just not asking for help too often. but i got a puzzler here. tv has no response when plugged in. no standby light, no response to button presses. so i open up to check voltages, and of course thinking i will try to force the PSU on and that will tell me if PSU and BL are ok and most likely that MB is bad. but this tv does not have 5vsb, only 18vsb, so how do i know what kind of voltage to send to PSON and BLON to trick the PSU into turning on without MB? if they still should get 5v, where do i pull that from?

thanks in advance!

[dick_barton]

You could use an external power supply set to 3.5 - 4.5V or use two/ three 1.5V batteries connected in series. I would also use a 1K resistor in series with the supply so nothing is damaged if you connect it to the wrong pins.

[CapLeaker]

some new tv's don't have a separate 5v stby circuit on the psu anymore. It goes like 18v to the main board, there is a buck converter(s) there, maybe check it?

[triplefour]

Originally Posted by dick_barton

You could use an external power supply set to 3.5 - 4.5V or use two/ three 1.5V batteries connected in series. I would also use a 1K resistor in series with the supply so nothing is damaged if you connect it to the wrong pins.

thanks for the response. i have a couple questions tho. firstly: why 3.5-4.5v and not just a straight 5v? i understand the 1k resistor part as thats what i normally do when taking 5v from the 5vsb pin.

thing is i know i have some 5v power supplies lying around i can canibalize to make this tool. but probly harder to find one that says 4.5

second question: do i just connect the negative of the psu to a ground screw on the psu and put the positive (with 1k resistor) to the PSON?

thanks again

[triplefour]

Originally Posted by CapLeaker

some new tv's don't have a separate 5v stby circuit on the psu anymore. It goes like 18v to the main board, there is a buck converter(s) there, maybe check it?

not sure thats such a good idea since i am trying to test only the PSU and for this to work i would have to have the MB hooked up as well, and i think its not good to force the PSU on with the MB connected?

[dick_barton]

Originally Posted by triplefour

second question: do i just connect the negative of the psu to a ground screw on the psu and put the positive (with 1k resistor) to the PSON?
thanks again

As you say the negative to the ground screw or ground terminal of the TV's power supply. You could use 5V but I would not go any higher. Some power supply adapters give out a higher voltage than stated on them so I would check the output voltage before carrying out the test.

[triplefour]

OK I made the tool. I used a usb block like for charging phones and I cut the end off of a usb cable and soldered the red and black wires to a couple pins. Now i can put 5v wherever I want!

So I put it to pson and blon but still it doesn't turn on the backlight.

The voltages on CN201 stay the same
But I do get a couple of new voltages on the other connector CN301
FAULT = 3.1V
CS = 2.8V
All other are 0v
Checking the led connectors I get between 7.5 and 8v on each of the LED pins and 84v on the VOUT pins.

I discovered by accident while testing the vsync pin for voltage that if I had my finger on the metal of the probe that it would give .5v to this pin and the backlights came on weakly. Testing the led pins under this condition I got around 16v on each and about 90v on the vout.
So then I tried to give the 5v to that pin and the backlight comes on to full brightness. The led pins voltage all drop to 1.1v which tho which is confusing to me...the vout pins measure 95v.

If I give 3v (borrowed from the fault pin ) I get a similar result. If I give the 2.8v I get a similar result as to the .5v from my finger. Not very bright and measuring 13v or so on the leds.

Not sure what it all means. It seems like ive succeeded in proving the psu can power the backlights...but what about everything else ?

[triplefour]

Pics

[triplefour]

im guessing (and also did google search to be sure) that the service manual is closely guarded in some vizio vault... would like to know what fault = 3.1v means

[triplefour]

just to follow up and close out this thread, i ended up ordering the main board, and it turns out my diagnosis was correct. but sad story: while i was waiting for the the part, the screen got badly damaged. i guess i didnt store it in the best place...someone must have kicked it or something. so i parted out the TV and will try to sell the parts on ebay. life goes on!