V042L no picture, but sound, bad MOSFET?

[djsmith]

I have a Vizio V042L. It's about 4 years old, and I had one inverter board replaced by a local repair shop about a year ago when it exhibited the same symptoms as these threads:

https://www.badcaps.net/forum/showthread.php?t=14667
https://www.badcaps.net/forum/showthread.php?t=18010

Now it's happening again: no picture, but all other functions in tact, including audio, but my problem seems different from those other threads (hence the new thread).

When I plug the set into the wall, the Vizio logo illuminates orange. When I press the power button, the Vizio logo illuminates white, but no picture. I tried the flashlight test, and can't see if there's any picture there at all, but there's definitely no backlight. About 2 seconds after powering on the set, a blue LED illuminates quickly on the master inverter board. After about 1 second, it slowly fades to dark over the course of another second.

So far, it seems the same as the other threads, right?

The devil's in the details here. I think my problem is different. Read on for the exciting ending!

After reading the very helpful information on those threads (and a few others), I began troubleshooting the set.

Here's what I found:

GOOD: Power supplies appear okay. 5v standby voltages are good when turned off, and all voltages are good when turned on. No capacitors appear puffed or smoked. The ICs and leads look good on each inverter board, but I graciously defer to the more masterful eyes on this forum to review my photos (attached, hopefully high enough resolution).

BAD: I measured the resistance across each of the 8 MOSFETs, 4 per inverter board. At least, I believe these are MOSFETs--please correct me if I'm mistaken. They all read 8.5 Kohm, except one MOSFET on the slave inverter board (circled in red), which reads in the 10s of Ohms (seems way too low compared to the other 7).

So I am suspecting this one MOSFET (circled in red).

My question to you, oh great masters of Badcaps:

1. Am I on the right track in diagnosing this problem?
2. If so, can this MOSFET be replaced?
3. My inverter boards are marked as Rev 1.2. Should I just buy 1.3 replacements anyway?

I have soldering experience, and this looks within my ability (the IC looks a lot harder by comparison).

Thank you in advance for lending your time and expertise.

[retiredcaps]

Re: V042L no picture, but sound, bad MOSFET?

The devil's in the details here.

1) First, thanks for a detailed post and uploading pictures using manage attachments.

A great change from the "help, it doesn't work" threads where the OP has no pictures, no model number, no punctuation, no grammar, no paragraphs, etc.

My question to you, oh great masters of Badcaps:

1. Am I on the right track in diagnosing this problem?
2. If so, can this MOSFET be replaced?
3. My inverter boards are marked as Rev 1.2. Should I just buy 1.3 replacements anyway?

2) Yes, that mosfet is shorted. Desolder and retest out of circuit to verify. If it is not shorted out of circuit, then something on the board is shorted.

3) Yes it can be replaced. You need a decent solder tip size (chisel or bigger probably).

4) You may want to order more than 1 in case these mosfets work in "pairs". If one is shorted, the other may be next to go.

a) If the part number is K4075, that is a 2SK4075 made by NEC.

b) You can get 2 of them for 99 cents + free shipping on ebay.com. Better yet, get 4 or 6 for that price in case it fails again.

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

I attempted to desolder the MOSFET for the purpose of measuring its resistance out of circuit. I'm embarrassed to say that my tools were not up to the task (certainly, my soldering skills couldn't be questioned, right!).

I used a wide-tip soldering iron, untinned (woops), and by applying a little too much torque to the leads to lift them from the pad while applying heat, I actually *broke* the leads free from the mosfet.

In doing so, I appear to have damaged the pad and trace where the leads connected. Photo attached. In the photo, you can see that one of the pads is actually gone, exposing a white material below (appears non-conductive).

Have I bricked this inverter board? Is there something I can measure to determine the extent of my damage?

There does not appear to be an emoticon to express my disappointment in myself.

[Scenic]

Re: V042L no picture, but sound, bad MOSFET?

desolder the rest of the MOSFET with lots of solder and the iron at the tab.
Try to re-tin the pad that looks like it's ripped off. Doesn't actually look like it's ripped off, more like cracked-away dull solder.

