Vizio sv421xvt

[landyboy]

Re: Vizio sv421xvt

Q2 collector voltage ranges from .0019 to .690VDC definatly not getting to 3-5vdc

[budm]

Re: Vizio sv421xvt

What DC do you get at the Base of Q2 with ref to chassis ground? It should toggle. If you check the resistance between the Collector (Middle pin) of Q2 to the #1 test point, does it show 0 Ohm?

[landyboy]

Re: Vizio sv421xvt

The DC on the base of Q2 toggles from 0 to .666VDC also From my test point 1 to the collector of Q2 I test 12Kohms

[budm]

Re: Vizio sv421xvt

12K is way high. I do not think we are at the right transistor for driving the PS_ON. I expect direct connection from #1 test point to the Collector of Q2.

[landyboy]

Re: Vizio sv421xvt

With set plugged in but not "powered on" I read 512ohms from Collector of Q2 to the PS_ON , Are you referring to my test point one on my Picture?

[landyboy]

Re: Vizio sv421xvt

From my test point 2 to the collector of Q2 I read 24ohms

[budm]

Re: Vizio sv421xvt

Looking at the picture, that Collector of Q2 is connected directly to the Base of Q1, the Emitter of Q1 is connected to ground so the Voltage on the Collector of Q2 can never swing higher than Vbe of Q1 which will be only arund 0.6~0.7V and that make sense what you are seeing. So I still do not believe we are looking at the correct point of the board.

[landyboy]

Re: Vizio sv421xvt

I do have 3.254 VDC at my test point 4. That is the closest to the PS_ON pin I have that voltage.

[budm]

Re: Vizio sv421xvt

Looking at your marked test points, it looks like test point 4 (left side of R1) has a copper trace connected directly to test point 2.
Can you draw out the connection from pin1 of that connector to see what resistors are in the path of that pin 1 connector? Pin1 for sure is the PS_ON pin?
I see R11, C19, R3, R1 in the area of the PS_ON pin.

[landyboy]

Re: Vizio sv421xvt

You are correct R11 C19 R3 (although there is no resistor present at R3)and R1 are in that path. I do believe pin 1 is the standby pin If I test to the standby pin on the PSU I have a solid short.

[budm]

Re: Vizio sv421xvt

OK, Connector pin1 is connected to the top end of R11 (561), the bottom end is connected to the left side of C19, the right end of C19 is connected to ground so this is the noise filter cap. Bottom end of R11 is also connected to the left side of R1 (no printing on the body) which is test point 2 (test points 2 and 4 are connected together). According to your test it has about 24 Ohms reading to the Collector of Q2 which does not make sense. The right side of R1 is connected to ????.
As the way the Q1 and Q2 are connected, Collector of Q2 can never go up more than 0.7V.
So find out the value of R1 and trace out to where both ends are connected to, it should be connected directly to a Collector of transistor some where.
I wish some one else has the same board as your to take some readings.

[landyboy]

Re: Vizio sv421xvt

The value of R1 is 3.3k The closest thing to a direct path I can find is 20ohms to the E of Q5 and Q7 it is also 20ohms to the one side of the Cap CE1

[budm]

Re: Vizio sv421xvt

Pictures the area of Q5, Q7?

[landyboy]

Re: Vizio sv421xvt

Here you go

[budm]

Re: Vizio sv421xvt

Emitter of Q5 nad Q7 are tied to Ground. Collector of Q7 is Connected to Base of Q5, so the Collector voltage of Q5 cannot go up any higher than 0.7V.
I wish I have the board in front of me.
You can also see VIA (round copper pad with very small hole, just above "R11" printing) by the pin 1 connector, it must be connected to something if they put a VIA there.

[landyboy]

Re: Vizio sv421xvt

The VIA you are talking about is a direct short with the Standby pin.

[budm]

Re: Vizio sv421xvt

But it must be connected to the trace on the bottom side of the board, on the bottom side, does it have any traces going some where else or just pin1? I am running out Idea since I do not have the board in front of me.

[landyboy]

Re: Vizio sv421xvt

Hre is a picture of the underside with that VIA circled in red

[budm]

Re: Vizio sv421xvt

OK, it does not look like it goes any where.
I am thinking about modification to get the PS_ON signal since we cannot find out exaclty where the transistor that generate the PS_ON is.
Can you tell me againg the DC voltage on the Collector of Q1 and Q5 when the TV is off and when the TV is on?

[landyboy]

Re: Vizio sv421xvt

OK
Q1 Off 5v On 0V
Q2 Off 0v On 2.713V