Vizio VW42lfhdtv10a - No standby

[uberdaan]

First post, so please be gentle

I have a 42" Vizio LCD that doesn't consistently go into standby when you turn it "off". It usually goes from white light to no light (no orange light). In order to get the orange light working again, I have to unplug it for a few seconds and plug it back in. (EDIT: To be clear, the TV turns on and works normally when you reset it and have the orange light on).

I took it apart to do some investigating (thanks badcaps!), and found the power board to have a very light humming/buzzing sound when the set is plugged in AND the orange light is on. I can't tell where on the power board the buzzing is coming from. When the orange light is off, there is only silence (and the TV cannot turn on).

By this point, you might already suspect bad caps in the 5v rail. I thought the same, so I visually inspected the tops of every cap on the power board...no bulging/burning (see photos) or other obvious indicators (to me) of a bad cap. I'm not sure if its possible for another component (xformer/vreg) to cause this specific issue...so I'm kindly asking for your thoughts on the issue before I start replacing every cap on the board.

Thanks for your input!


Mainboard measurements are below, although I don't think the problem is on the mainboard (photos attached also).

---------------------------------------------------------------------

Fuses (each side):
5.22v/5.19v
12.27v/12.25v

u16
11.51v
6.5mv
5v

u44
8.41v
9.67v
11.52v

u22
2.09v
3.3v
5.2v

u15
2.08v
3.3v (big tab...center pin is cut)
5.16v

u19
1.2v
2.5v (like u15)
5.16v

u17
2.1v
3.3
5.16v

u21
5mv
1.25v
2.5v

q19
12.19v
12.25v
11.53v

Power board output connector to mainboard (with tv on):
>space
>4.99v brown (B1)
>3.36v black (A1)
>5.23v white
>5.23v gray
>5.24v purple
>(space)
>0v blue
>0v green
>0v yellow
>(space)
>12.27v orange
>12.27 red
>12.27 brown
>1.93v black (PSON)

[martino]

Re: Vizio VW42lfhdtv10a - No standby

To keep you busy till somebody smarter then me jumps in: When it's in failmode(no amber light) do you still have the 5v standby ?

[uberdaan]

Re: Vizio VW42lfhdtv10a - No standby

Ah - excellent question. I did check the voltages at the power board output in ON, OFF, and FAIL modes. The ON voltages are listed in my first post, but I didn't list the others.

In the OFF mode (w/ orange light), there are a few mV on some of the pins.
In the FAIL mode, there is 0v on all the pins. It's as if the main fuse blew - but I checked it, and the TV comes back on (I also replaced it anyway).

I should also note that this seems to be an intermittent problem. It doesn't always fail hard - it does go into standby mode sometimes.

[martino]

Re: Vizio VW42lfhdtv10a - No standby

In standby WITH amber light you should have 5 volts standby....
When amberlight is off and 5volt standby is zero,I would be curious to see if it comes back up by pulling connector to mainboard out...(to see if its the mainboard pulling 5volt down or if powerboard just fails)

[uberdaan]

Re: Vizio VW42lfhdtv10a - No standby

Interesting idea - I'll give it a shot.

Is there a feedback circuit that would require the mainboard to be plugged in for the power board to output the voltages?

[martino]

Re: Vizio VW42lfhdtv10a - No standby

Yes and no. Power supply has a 5 volt life line that's always on so that the mainboards brain can function( standby light, remote function etc).
Rest of power supply comes alive through a feed back from mainboard, backlight etc.

[budm]

Re: Vizio VW42lfhdtv10a - No standby

May be you should read this:
https://www.badcaps.net/forum/showthread.php?t=25810

[uberdaan]

Re: Vizio VW42lfhdtv10a - No standby

Thanks budm. I read the basic troubleshooting. The power board troubleshooting does apply. I measured the PS_ON output voltage with the TV on at 1.93v...does this sound like the correct voltage, or could it be just enough to get things going? (You quoted 3.3-5v in the tutorial).

I measured everything I could coming off the mainboard - and all looks normal to me except U21. Problem is, I have no idea what u21 is supposed to do.

I keep circling back to why this problem would be intermittent (and slightly noisy), unless it were a bad component (e.g., U21) in the 5v circuit. Wouldn't this failure mode cause the front standby light to die, and system to not turn on?

[budm]

Re: Vizio VW42lfhdtv10a - No standby

So, then when it is in failed mode, do you have the 5VSTANDBY or not? You can also tell if the PS_ON is high enough by measuring the DC Voltage RIGHT AT THE TWO LEGS OF THE MAIN filter cap (big blue cap laying on its side), it should have about 160~170VDC when TV is OFF and about 360~400VDC when TV is ON. See if that voltage also drops down to 170V when the TV failed.