If the center part of the pad is really gone, you could scrape off the protective top layer on the surrounding area with a small flat bladed screwdriver (carefully!) till you see the copper, and tin that, making sure it does make contact to the resistor (?) at the right and the other part of the trace that goes under where the legs would have been.

It's not pretty, but fixable (except the MOSFET itself)

[retiredcaps]

Re: V042L no picture, but sound, bad MOSFET?

Have I bricked this inverter board? Is there something I can measure to determine the extent of my damage?

IF the 2 inverter boards are symmetrical, you can "ohms" out the various pin connections to make sure you didn't damage anything when you install the new mosfet.

There does not appear to be an emoticon to express my disappointment in myself.

Maybe?



Of course, just having a bit of fun. No disprect intended.

[bbjunkie]

Re: V042L no picture, but sound, bad MOSFET?

To help a user in another thread, can you tell me the values (or colors) of R202 and R206 on the PSU board near the bridge rectifier?

Thanks!

Jim

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

So what do you think about this mosfet site now that I removed the mosfet?

I added a little solder to the two pads and the mosfet site itself. I took this photo with my iPhone through my microscope (thought that was pretty cool).

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

bbjunkie: here's the photo you requested of R202 and R206 (unrelated to this problem for future readers).

[retiredcaps]

Re: V042L no picture, but sound, bad MOSFET?

So what do you think about this mosfet site now that I removed the mosfet?

It looks okay to me, but my soldering skills aren't the greatest.

At worst, if you damaged something, you might be able to run jumper wires to the nearest component that is in same circuit path. That was my suggestion for "ohms" out.

I had to do that once for a Benq monitor. Not pretty, but it works.

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

The plot thickens!

I received my MOSFET by mail today and soldered it on to the board. When I measured the resistance across the two leads, it measured the same as before (much lower than the others), which concerned me. And sure enough, the TV still exhibits the same symptoms.

What could this mean?

[tom66]

Re: V042L no picture, but sound, bad MOSFET?

With the MOSFET removed, did you check the resistance?

These MOSFETs are ESD sensitive - handle them carefully. If you touched it too much, it could have shorted. Then again, I've only seen small signal MOSFETs go short circuit; bigger ones are more robust (but not completely immune.)

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

Before soldering the new MOSFET on to the board, I believe the two pads (where the MOSFET leads connect) actually read low resistance between them. That surprised me, but I am an electronics newb, so I let it go.

Would that indicate a problem somewhere else (not the MOSFET)?

[caphair]

Re: V042L no picture, but sound, bad MOSFET?

Sounds like something else is shorted giving a low ohms reading at the MOSFET. I'd trace back from the leads to any corresponding components and test for shorts.

[kc8adu]

Re: V042L no picture, but sound, bad MOSFET?

the ic bd9897 is bad.it looks to have been replaced before.
there is a mod out there to fix the repeat failures.

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

kc8adu: How can you tell that IC is bad? It looks fine to me.

[retiredcaps]

Re: V042L no picture, but sound, bad MOSFET?

Would that indicate a problem somewhere else (not the MOSFET)?

In post #2, point 2, did you verify that the original mosfet was shorted out of circuit?

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

In post #2, point 2, did you verify that the original mosfet was shorted out of circuit?

No, because I damaged it during removal.

[kc8adu]

Re: V042L no picture, but sound, bad MOSFET?

experience in fixing these.design flaw will cause repeat failure.i will try to find the mod.

kc8adu: How can you tell that IC is bad? It looks fine to me.

[PlainBill]

Re: V042L no picture, but sound, bad MOSFET?

experience in fixing these.design flaw will cause repeat failure.i will try to find the mod.

As a gesture of appreciation, this thread has a link.

PlainBill

[djsmith]

Re: V042L no picture, but sound, bad MOSFET?

How can I tell if the IC needs to be replaced? Also, if I do the mod linked above by PlainBill, will I still have to replace the IC?