Vizio M420NV no backlight

[jman100]

Hi guys,

Looking for some advice on a Vizio M420NV. The screen comes on and the tv functions fine, audio and video, but the backlight is completely out. I was only able to tell the picture was still there by holding up a flashlight to it.

There is never any flicker at all from the LEDs, they completely do not come on. I'm a novice on hardware stuff so any advice on what to test is appreciated. I looked for a BL_ON pin but I cannot find one labeled that, and don't know what it would look like.

Thanks in advance!

[budm]

Re: Vizio M420NV no backlight

We need good clear pictures of the whole backside of the TV showing all the boards, and then pictures of each board.

[jman100]

Re: Vizio M420NV no backlight

Sorry for the delay, here are a bunch of pics.

[CapLeaker]

Re: Vizio M420NV no backlight

turn the tv on and check the DC V on the BLon pin.

[jman100]

Re: Vizio M420NV no backlight

turn the tv on and check the DC V on the BLon pin.

Once again sorry for the delay.

BL_ON shows .03 volts while the tv is off, and goes to .12 when the tv is turned on.

[CapLeaker]

Re: Vizio M420NV no backlight

That sux. Main board isn't sending BLon signal...
since the tv comes on and the backlights stay off, it's a bad EEPROM on the main board.
Your choices are to get a new replacement main board, or buy the repair kit at Shopjimmy for your board number 0171-2272-3174. You would have to change 2 two ICs (U14 and U33) on the main board. U14 fixes the frozen on welcome screen, U33 is for the backlights.

[jman100]

Re: Vizio M420NV no backlight

That sux. Main board isn't sending BLon signal...
since the tv comes on and the backlights stay off, it's a bad EEPROM on the main board.
Your choices are to get a new replacement main board, or buy the repair kit at Shopjimmy for your board number 0171-2272-3174. You would have to change 2 two ICs (U14 and U33) on the main board. U14 fixes the frozen on welcome screen, U33 is for the backlights.

Darn. Oh well, I did acquire the TV for free. I might get the repair kit.

I assume there's no way to bypass or manually try to turn the backlight on to make sure?

[CapLeaker]

Re: Vizio M420NV no backlight

To force the PSU on, only leave the backlights connected. Take two 1k ohm resistors and jump STBY voltage to PSon and STBY voltage to BLon. Plug it in and the backlights should be up. If not, try to put a third resistor between STBY and ADJ.

[sricechex]

Re: Vizio M420NV no backlight

I know this was a while back, but I thought this may be a source for diagnosing an almost identical problem.
I have a m420nv with no backlight, good image, white logo.
VLED voltages of 164v.
BL_ON from 0.4v off to 0.8v on.
Does this sound like an eeprom failure on the main board?
Is there further diagnostic information necessary or suggested?

Thanks!

[nomoresonys]

Re: Vizio M420NV no backlight

Unplug tv, then unplug powersupply board from mainboard, plug tv back in turn tv on see if backlights come on, that will work if it has pull up resistors if not you have to do like CapLeaker instructs in post # 8.

[sricechex]

Re: Vizio M420NV no backlight

Great! Thank you, nomoresonys. I'll give that a shot.

If I can confirm that the backlights can be forced on successfully, can I be relatively confident that there is an eeprom failure on the mainboard? (is there a source for ideal voltages at those signal pins?)

[nomoresonys]

Re: Vizio M420NV no backlight

As CapLeaker says in post 6, if you don't have ps_on coming from the mainboard, most likely a mainboard fault, you could try heating the mainboard a little with a hair dryer, see if it trys to come on or anything changes.

[sricechex]

Re: Vizio M420NV no backlight

I think he was taking about BL_ON , Not PS_ON?
That certainly makes sense, I'm just inexperienced as to what voltage constitutes "getting no signal. "
I'm getting 0.8v, but should it be a known board signal voltage like 3.2 or 5v?

[nomoresonys]

Re: Vizio M420NV no backlight

http://www.shopjimmy.com/vizio-3642-...for-m420nv.htm The board is 134 bucks there but they have eprom kits for around 20.

[nomoresonys]

Re: Vizio M420NV no backlight

https://www.ebay.com/itm/Vizio-3642-...ty!43616!US!-1
One there for 80 bucks, looks like these boards are pretty costly.

[nomoresonys]

Re: Vizio M420NV no backlight

I think he was taking about BL_ON , Not PS_ON?
That certainly makes sense, I'm just inexperienced as to what voltage constitutes "getting no signal. "
I'm getting 0.8v, but should it be a known board signal voltage like 3.2 or 5v?

My bad I meant to say BL-ON.Yes I would expect about 3 or so. budm usually says 2.5 or greater equals backlights on and if I recall 1. or lower equals backlights off, something like that.

[nomoresonys]

Re: Vizio M420NV no backlight

I think he was taking about BL_ON , Not PS_ON?
That certainly makes sense, I'm just inexperienced as to what voltage constitutes "getting no signal. "
I'm getting 0.8v, but should it be a known board signal voltage like 3.2 or 5v?

Yes usually around 3, you are way low there.

[sricechex]

Re: Vizio M420NV no backlight

If the backlights do work when forced, I'll give the eeprom-swap a try. I'll report relevant information as I find it.
Is the standby voltage what should be expected in a live signal?

[nomoresonys]

Re: Vizio M420NV no backlight

Did you try test in post #10.

[nomoresonys]

Re: Vizio M420NV no backlight

https://www.youtube.com/watch?v=mUvclBfaMXg Testing powersupply by itself, unplug all other boards, he's using the ground pin for testing because he has board out of tv, you can just use the metal back of tv or other suitable ground for testing, IF you can't get a steady standby reading while testing with this methed, it's a powersupply board problem. Could try that to verify powersupply. CAREFUL HIGH VOLTAGE FROM THE HOT SIDE CAN BE DEADLY.