Vizio VX42L NO picture, good sound and LCD

[charlesflynch]

My vizio VX42LHDTV10A had the screen go blank.
No previous flickering, etc.
Yellow logo on standby, white when powered on.
TV turns on/off normally and stays on or off.
LCD is working, verified with a flashlight shining into the screen.
Sound is working.

Checked power supply board for bad caps, but all look good.

with TV on, voltages from the power supply board are:
24V is 24.2V
5V stby is 5.17V
12V is 11.69
"A" is 2.89V (bl_on?)
"B" is 5.02 (brightness?)
PSON is 3.12

Did a quick check of the LDOs on the main board, but all had the appropriate differential and/or fixed output.

Looked at the Master/Slave inverter boards, and noticed the 4 large caps were bloated/blown, so i replace them with the same value, but 50v instead of 35v.
Master Board:
Philips LCD LC420WX7 6632L-0448A PNEL-T702A
Slave Board:
Philips LCD LC420WX7 6632L-0449A PNEL-T703A

Unfortunately, that didnt help.

Found 1 fuse on each inverter board and they were good.

I tried jumpering the 5VSB to PSON before turning on tv to see if it was a timing issue, but no luck.

I checked the 4 MOSFETS on the boards
MQ1,MQ2,MQ4,MQ5
1H46TA
FDD
8447L

The top two (MQ1 and MQ4) have +24V on D, and 0 on both G and S
The S of MQ1 feeds the D of MQ2, which has all pins at 0v.
The S of MQ4 feeds the D of MQ5, which has all pins at 0v.

The G of MQ1 and MQ4 are both traced back different pins on MU1, the main IC
BS9897FS
746 T49

how likely is it that the main IC would go bad? It might be above my abilities to fix, but I might be able to get someone at work to help.

Is there a way to make sure it's the inverter boards and not the actual CCFLs?

I saw mentions of a mod for other phillips inverter boards, but not mine specifically, anyone know of a similar mod?

Everywhere I looked on google, no one had the boards in stock. Anyone have any recommendations for a reputable store?

Thanks for any help

[Agent24]

Re: Vizio VX42L NO picture, good sound and LCD

Probably not the lamps unless they were (somehow) shorted to ground. If the lamps are old or one is bad, they usually flicker momentarily then the inverter senses an error and shuts down. Sounds like yours isn't even trying to come on.

If the MOSFETs have 0v on their gates they are probably getting no drive signal from the controller ICs, unlikely both boards went bad at once, so probably the controller ICs aren't getting signal to turn on.

Where do the inverter's control signals come from? Can you add photos showing what connects to what etc?

[nomoresonys]

Re: Vizio VX42L NO picture, good sound and LCD

I replaced some caps on philips inverter board, had to go back over my solder joints about 3 times to get it working, finely got it right, but I'm a pretty bad solderer.

[nomoresonys]

Re: Vizio VX42L NO picture, good sound and LCD

on a lot of tvs, if I remember right, if the power supply has pull up resistors, try unplugging the ps-on plug from power supply or mainboard and see if backlights pop on, this might confirm inverters are good, someone please correct if thats wrong.

[charlesflynch]

Re: Vizio VX42L NO picture, good sound and LCD

Probably not the lamps unless they were (somehow) shorted to ground. If the lamps are old or one is bad, they usually flicker momentarily then the inverter senses an error and shuts down. Sounds like yours isn't even trying to come on.

If the MOSFETs have 0v on their gates they are probably getting no drive signal from the controller ICs, unlikely both boards went bad at once, so probably the controller ICs aren't getting signal to turn on.

Where do the inverter's control signals come from? Can you add photos showing what connects to what etc?

I've tried looking at the IC's spec sheet, but couldn't find a good idea which one is the "on" line for the IC.

for the inverter boards:
There is an 11 pin ribbon cable connecting the master-slave inverter boards:
1:SS
2:VREF
3UTY_OUT
4:GND
5:CT_SYNC
6:GND
7:SRT
8:ON/OFF1
9:ON/OFF2
10:STB
11:GND

There is also a 4 pin ribbon cable going from the master to the TCON bd, but no clues as to what's on those pins.

The master has a cable going to the pwr board with the +24, GND, ON/OFF, VBR-B

The VBR-B is brightness, I'm assuming and ON/OFF is the BL_ON 2.9v

The slave's to the pwr board only has the +24 and GND populated

[charlesflynch]

Re: Vizio VX42L NO picture, good sound and LCD

I replaced some caps on philips inverter board, had to go back over my solder joints about 3 times to get it working, finely got it right, but I'm a pretty bad solderer.

