Vizio M421VT LED no display

[ryan74]

Re: Vizio M421VT LED no display

I am surprised that your meter can read the AC voltage on those test points since that transformer is driven at 50KHz to 80 KHz range, most cheap meter does not have the bandwidth to measure those high frequency on that switching power supply transformer.

It's a Klein MM1000. $60 new

[ryan74]

Re: Vizio M421VT LED no display

I just got part put in but while doing it I noticed I didnt verify spacing of leads when I ordered.
Besides the spacing I had to install it upside down so pos. and neg. matched.
See pic
I hope it will be ok. I will update results after I test it.

[ryan74]

Re: Vizio M421VT LED no display

I still have issues.
I left backlight disconnected (CN902,CN905) when I tested VLED voltage.
After pressing power it was 40vdc. for a second then dropped off.

I had the same results checking voltage with backlight connected.

From BudM-I think once the bridge rectifier is replaced to get the voltage for the LEDs you will be OK unless the circuts that drive the transformer also failed due to shorted out bridge.

What next?

[budm]

Re: Vizio M421VT LED no display

So what voltage do you get at the +/- terminal of the bridge, it must be quite high since the filter caps are rated at 33uf 200V.

[ryan74]

Re: Vizio M421VT LED no display

Grounded to chassis:
All I get is the 40vdc on the + of bridge just like above post.(drops off)
The - is 0-9.9mV.

[tom66]

Re: Vizio M421VT LED no display

So, next step is to see if BLON comes on and stays high and also maybe DIM (I am not sure if this set modulates the voltage to vary brightness or uses PWM; it may use a combination of the two.)

[ryan74]

Re: Vizio M421VT LED no display

I pulled and tested bridge just to be sure. Everything tested as below except:
Pin 4 to 1 was .879V.

So, testing the bridge: Looking at device from top label side pin 1 is +, 2 and 3 are ~ and 4 is -.

Meter probes in order (red) (black)
Pin 2 to 1 you should see ~0.4V
Pin 3 to 1 you should see ~0.4V
Pin 1 to 2 you should see OL
Pin 1 to 3 you should see OL
Pin 4 to 1 you should see ~0.4V
Pin 1 to 4 you should see OL
Pin 2 to 3 you should see OL
Pin 3 to 2 you should see OL

[tom66]

Re: Vizio M421VT LED no display

My error 4-1 is correct as there are two series diodes.

[ryan74]

Re: Vizio M421VT LED no display

I get the same reading as in post 1

Cn904
on-off = 4.91V
DIMM = 2.93V
12v = 12V
12v = 12V
G = 20mV
G = 20mV
G = 20mV
24v = 24.73V
24v = 24.73V
PS_on = 3.09V
5.2v = 5.09V
5.2v = 5.09V
ACD = 2.8mV

[budm]

Re: Vizio M421VT LED no display

The 40V reading at the output of the bridge seems too low since the filter caps are 200V rating.

[ryan74]

Re: Vizio M421VT LED no display

The 40V reading at the output of the bridge seems too low since the filter caps are 200V rating.

I agree, I don't think it is getting up to correct voltage before shutting down.
I know transformers rarely go bad but could it be a bad transformer or most likely something else?

[tom66]

Re: Vizio M421VT LED no display

The capacitors on the output could be bad.

[budm]

Re: Vizio M421VT LED no display

or the drive circuit that drives the transformer is not driving it correctly, that is why I wonder if it gets damaged when the bridge shorted out.

[tom66]

Re: Vizio M421VT LED no display

I'd be surprised. Most drive circuits fail or work. Like shorted FETs. Won't work. A few exceptions, like bad coupling caps and current shunt resistors, of course.

[ryan74]

Re: Vizio M421VT LED no display

when I press power I get the 40v for a split second.
This may sound stupid but I don't know. The drive circuit for transformer- Is this the cold(north) side? or are you refering to the circuit before transformer?

I pulled all 4 caps in upper left of board(2-200v) and tested good.
200v 33uf=35uf
50v 47uf=48uf

Im stuck at this point but I think I found a board for $120 but Im not sure I want to give up yet.

[budm]

Re: Vizio M421VT LED no display

The hot side, there are two MOSFETs and little IC for driving them, you can look up the spec of that IC.

[ryan74]

Re: Vizio M421VT LED no display

ok I guess I am starting from scratch again. I purchased a power supply board and still have the same problem.

[ryan74]

Re: Vizio M421VT LED no display

I rechecked voltages:
------------
on/off-4.91v
dim - 2.93v
12v - 11.91v
12v - 11.91v
G
G
G
24v - 24.67v
24v - 24.67v
ps_on-3.09v
5.2v - 5.11v
5.2v - 5.11v
acd -

------------
Vled-194.7mV
all #- 10.65v

-------------
Vsync- 25mV
DIM- 58.5mV
G
DCLK- 49.1mV
G

I assume vsync and dim should be giving a reading and im not sure about ACD.
Now im thinking it is the middle board(pc/tcon?) or possibly main board.
http://www.shopjimmy.com/vizio-cbpft...2-pc-board.htm

these are the original readings from old board:
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

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

Any idea how to test pc board? Theres a connector from main board but nothing listed by it.

[ryan74]

Re: Vizio M421VT LED no display

I ordered the pc board to try. If it still doesn't work I don't know what I will do.

[ryan74]

Re: Vizio M421VT LED no display

I have tried power supply board and pc board with no luck. Not sure if I should try main board or dig into panel lighting.