LCD TV Powersupply problem +Intro

[FrenicX]

So I've been searching around the web to try to figure out what I can do to fix my TV. I found this forum and figured I'd try it out.

I have always preferred to do things myself because I love the learning processes and the pride that comes from the accomplishments. I am already somewhat of a techy and I can build/repair a computer with the best of them but I have never tried a board level repair like this. Some of my other interests including rocketry, RC cars, Automotive work, DIY home projects, PC games, ps3.. well the list goes on.

So onto my problem, and I thank you guys in advance for any info you might offer.

I have a Toshiba LCD TV model# 42RV530U. It doesn't turn on at all. No sound, no lights. Nothing. I did all the outlet testing, waiting hours while unplugged, so I went ahead and opened the TV up.

There were two fuses on the power supply board. One was for 250v/8A right near the power cord and the other was 250v/2.5A which seems to come after a lot of items on the board. The 2.5A fuse was blown.

I went to Radio Shack and got some new fuses of the same Specs. Each time I plugged the power cord into the wall, the new fuse would blow. I removed power supply board from the TV and found that the fuse will blow while outside of the TV as well.

I looked for any broken or loose solder joints but they all seem fine except one that has too much solder in the joint, but it's solid and isn't spilled over if you know what I mean. I can take good pictures of both sides of the board if that helps.

I have a good multimeter, and I played around with the continuity test a bit, but I have no idea what I should test. Keep in mind I am very inexperienced in this field but I do know what most items on circuit board are, such as diodes, resistors, capacitors, transformers.

 

[(*steve*)]

Take a photo of the board. It is likely a switchmode power supply, and I would suspect that you possibly have either a shorted diode or transistor.

I would measure between the pins of all the devices attached to heatsinks. See if there are any pins that have zero (or very low) resistances between them. Note that external circuitry may cause this, so don't be overly surprised -- but it may indicate places to spend a little more time on.

Also check any large diodes you see for diode action. (resistance much higher in one direction than the other) Even better to use a diode test function if your meter has one.

 

[FrenicX]

One side of the fuse goes directly to a few small capacitors which connect to the heatsinked transistors. That side of the fuse is also connected directly to the 400v capacitor. From there it goes to a high voltage diode, one biggest on the board. The other side of the fuse goes to a small capacitor and an item marked with an "L." from there it's onto huge item that looks like a transistor but has 11 pins and a heatsink.

 

[SeasonedTech]

As a tv repairman/bench tech, it's difficult to diagnose, let alone do repairs on these SMPS boards, .....we mostly just order a new one and be done with it.
In the rare cases where we techs DO manage to do a successful repair on an SMPS, it's usually a matter of replacing caps, maybe a diode.

But now imagine a novice/tinkerer..... with no understanding of SMPS design/operation.......good luck.

 

[FrenicX]

Well no harm in trying I guess. But at this point the extent of my knowledge would be to remove and test, but really there's so much to test and some things I am not sure I can even test.

I'll will still hold out though. If there's more ideas I'll gladly take them.

To be honest though, I was ready to have it repaired at a shop but really not in my budget. I saw how easy it was to remove the board and thought I might have a go at it. Then I checked out who sells boards and I can't find the board in stock anywhere so I went back to trying board level. But if it's a lost cause a may have to keep looking.

Does anyone know where I might find the specific board for sale?

 

[(*steve*)]

SeasonedTech -- you'd be surprised. We have talked people through the types of repair that I thought were almost impossible.

FrenicX, what SeasonedTech says has a lot of truth behind it. If you value your time highly (or even not so highly) then it may be better buying a new unit. There is no guarantee that you will be able to get it going again.

Also, it is better to take photos like the first and second without a flash. Take the board outside, or somewhere with lots of light and/or use a tripod to keep the camera still during the longer exposure. Try to have the board fill the frame and don't downsize the image. If the image is not sharp enough to read all of the writing on the board -- go take another one. Upload it to something like http://imageshack.com if it is too large to upload here (and give us a link to it)

I am presuming that the TV died suddenly, and had not had a history of being hard to turn on

However, having said all of that... For each device attached to heatsinks, do the following:

1) Give them a number from 1 to n, and mark this on you photo taken from the top of the board

Note down and tell us
2) The part number on the device (if you can't figure it out, tell us everything)
3) how many leads
4) The part designator on the board (e.g. D201, Q7, IC31)

Then, for each part that has 4 or less leads, and with your multimeter on a 10k or 20K resistance range, or a diode test range if you have it (and yell us what range you're using):
5) Call the leads on the device 1, 2, 3, 4 from left to right from the direction you can read the part number.
6) With the RED lead on pin 1, measure and tell us the reading to pin 2, 3, ...
7) with the Black lead on pin 1, measure and tell us the reading to pin 2, 3,...
8) repeat steps 6 and 7 for the remaining pins (pin 2 to pins 3, 4; then pin 3 to pin 4)

9) post a reply with all of this data.

