Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

[not-a-real-repairman]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

It looks like it goes something like this:
BL_On pin <- Capacitor (to ground) <- Resistor <- Capacitor (to ground) <- Feed from MCU.

I've got the RF shield off the MCU, but it looks like there is a large heat-sink that I can't seem to remove. My guess is some sort of double-sided tape.

Any suggestions on the best way to get the heat-sink off so I can peek under it?

[fzabkar]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

Without seeing under the shield, ISTM that there may be a single SoC that does all the work. AISI, the SoC has a GPIO pin that directly drives the BL_ON signal. Moreover, this GPIO pin appears to have partially failed. I've seen this happen on several occasions in other equipment where an MCU or one of its peripheral chips drives a long cable run without any buffering. In fact this was a common problem in IBM's PS/2 Model 25. In that case the S-MOS graphics controller would develop weak H/sync and V/sync outputs which would cause the display to roll or tear. My solution was to buffer these "failed" outputs with an LM393 dual comparator. Perhaps you could buffer the BL_ON signal with a transistor or two?

BTW, I wouldn't try to remove the heatsink. Once when I was salvaging heatsinks from dead motherboards I managed to rip one of the chips off the board.

[budm]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

Worst come to worst you can build a delay circuit using the PS-ON signal to drive this delay circuit to produce the BL-ON signal after the PS-ON is present.

[not-a-real-repairman]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

How much of delay are we talking? I watched the voltmeter as I powered up the TV and timed how long it took the BL_On and PS_On pins to energize, and they both seemed to turn on as soon as the button was pressed.

[budm]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

Less than a second. The sequence is when the power switch is activated, the PS-ON is sent, then the on command for the switched MOSFET that supply the B+ to the T-CON is sent, then the BL-ON command.
You may be able to use the T-CON switched B+ to turn on the backlights (you just have to scale down the B + 12V for running the T-CON to be <5V using resistors Voltage devider) but you may see the pictures affect (fading effect) on the screen when the backlights are on before the T-CON is stable.

[not-a-real-repairman]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

Playing "Blue-Smoke Advocate" here -- is there a risk in just hooking up BL_On to PS_On? What components could possibly be harmed in the LED driver / LEDs?

[not-a-real-repairman]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

... also, is it worth looking into converting this into a SV472XVT (since those main boards are more prevalent)?

Looks like the same video specs, uses the same T-Con, power, and LED driver boards with only slight differences in the main board(SV472... adds WiFi/Ethernet, but it looks like the back of the XVT472 is already has a hole to accept the network jack -- label just covers over it on the back plate since this model doesn't use it.)

[budm]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

Playing "Blue-Smoke Advocate" here -- is there a risk in just hooking up BL_On to PS_On? What components could possibly be harmed in the LED driver / LEDs?

You do not want to run on the Backlights circuit (heavy load) on at the same time you turn on the power supply, you want the power supply to stabilize first and then apply the heavy load.
But if you want, you can put 1K resistor in series with the BL-ON wire, then another end of the resistor to the PS-ON pin on the power supply board.

[not-a-real-repairman]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

But if you want, you can put 1K resistor in series with the BL-ON wire, then another end of the resistor to the PS-ON pin on the power supply board.

I'm not sure I follow you here. Do I put the resistor on the pin coming from the main board, or on the pins where BL_On and PS_On go into the power supply?

[budm]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

I'm not sure I follow you here. Do I put the resistor on the pin coming from the main board, or on the pins where BL_On and PS_On go into the power supply?

Between the BL_On and PS_On pin of the power supply, but you also must remove that BL-ON wire going back to the main board because we do not want to back feed the force on signal back to the main board.

[not-a-real-repairman]

Re: Vizio XVT472SV No Backlight - 1.0v on LED Driver ON/OFF pin

I rigged the resistor like you said, and the backlight came right on when I fired it up!

Only problem now is that none of the AV or Component inputs work, and the HDMI work on my Roku for showing the menu, but any shows I try to play shows a purple screen. There is sound, but no pic. I think from reading other forums that it means the HDMI signal couldn't get a secure channel.

TV tuner works fine for both digital and analog signals.

The MCU chip gets pretty toasty too, and I think that's the problem. If it's only giving 1.0v on the BL_On pin and can't HDMI authenticate or handle AV/component, it's probably got something inside that's fried.

Or ... could there be another component that feeds the MCU that might be lowering the voltage and making it not function??

It's soooo close to working, and I might just stand it up and watch live TV with it until I can find a new main board.