I'm not so good either, but one of the tech's at work called it "ugly but functional"

I measured res between the cap's leads and the +/- power rails and got 0.00 ohms. Is there a better way to test?

[budm]

Re: Vizio VX42L NO picture, good sound and LCD

So let me get this correctly, you did not see the backlights flash on at all? and the fuses are OK on the inverter board?
"I measured res between the cap's leads and the +/- power rails and got 0.00 ohms." I do not understand the readings, do you mean the resistance between the two legs (+, -) shows 0 Ohms?
The IC on the Inverter looks OK?

[charlesflynch]

Re: Vizio VX42L NO picture, good sound and LCD

8:ON/OFF1
9:ON/OFF2
10:STB

Ok, looked a bit closer and it seems as though on the master bd the On/Off from the pwr supply board goes to ON/OFF2 on the mas/slav ribbon. On the slave these two are connected together, so ON/OFF1 on the master will have the ~3v when the slave is connected. ON/OFF1 goes through a 10k resistor an then to pin16 on the IC (labelled as STB) which is also pin 10 on the mas/slv ribbon.

Just checked and on both the master and slave, ON/OFF1 and ON/OFF2 are both at 2.88v and STB is at 0.7V

looks like i need to figure out what else is on the STB line

[charlesflynch]

Re: Vizio VX42L NO picture, good sound and LCD

So let me get this correctly, you did not see the backlights flash on at all? and the fuses are OK on the inverter board?
"I measured res between the cap's leads and the +/- power rails and got 0.00 ohms." I do not understand the readings, do you mean the resistance between the two legs (+, -) shows 0 Ohms?
The IC on the Inverter looks OK?

no flash or any signs of life from the backlights.

found 1 fuse on each inverter, both boards measured to be good.

+ lead of each cap to +24V is 0.0 ohms
+ lead of each cap to the other + cap lead is 0.0 ohms
- lead of each cap to GND is 0.0 ohms
- lead of each cap to the other - cap lead is 0.0 ohms.

We have capacatince meters at work, so if I get time, I'll try to measure the boards tomorrow. obviously will have the 2 big caps in parallel as well as other board stuff, but should give a rough idea of if i'm close?

[budm]

Re: Vizio VX42L NO picture, good sound and LCD

The caps are connected in parallel, the minus is connected to the circuit ground (24V return), the Plus are connected to the fuse which is fed by the 24VDC B+.
The master-slave inverter are setup as push pull to drive the lamps, The master inverter send the drive signal to the slave to synchronize the circuit of the master and the slave. If the Master inverter board is working but the slave is bad, you will see one side of the screen lights up only, if the Master is bad but the sync signal is still good, the you will see the other half of screen lights up only.
So right now the MASTER is not active.
There is a small flat white cable between the Master inverter and the T-CON board, remove it to see if you will get the backlights, this TV has what is called Dynamic Contrast function so they can claim that the TV has high contrast ratio (B.S), the brightness of the backlights is controlled by the T-CON which depends on the scene of the Videos.

[charlesflynch]

Re: Vizio VX42L NO picture, good sound and LCD

Took your suggestion and disconnected the ribbon from MAS to TCON, but still nothing on the tubes at all. I even turned the light out in the garage to see if there was something faint, but no luck.

I found 1 fuse on the TCON and it was good.

Found 1 LDO and is seems good.
RT9164A
15CG9L OK
1:0v
2:1.5v
3:3.3v

There is an LED MD1 on the master only, that lights up bright when you power on the TV, but fades off within 1-2 seconds. Unfortunately I dont know if that's normal operation or not.

[budm]

Re: Vizio VX42L NO picture, good sound and LCD

"There is an LED MD1 on the master only, that lights up bright when you power on the TV, but fades off within 1-2 seconds." That is correct.
I expect the lamps to flash on then what ever is wrong with the circuits is then detected and goes into shutdown, so we must be missing something here.

[Agent24]

Re: Vizio VX42L NO picture, good sound and LCD

Something dead in the inverter that didn't blow the fuse? The inverter controller tries to start, and shuts down? The backlights never flicker because whatever is bad stops them from getting any power?

[budm]

Re: Vizio VX42L NO picture, good sound and LCD

He can try replacing the IC that will be my first choice.