If this seems like taking too long for you, start with the components closest to the fuse which blows and post the results after you've done the first few.

The device with many leads is almost certainly an offline SMPS controller. It can't be meaningfully tested the way I'm asking you to test the other devices.

 

[FrenicX]

Thank you Steve, I was a bit discouraged there for a moment.

I will take better pictures tonight and hopfully get things numbered and setup too. I think I can get the testing done tomorrow sometime hopefully. It shouldn't Be too hard since there are only 10 devices on heatsinks and one is a large 4 lead, the other is the SMPS controller as you put it. The rest have 3 leads.

Just to add, I think the large 4 lead item has two AC leads and a pos/neg lead based on the symbols I see. There is also only a diode and the 400v cap that separate the fuse that blows from the positive lead on this item.

 

[(*steve*)]

I was a bit discouraged there for a moment.

But don't get too encouraged either -- at least just yet.

Just to add, I think the large 4 lead item has two AC leads and a pos/neg lead based on the symbols I see. There is also only a diode and the 400v cap that separate the fuse that blows from the positive lead on this item.

Start around this area. It is fairly likely that the 4 legged device is a bridge rectifier and failure of this can lead to pretty much exactly the fault you describe. That's not to say that a failure in other components won't do the same.

 

[SeasonedTech]

I'm anxious to see how this plays out. I really am.

Particularly since I read:
"I went to Radio Shack and got some new fuses of the same Specs. Each time I plugged the power cord into the wall, the new fuse would blow. I removed power supply board from the TV and found that the fuse will blow while outside of the TV as well. "

First/original fuse that blew was because of a massive fault.

Repeated attempts at replacing fuses most likely took out more and more parts on the panel.
Good chance some of the non-replaceable surface mounted chips underneath got fried.

I've seen 'em come in my shop with tinfoil wrapped around the fuse and a board fried beyond recognition.

Hint: never replace a fuse..... 90% of the time it's a waste of time. (as this case shows)
The original problem HAS to be repaired FIRST... there's a REASON the fuse blew in the first place.
Otherwise more damage will be done.

But good luck.

 

[FrenicX]

I'm anxious to see how this plays out. I really am.

Particularly since I read:
"I went to Radio Shack and got some new fuses of the same Specs. Each time I plugged the power cord into the wall, the new fuse would blow. I removed power supply board from the TV and found that the fuse will blow while outside of the TV as well. "

First/original fuse that blew was because of a massive fault.

Repeated attempts at replacing fuses most likely took out more and more parts on the panel.
Good chance some of the non-replaceable surface mounted chips underneath got fried.

I've seen 'em come in my shop with tinfoil wrapped around the fuse and a board fried beyond recognition.

Hint: never replace a fuse..... 90% of the time it's a waste of time. (as this case shows)
The original problem HAS to be repaired FIRST... there's a REASON the fuse blew in the first place.
Otherwise more damage will be done.

But good luck.

Noted. Thanks.

 

[FrenicX]

Hey everyone who read this thread back in the day. I just thougt I would tell you all what happened.

So I ended up forgetting the board level repair and instead found a damaged part for 30 bucks on eBay. It was missing 3 caps. I got caps from radio shack and put it all back together and it worked, no problem.

This all got me thinking though, so I started going to campuses and touring schools electronics programs. I then enrolled in a "computer electronics engineering & technology" program and graduated with an associates last month, continuing on to the bachelors program now. My career situation is much better and my TV is still running, thanks to those 3 caps.

Thanks to all who responded or even read the thread, for sparking my interest.

 

[(*steve*)]

Well, I think that's a great outcome, and I think even SeasonedTech would agree with that.

 

[elebish]

Just replace the board, probably cheaper in the long run and much faster. Sometimes we're the windshield and sometimes the bug! Ed.