Vizio SV422XVT no backlight, photos and details inside

[budm]

Re: Vizio SV422XVT no backlight, photos and details inside

Sorry about the designator mixed up. If that fusible resistor is open (check it with the TV unpluged and the the two large caps are discharged), then you need to take the resistance reading of all 8 MOSFET to see if they are shorted out. Looking at the MPSFET with tab on top, call the left pin as pin one, big tab is pin 2, right pin as pin 3. Take the reading and report them like this:
1-2 = xxx Ohms
1-3 = xxx Ohms
2-3 = xxx Ohms

FBxxx is piece of wire fed through a small Ferrite bead (it is basically a very small inductor), it will show as 0 Ohm.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

I'll report back with the transistors' resistance measurements as well as confirmation of the fusible resistor's condition.

What's the best way to discharge the large caps? In the past I've shorted small caps with an insulated screwdriver. I don't have many spare parts available, so I don't have a resistor to discharge them through. Would a household 120V lightbulb or 12V automotive bulb work?

[selldoor]

Re: Vizio SV422XVT no backlight, photos and details inside

Would a household 120V lightbulb or 12V automotive bulb work?

We usually suggest a 10k 2 w resistor ( or Higher), If you dont have one most modern caps do self discharge within a few minutes but given that this is on a faulty board I would set my meter on 600vdc and just test the caps after it has been off for a few minutes- puts it beyond doubt.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

Before testing, I measured the voltage across both C909 and C910. Both had discharged since my last session of testing.

I then measured for resistance across the points contained within the box marked "GR3." I am using an auto-ranging multimeter, and it read "0 L," which seems to indicate "infinite resistance" according to my Googling.

I also measured the resistances of eight MOSFETs on the rear of the board. I created a spreadsheet of the values and have attached a screenshot.

[selldoor]

Re: Vizio SV422XVT no backlight, photos and details inside

Suggests q 708 and Q709 are faulty - procedure is to remove them from the board to re-test - what are the part numbers on those two?

EDIT - I see you have posted the part no.


Probably one of these

http://www.alldatasheet.com/datashee...D/FDD5N50.html

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

I thought the values for Q708 and Q709 looked suspicious. I will report back when I've located my soldering iron and desoldering braid for testing again off the board.

Does my testing indicate that fusible resistor F601 is also faulty? I've attached a photo for help with identification. I haven't had any luck finding it by Googling the text "ICP250 4A00SMT" printed on its side. The closest I came is this TV repair blog post, which features an "ICP250 3A00SMT."

[budm]

Re: Vizio SV422XVT no backlight, photos and details inside

Yes, that fuse resistor is blown due to shorted out MOSFET.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

Great, sounds like I have some parts to order.

The blog post I mentioned earlier suggests TWW5JR15E as a substitute for ICP250 3A00SMT. Note the differing digit on the part numbers, "3" rather than "4" on this TV's ICP250 4A00SMT. I haven't been able to locate specs on this fusible resistor.

[tom66]

Re: Vizio SV422XVT no backlight, photos and details inside

It's this part (search ICP):


4A rated = 0.11 ohm.

[budm]

Re: Vizio SV422XVT no backlight, photos and details inside

Great job finding the spec Tom.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

Thanks, Tom! According to that spec sheet ICP250 4A00SMT is 250V, 4A, 0.11 Ohm.

Before your post, I chatted with a Digikey representative and he offered this TWW5JR10E as a possible alternative. It is rated at 5W and 0.1 Ohm. Does this look like what I need?

[tom66]

Re: Vizio SV422XVT no backlight, photos and details inside

Only problem is it's not fusible, I'm not sure if that will matter too much though as it will likely only move the failure from that resistor to the main fuse.

The lead spacing is correct, but the replacement device is a little wider by 4mm. Do you think you could still fit it in?

[tom66]

Re: Vizio SV422XVT no backlight, photos and details inside

Personally, I'd consider one of these devices:
http://www.digikey.com/product-detai...MCT-ND/2813098

If a 0.1 ohm resistor carries 4A max, it will dissipate 1.6W. This device is fusible and can handle 2W. The TV should use under half an amp for the backlight (@400V) as it's a fairly modern, efficient LCD TV. So I reason this device should be OK, especially as it provides the fusible option. Inrush is probably provided by the main NTC thermistor, so it's not a necessary feature.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

I like your suggestion of the fusible resistor vs. the one I had offered. I should have no problem fitting an axial lead component in place of the existing radial one.

I admit I'm quite out of my element at this point... Any suggestion as to which MOSFETs I may need? I found this list at DigiKey and noticed some differences in the specs.

I will attempt to re-read the part numbers on the defective MOSFETs and report back.

[tom66]

Re: Vizio SV422XVT no backlight, photos and details inside

If you get the part # we can look up the specifications and find an equivalent device.

[budm]

Re: Vizio SV422XVT no backlight, photos and details inside

Try using Alcohol to remove the flux off the body then you may be able to read the numbers on it.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

I've attached a macro photo of Q709. As far as I can tell, it says:

1K32CB
FDD
5N50

A DigiKey rep was unable to cross reference that number. I understand the lowercase script 'f' indicates a Fairchild part.

[selldoor]

Re: Vizio SV422XVT no backlight, photos and details inside

in post 16 gustagen said
The eight transistors you've circled all seem to have the same part numbers. They are all very difficult to read because they seem not to have been stamped hard enough for all digits to be clear, and are coated with a shiny, gummy substance. Using a magnifying glass and piecing the part numbers together from each one, I'm confident they say:

1K32CB
DD
5N50

from that I said post25
Probably one of these

http://www.alldatasheet.com/datashee...D/FDD5N50.html
Last edited by selldoor; Today at 12:34 PM..

Which suggests one of these?

http://www.digikey.com/scripts/dksea...eywords=DD5N50

[retiredcaps]

Re: Vizio SV422XVT no backlight, photos and details inside

Selldoor's URL didn't work, but it is here

http://pdf1.alldatasheet.com/datashe.../UTC/5N50.html

Sometimes when looking for part numbers, the first line is not always the actual part number. I, too, tried 1K32CB first but couldn't find anything.

[gustagen]

Re: Vizio SV422XVT no backlight, photos and details inside

I had previously reported the second line of text as DD, instead of FDD.

I found this page at Fairchild Semiconductor which seems to indicate the top line of text refers to factory and date codes.

EDIT: Looks like retiredcaps beat me to this!