Vizio sv421xvt

[landyboy]

I just picked up a Vizio sv421xvt. Vizio emblem goes white but no picture no backlight. During my initial trouble shooting I found that I am able to get the set to power on and access a menu if I jumper 5v to the standby on the power supply.

[johnboy1313]

Re: Vizio sv421xvt

When the emblem turns white, is there sound? Can you see a picture with a flashlight held to the screen?

[budm]

Re: Vizio sv421xvt

"I found that I am able to get the set to power on and access a menu if I jumper 5v to the standby on the power supply." So the backlights come on, and you can see pictures/menu on the screen. It sounds like the PS_ON signal is no being generated by the main board, but did you disconnect that STBY wire from the main board to allow you to force on the PS_ON pin? The main board will keep that line to ground and not allow it to go HI if if the wire is still connected. How are the voltages on the LDO regulators on the main board?
If you can force on the power supply and everything are functioning, then you may just have lost the PS_ON signal only.

[landyboy]

Re: Vizio sv421xvt

Ok so I disconnected the standby and same results if I jumper 5v to the standby pin on the psu connector set powers on. Here are the results of the reg's:

U2 1117L 2.024, 3.255, 5.094
U3 1084 2.069, 3.322, 5.056
U4 1117L 0.002, 1.234, 3.322
U5 1084 2.054, 3.311, 5.056
U6 1117L 0.002, 0.002, 0.002
U7 1117L 2.022, 3.252, 5.056
U8 AZ7805T 0.001, 0.001, 0.001
U9 1084 0.565, 1.820, 3.309

[budm]

Re: Vizio sv421xvt

Are those reading with the force on activated? If it is, the 12v power supply is not on so you will not have Vin for the U8 and U6.

[landyboy]

Re: Vizio sv421xvt

No the reading are not with set forced on. I forced unit on and I got:

U6 6.86 8.10 1.62
U8 11.62 0 5.037

[budm]

Re: Vizio sv421xvt

OK ,all the readings are good, except U6 6.86 8.10 1.62. That must be an error, the 1.62 is the Vin pin (PIN 3) which is very low and you have adj pin and the Vout pin at much higher voltage.

[landyboy]

Re: Vizio sv421xvt

yes the 1.62 is a typo it should read 11.62

[budm]

Re: Vizio sv421xvt

Then it is OK, 6.86V Adj +1.25Vref = 8.11V= Good.

[landyboy]

Re: Vizio sv421xvt

So the mainboard is not sending the "turn-on" voltage to the psu? What controls that on the main board?

[budm]

Re: Vizio sv421xvt

The MCU sends the signal to the interface Transistor (or MOSFET) that will keep the PS_ON LO (PS-ON <1V), it will go high (PS_ON >2V) when transistor is turn off. For example:

[landyboy]

Re: Vizio sv421xvt

Do you think the MCU is bad? Firmware? Should I look for a mainboard?

[landyboy]

Re: Vizio sv421xvt

I tested the standby voltage it is .0042 before I hit the on button but it does increase to 0.658 VDC when I attempt to turn the unit on. So it does send voltage but apparently not enough to turn the set on.

[budm]

Re: Vizio sv421xvt

See POST 11, the transistor may be leaking (low resistanace between Collector and Emitter). I would trace out the connection from the PS_ON pin to see if it ends up connecting to the Collector of the transistor some where on the board.

[landyboy]

Re: Vizio sv421xvt

Ok on the attached picture I have traced the voltage .680VDC (same voltage on standystandby pin) to the collector of transistor marked 1 (Q2) when on button is pushed. Collector of 2 (Q1) is 0 when power button is pushed. When power button is pushed again (turned off) Q2 goes to 5.112VDC and Q1 goes to .0042vdc

[landyboy]

Re: Vizio sv421xvt

I can't figure out where the .680VDC is coming from feeding Q1 and why it is only .680 volts

[budm]

Re: Vizio sv421xvt

Is the PS_ON PIN connected to the Collector (middle pin) of Q2? It looks like there is resistor in series with the PS_ON PIN.
"I have traced the voltage .680VDC (same voltage on standystandby pin) to the collector of transistor marked 1 (Q2) when on button is pushed. Collector of 2 (Q1) is 0 when power button is pushed. When power button is pushed again (turned off) Q2 goes to 5.112VDC and Q1 goes to .0042vdc" This statement does not make sense since you are seeing 5.112V. The circuits of those two transistors are correct, when Q1 is ON, Q2 will be off, when Q1 is OFF, Q2 will be ON.
Are you sure that the PS_ON is traced back to Q2 Collector?

[landyboy]

Re: Vizio sv421xvt

Ok on the attached photo with power button pushed I test 3.254 VDC at test point 1 and .680VDC at test point 2. And at test point 3 .680VDC and test point 4 3.254VDC

[landyboy]

Re: Vizio sv421xvt

budm I apparently need better reading glasses. Q1 has the 5VDC Q2 has the .680V that is also present at the PS_ON pin so based on that I am thinking the PS_ON Pins is connected to Q2

[budm]

Re: Vizio sv421xvt

If the PS_ON is generated by Q2, check Q2 Collector voltage, it should swing high (3~5V) and low (<1V).
Or we may be looking at the wrong section since it looks like Q2 (Collector) is connected to the Base of Q1.