VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no picture

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

U37 is the MOSFET that we put the jumper wire between the Source (pin 1& 3) pin and the Drain pin (pin 5~8). The Gate pin (pin2 & 4) of U37 is connected to transistor Q26 pin C (C = Collector).
The Q26 is getting the correct voltage to bias it on (0.6V on the E (Emitter) and B (Base) but the Collector voltage did not go down like it should and stuck at 12V, that means that Q26 is not functioning.

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

i am confused again...we put the jumper wire between pin 1-4 of the mosfet and the + side of the capacitor

but you seem to be saying i put it between the source and the drain? on the same mosfet?

also i tested the voltage on C (collector) and it was .3v not 12...

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

"i am confused again...we put the jumper wire between pin 1-4 of the mosfet and the + side of the capacitor" No.
If you look at that MOSFET U37, the bottom right has a little dot to indicate pin 1. Pin 1 and Pin 3 are the Source pins (which the 12v is fed into) which are connected to the + of the cap. pIN 5~8 (the left side) of U37 which are all tied together are the Drain pin (this Drain pin which where the 12v comes out, is then connected to pin 30 of J8), so what we did is to connect the S and D together to by pass U37. Pin 2 and 4 are the GATE pins which are tied together and connected to the Collector pin of Q26, Q26 get the ON/OFF signal to the BASE pin from processor, but the Q26 is not going on and off it is stuck in the off mode which causes the U37 to be stuck in the OFF mode.
Now I may be wrong about the Q26 connection since now you are saying that the Collector pin is at 0.3V not 12V, then we need to find out if the Collector pin is actually connected to pin 1 and 3 of U37 MOSFET or not. That Collector pin should change state when TV is powered on and powered off.

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

ok much clearer now i was thinking pin 1-4 were on the left side for some reason... but u say the source (1 and 3) get 12v but i only measured 11.95...is that close enough? pins 2 and 4 (the gate pins) are getting 12.05 each... what should q26 be doing to modify those voltages? what voltage does the gate want to see before it will push 12v out the drain?

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

cause as i said, q26 puts .3v out its collector and i guess i just dont really see what is happening there...

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

ok im gonna check some voltages on the collector and try some resistances to see where its going to?

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

11.95Vdc is good enough for 12V, less than 1% off. The minimum turn on threshold Voltage between G and S is typical around 3~5V for the MOSFET to be on, right now the G and S voltage are about the same that is why it is not on. So far I cannot find the spec for the 4853 yet.
So in 2 (or 4) of the U37 does not change state when TV is off and when the TV is on? Same for the Collector of Q26?

http://electronics.stackexchange.com...et-as-a-switch

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

the collector does change states. when the tv is off, (standby) it is 0, when i push the power button it goes to .02v

with the tv off i measured high resistances between the collector and each of the 4 pins (source and gate) on 20k ohms scale i got 11.99, 8.97, 11.99, 8.97
funny thing about resistance with my meter if i reverse the probes i get a slightly different resistance.
in this case when i held the red probe steady on the collector and use the black probe to test resistances i got 7.10, 7.42, 7.10, 7.42

any idea how that would be happening? i thought resistance shouldnt matter which way the probes were?

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

If the Collector of Q26 is connected to pin 2 (4) then it should show <1 Ohm, so the Q26 may not be the one that is used for controlling U37 MOSFET.
At this point, I would use the TV as it is for now. This is getting long to troubleshoot further more for me.
the resistance reading will be affected if there is semiconductor device connected to the circuit you are measuring the resistance.

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

yeah i think its fine without that mosfet in the way actually.... you were saying that there would always be 12v present there even in standby but there isnt. i think the capacitor doesnt get any power until you push the power switch, so theres no 12v going into the mosfet or coming out of it... ill just tidy up those solder connections and call it good. maybe ill try figure out whats feeding the mosfet some other day but if it doesnt affect the way the tv works... anyway thanks a lot for your help and i hope you will help me again with this other tv i got...it just doesnt turn on at all, making me think it must be the power supply...but i guess we'll see! Thanks again! one more TV saved from the dump... great job!

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

The only time that the 12V will not be present is when TV is OFF, in standby mode (not off), the 12V will be present, normally that 12V feeding the T-CON will be off when TV goes into standby mode (not off mode, TV goes into standby when it does not detect video signal from PC or from any video source so it goes into standby mode (not the same as off mode), thati s why the MOSFET is used to swithc off the 12V feeding the T-CON, this is done to save power consumption when TV is in standby.
I also thing that U37 is 4953GM, not 4853GM.

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

ooh i just figured out tho that pin 2 and 4 have .3 ohms resistance to the left side of r351 and .4 ohms to the right side of r351 ....could that be a clue? should i see voltage coming from that...is it a resistor (hence the R) ?

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

Yes, R = Resistor, C (or CE in this board) = capacitor for this board.
See my update on post 151.

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

and no i have double and triple checked it, the j30 is not getting power, the u39 is not getting power, when the tv is in standby these things are defnitely at 0 volts. then go up to 12 when i push power

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

oh wait i think youre saying that there is a difference between plugging the set in and not turning it on, and plugging it in and leaving it on until it goes into standby on its own with no input...i thought those 2 standby modes were the same...

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

but i guess theyre not...as one is waiting for video signal input, and the other is waiting for a push button input before it will turn on...hmm maybe i will have 12v always present in standby like u say...but thats not a big deal im a big fan of turning things off when im done using them

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

No it is not the same. It all has to do with power saving. So you are fine, just turn off the TV when you are done.

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

yeah but if i wanted to sell the tv, i should repair the component or replace the board...
so if i can ask just a couple more questions and then ill try to figure it out on my own from there.... u said the mosfet needs to see a different voltage on its source and gate... but the gate is being fed 12v from that r351 ... the source has to be 12v for it to be able to push 12v out the drain, so what im trying to figure out here is how to get the gate pins to read a significantly lower voltage, right?

[triplefour]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

also i think because this mosfet is unable to do its job, even though im waiting forever, i dont think its ever going to go into standby on its own...or that is to say i dont think it can go into standby mode anymore?

[budm]

Re: VIZIO V037L FHDTV20A powers up, backlight on, sound works, buttons work, no pict

Put it into PC mode or disconnect the antenna/cable from the tuner when in TV mode, it should then go into STBY mode after a few minutes, some you can setup the time in the user menu for how long before it will go into standby if it does not detect signals.