Vizio M421VT LED no display

[ryan74]

I can't find much info or a service manual for this tv and it is my first Led model.
When I plug this in the vizio standby/power logo flashes white about 1 minute, shuts off(typical vizio) and nothing else. If I then press power the light stays on white but nothing else happens.
No sound, no pic, no backlight at all and I do not hear any relays click when plugging in or pressing power.
Iam not sure on correct voltages and everything looks ok visually.
Anyone have a similar issue or suggestions?
Here is what I checked(dc).

CN902
Vled = 235.6mV
#4-1 = 10.69V
#4-2 = 10.27V
#4-3 = 10.67V
#3-3 = 10.65V
#3-2 = 10.66V
#3-1 = 10.64V
Vled = 236.2mV

Cn905
Vled = 237mV
#2-1 = 10.25V
#2-2 = 10.63V
#2-3 = 10.64V
#1-3 = 10.66V
#1-2 = 10.27V
#1-1 = 10.67V
Vled = 237.0mV

Cn904
1 = 4.91V
2 = 2.93V
3 = 12V
4 = 12V
5 = 20mV
6 = 20mV
7 = 20mV
8 = 24.73V
9 = 24.73V
10 = 3.09V
11 = 5.09V
12 = 5.09V
13 = 2.8mV

Power supply # 080GL1927LT
PCB # 3PCGC10021A-R
Panel # LC420EUH

[ryan74]

Re: Vizio M421VT LED no display

CN904 connects to main board and had some voltages listed so I relisted it.

Im still not sure but shouldn't Vled(power supply) and pwr_ok(main board) have voltage?
Cn904
1)BL_on = 4.91V
2)BL_DIM = 2.93V
3)12V. = 12V
4)12V. = 12V
5)G = 20mV
6)G = 20mV
7)G = 20mV
8)24V. = 24.73V
9)24V. = 24.73V
10)BS_on = 3.09V
11)5V. = 5.09V
12)5V. = 5.09V
13)PWR_ok = 2.8mV

main board # is TXACB5K05304(barcode sticker)

[ryan74]

Re: Vizio M421VT LED no display

After reseating plugs and playing with inputs I get a display, menu, and sound but no backlight.
So the only issue is no backlight at all.

I don't know if it is power supply inverter issue to led backlight or a bad led backlight.

CN902 and CN905 go to backlight. I assume 10v is ok but I have no clue about VLED.

Im not sure if pwr_ok should have voltage.

I hate to open panel to look at backlight if I don't have to.

[tom66]

Re: Vizio M421VT LED no display

If you got 24V and BLON going to backlight inverter, check the output going to the LED strips (around 90V or so, probably.)

[ryan74]

Re: Vizio M421VT LED no display

If you got 24V and BLON going to backlight inverter, check the output going to the LED strips (around 90V or so, probably.)

The power output to led strips I think is CN902 and CN905 on power supply (see above post).
Nowhere neer 90v.
So it must be inverter/power board or im confused?

[tom66]

Re: Vizio M421VT LED no display

10V doesn't sound right at all; test it without the LEDs connected, there may be a short in one of the strips (seems fairly common...)

[ryan74]

Re: Vizio M421VT LED no display

with led wires disconnected I still get 10v on all the # pins.

vled showed 15mV (connected it was 235mV)

Looking at the back of power board all the # pins lead to the board atached to top side
-top side board is labeled:
LGIT PCB P/N: 3PHAC20004A-R
LED DRV-12CH
Barcode # FF4610BPV20L12304

Could there be 12 led strips(12CH) each being supplied 10v?

[ryan74]

Re: Vizio M421VT LED no display

Since Iam still unable to confirm if this is power board or led backlight problem, is there an easy way to test or bypass led lamps without opening panel?

[tom66]

Re: Vizio M421VT LED no display

What's the highest voltage ISOLATED power supply you've got?

[martyparish]

Re: Vizio M421VT LED no display

Nobody has mentioned it yet, but I'd be leery of those "sam young" caps. May or may not help, but they are known junk right?

[ryan74]

Re: Vizio M421VT LED no display

What's the highest voltage ISOLATED power supply you've got?

That would be a 12v 1.0A charger or a computer power supply. So testing led probly wont work for me.

I may try replacing caps but testing them seems pointless with my multimeters capacitor test. If a cap is bad but looks ok visually my multimeter reads it as good.

Better quality pic from shopjimmy below(not my board)

[tom66]

Re: Vizio M421VT LED no display

If you can beg borrow or steal a 90V, constant current limited power supply that may work.

[ryan74]

Re: Vizio M421VT LED no display

If you can beg borrow or steal a 90V, constant current limited power supply that may work.

That will be difficult.

Im not sure what I should do at this point. It's a repair for a friend so I have to try and keep cost down so I cannot buy boards to just swap and try.

If I understand right, there is 12channels feeding power to led strips at 10v. each.
How possitive are you it should be more like 90v.?
I found a board for $119

[tom66]

Re: Vizio M421VT LED no display

Probably somewhere in range of 40~90V.

Each strip is basically lots of LEDs in series, usually with a resistor to balance the current (and limit fault current.)

The controller (green "addon" board) monitors the current overall and regulates the voltage to compensate for small changes in LED operating voltage as the set warms up.

That's at least how I think it works.

[budm]

Re: Vizio M421VT LED no display

It does have 12 strings of LEDs, the LED's brightness is also controlled by the T-CON board for the dynamic contrast so they can say it has high contrast ratio. You can try disconnecting the cable between the T-CON and the power supply board by that big green board for the LED power supply to see if the LEDs will come on.

[ryan74]

Re: Vizio M421VT LED no display

Thought I would list caps on cold side of board. Maybe it will help confirm voltages I should have.
upper left side:
2-Samwha BL 200v 33uf
2-Samwha ZS 50v 47uf

upper right side:
2-Samwha MK 10v 1000uf
4-Samyoung NXB 25v 330uf
2-Samyoung NXB 50v 150uf
4-Smawha MK 50v 47uf

[tom66]

Re: Vizio M421VT LED no display

It looks like the output (maybe up to 200V) is on VLED. Not sure what the individual points are, then. Have you tested that bridge rectifier on the secondary for shorts?

[budm]

Re: Vizio M421VT LED no display

Those 33uf 200v Caps are for the LED power supply primaRy drive circuits which is fed by the bridge rectifier, they are not for the LED power supply out put side.
Those 50V caps in the leaft side where the output connectors for the LEDs are for the LED power supply output voltages. So I expect the LEDs to be driven by less than 50V.

[ryan74]

Re: Vizio M421VT LED no display

You can try disconnecting the cable between the T-CON and the power supply board by that big green board for the LED power supply to see if the LEDs will come on.

no change/no led

[ryan74]

Re: Vizio M421VT LED no display

It looks like the output (maybe up to 200V) is on VLED. Not sure what the individual points are, then. Have you tested that bridge rectifier on the secondary for shorts?

no I haven't. Ill check it.