Vizio VW42L -No Power/Fuse keeps blowing

[vote4pedro]

Hey Everyone,

I have a Vizio VW42L HDTV10A Flat screen TV.
This TV just one day cut off on its own and now will not power up.

I took the back off and stated looking for a blown fuse. The only fuse that I could find was near were the power comes into the TV was blown.

I replaced the fuse with an exact rating fuse, only the original one was ceramic and I used a glass fuse.

The problem is that after I replace the fuse and plug the power into the TV the fuse blows instantly.

I was wondering if you guys have experienced this problem with the Vizio TV's and if you would have any advise on troubleshooting this problem?

I was thinking that there has to be a short somewhere, but I never had any trouble with this TV until this happened.

Thanks for helping.

[tom66]

Post some pictures.

A fuse which blows repeatedly generally indicates:

- a shorted bridge rectifier; or
- a shorted power MOSFET

Both defects are on the power supply. Hopefully it is the bridge, as this is easy to replace.

[vote4pedro]

Here are some pics from the inside of the TV.

I appreciate you looking at this for me.

Can you point out to me were the bridge rectifier or MOSFET
is located. How would I test these to check if they are bad or good?

Thanks again..

[tom66]

Can you unplug the PSU and unscrew it, and post a picture of the underside (green side)?

[clean-machines]

there is the bridge rectifier right next to the big cap on its side go past the 4 blue bits and its the large flat black thing on the heat sink it has 4 pins going down to the board thats the bit toms on about check it for shorts like tom said the drive transistors are the other large black squares on the heat sinks with 3 legs set meter to diode and test to see if theyve shorted that will be a good start

[clean-machines]

if the meter beeps at you a lot on one or more of the parts your probing or you get a verylow voltage drop across the components then that point toward the failed component un solder it from the board and check again if its still shorted then you know its that, once youve found the shorted part if its the bridge just change it and your fixed if its the transistor then trace the driver by tracing the track from the gate pin of the shorted transistor and again check the driver to the transistor it may be a small ic or a small sm transistor looking part but again put some more pics on so tom can see the whole board and he will point this out to you , as chances are the blown fet could have damaged the fet driver too

[vote4pedro]

Here is a pic from the backside of the PSU.

Thanks again for your time with this. I marked on the pic the area of the fuse.


Thanks

[tom66]

This is your bridge rectifier.

To test:
- Unplug board.
- Mentally number the pins 1 thru 4.
- Set meter to DIODE test; red probe in VOLT/OHM, black in COM.
- Test from 1 to 2, 3, 4.
- Test from 2 to 1, 3, 4.
- Test from 3 to 1, 2, 4.
- Test from 4 to 1, 2, 3.

If the bridge is good, on all readings you will either get a single "1" or "OL", or a reading around 400 to 800. If something is bad with the bridge or something further along the line is shorted you will get zero or nearly zero.

[vote4pedro]

let me know if this pic is not clear enough and I will post another one, I didn't know how large of a photo I could post.

[tom66]

Gah! I'm an idiot!

I forgot to upload the attachment.

Try my test this time with the attachment to help.

[vote4pedro]

Thanks alot, I will do the test and get back to you.

[vote4pedro]

Hey Tom,
I did the tests on the bridge rectifier and here is what I got.

Test #1
Pin 1 to 2 = 482v
Pin 1 to 3 = 486v
Pin 1 to 4 = 0v

Test #2
Pin 2 to 1 = 482v
Pin 2 to 3 = 933v
Pin 2 to 4 = 482v

Test #3
Pin 3 to 1 = 486v
Pin 3 to 2 = 933v
Pin 3 to 4 = 486v

Test #4
Pin 4 to 1 = 0v
Pin 4 to 2 = 482v
Pin 4 to 3 = 486v

Since I got a 0v reading on pins 1 to 4, I suppose this makes the bridge bad.

Do you have a good source to where to purchase the bridge rectifier from?

Is there a voltage rating on the bridge? I wanted to make sure I get the correct one.

Thanks again for your help and time with this.

[budm]

Pin 1 and 4 are the output pins, since they are connected to the load, if the load is shorted, I.E. shorted MOSFET, then you will get 0 reading. You should remove the bridge and check pin 1 and 4 of the bridge out side the board, and also check pin 1 and 4 of the board to see if it still read 0 without the bridge in place.

[tom66]

If you have a short on 1-4 you have a dead load side, so a shorted FET or similar.

edit: Budm beat me!

[vote4pedro]

let me make sure I understand.

remove the bridge rectifier from the board and perform the same test on the pins.

are you saying were the bridge is connected to the board it will show a short on pins 1 & 4 so removing it from the board will eliminate this and give a more accurate reading?

so far is the reading on the pins within normal range?


thanks

[tom66]

Remove the bridge from the board (unscrew it from the heatsink and use a flat headed screwdriver to separate it from the thermal paste, then desolder each pin using a soldering iron and wick or a solder sucker.)

Then test the bridge, if it passes the test then it's not at fault. Test between 1 and 4 (without the bridge in place, on the board itself) and if there is a zero reading there then there's a fault further down the line.

[vote4pedro]

OK I got it, I will have to go out and get some solder wick.


Just wondering, If the bridge is bad what voltage reading would I get on pins 1 & 4 with it out of the board?

I will post the results later.


Thanks a bunch

[budm]

The meter should show low Ohm and then rises up as the main DC cap starts charging by the meter into K ohms range.

[vote4pedro]

OK, I removed the bridge from the board.
Here is what I came up with.

Pin 1 has a (-) symbol
Pin 2 has a (~) sign wave symbol
Pin 3 has a (~) sign wave symbol
Pin 4 has a (+) symbol

I don't know if this matters or not, but If I put the black lead on pin 1 since it has the negative symbol on it and the red led on pin 4 I get a "OL" reading.

If I put the red lead on pin 1 and black lead on pin 4 I get a .885v

If I test pins # 2 and 3 that have the sign wave symbol I get a "OL" reading.

What do you think? Does the bridge appear to be bad?

Thanks 4 helping

[budm]

Please put the Ohm meter where pin1 and pin4 on the board and tell us what you read in Ohm (put meter in 200 Ohm range).