Old Samsung LCD blowing fuse. Where to begin?

[JPicasso]

Hello,  First time post, impressed by how active these boards are.

I have an old Samsung LN52A750R1F LCD TV that is currently blowing the fuse when it's plugged in.  The TV was operating normally and then it just turned off.  I replaced the fuse and it blew within seconds of being plugged in.

About a year ago, I replaced two of the capacitors (CM803 and the one next to it) to fix the intermittent power on issue and it has been running fine since then.  We have had zero other issues with this TV, it's been a champ and I'd like to try and get it back running.

So aside from the fuse there are no other visible problems, like burn marks or puffy capacitors.  I don't know where to begin.  It would also seem that replacement BN44-00201A boards are no longer much available.

Any help is appreciated.

[Armadillo]

Hi;

I am Armadillo from a small country; Welcome;   

Break down the power supply into section, switching, pfc, backlight and the incoming section.

Take out fuse FI801 to break switching from PFC, at the same time check if the fuse is blown,
take out D1810 [supplying to backlight].

and then you check for shorts by measuring ohms,

Suggestion;

First you unplug, verify uplug and discharge all capacitors.

QP801 and QP802 and across Bridge rectifier BD801S Pin 1 and Pin 4; [PFC]
QM803 [Switching]
across CX803S; [Power incoming] and also cross Bridge rectifier BD801S Pin 2 and Pin 3
Q1813 to Q1816 [backlight]

identify the faulty section and troubleshoot from there.

[JPicasso]

Great info in your post.  we'll see how this goes.  My skills are def beginner level.

FI801 is the fuse that is blowing.


I found DI810, it's screwed to a heat sink, and looks more complicated than those capacitors, but I think I can get to it.  Is this the best course of action considering what fuse is blowing?

Thanks.

[Armadillo]

You can go straight to switching.

[Armadillo]

For shorts, after you replaced the faulty components,
cursory measure ohms before you conclude ready for test.
And use the light bulb in series method. 

[JPicasso]

Okay.  Took me forever to find what I was looking for, and apparently I need bifocal glasses.   

So, CM814 has shorted.  It's a 1kv 470pF ceramic capacitor.  I can get a replacement, no problems.  It does have some caulk on it, I'm presuming its to help with heat.  What is the best way to remove, and do I need to apply some caulk after I've replaced the part?

All the other parts in the vicinity seem good.  Resistors are all showing the ohms I'd expect.  Neat.

[Armadillo]

Screw Driver poke and pry would do but don't pull onto other components. You know what to do, common sense.
If you want to apply it, use the hot melt polyimide adhesive for pcb, but since you are not transporting it around, I don't think it's absolute. The next time you want to look, you know where to find.
That's fast. No beginner. Great; 
Update us when finally done.   

[james_s]

It's unusual to see a ceramic capacitor shorted, are you sure it isn't something in parallel with the capacitor?

[JPicasso]

Oh sure, james just had to ruin things! 

I pulled the capacitor off the board (and lets not talk about my solder wick skills) and tested it separate.  no short.  after I got the goop off of the cap, it's visually fine.  methinks it's not this piece.

So I check around the QM803 thing.  Two of the leads short out against each other.  surprise surprise, they are in parallel with the CM814 cap.

In the attached pic you can see my CM814 sittin on its side.  QM803 shorts out between the center and rightmost pin.  Is that broken? Not exactly sure what that is.  What do I replace it with?

Thanks

[james_s]

I can't see the number on it but that looks like the chopper mosfet in the power supply, which is exactly what I'd expect to find shorted if the line fuse is blowing. There's a reasonable chance that the IC driving it has also been damaged.

[Armadillo]

It would serve as a good reference material for others in this forum, if you post a picture of the measured ohms across the removed CM814 on the bench. You can use crocodile clips.

[JPicasso]

The Schematics that were posted (Thanks Armadillo!) indicate that it's a 2SK3532(600V/9A)_TO220F  I can find data sheets so far, but no parts.  Guess I'll keep looking.

So we would expect the IC... ah, ICM801A, which is a ICE2QS01 to need replacing also?  Can I check that, or is it just another part that is worth replacing?  Seems like if I have to desolder this bad boy, I might as well replace it. 

Where does one find these parts in the US?  The only luck I'm having is Australia or European sites.

Continued thanks for all the replies!

[Armadillo]

if you are not in a hurry, you can try ebay, from HongKong or China. Check carefully, some are salvaged parts.

But if you are in a hurry, you need to get equivalent from example mouser. STF7N90K5 I think is a better part than original, but it's expensive plus delivery I don't know how it will turn out.

Regarding the IC, that depends if the shorts goes to Pin 1, the Gate. To save the trouble and risk, you can buy the IC but only install if necessary, otherwise keep as spare.   

[JPicasso]

Parts have arrived!  I ordered some spares, from mouser.  AFAIK, they are close matches to what was originally installed.  Love how the shipping costs more than the parts, but I'm glad they are able to ship such small quantities.

How best to remove the mosfet from the heatsink behind it? Check the picture above in replay #8.  It's glued (or epoxied?) to the aluminum frame behind it.  Do I need to desolder both, and use some better persuaders, or is there an easy way that I'm unaware?  I'm afraid if I try and wedge a razor between, I'll start damaging the board.

And yes, I've removed the screw.

Thanks!

[Armadillo]

Have you not removed transistor from a heatsink before?

The white creamy white thing directly behind the mosfet is called the "Heat Sink Compound". Just pry it away, no need to worry a thing. It's not made of "toufu" aka bean curd.
BUT, be aware. If the screw is insulated with a sleeve, you need to put the sleeve back. If the transistor back is completed with a mica sheet, same you need to put it back.
You need to use IPA to clean away old heat sink compound from the heatsink and apply new compound. A tiny bit, even spread thin layer will do. Thick layer is the worse.
Don't say you do not have heat sink compound.   

[JPicasso]

Oh, yeah, sure, I've removed lots of these black thingies from the silver thingies.  Like, all the time. 

So there's no sleeve on the screw,and I don't have an IPA, but I do have some chocolate stout in the basement fridge, so I'll use that.  .

Seriously, I'm concerned I'll wedge it out and destroy the board.  I think I'll clip the leads first, so there's less pressure on the board.  I"ve got some alcohol to clean the stuff off, but I don't think I have any to secure the new pieces.  I've got some thermal paste around here somewhere.

[amyk]

It would also seem that replacement BN44-00201A boards are no longer much available.

It has a 13V and 5.1V output along with a 5.1V standby rail, and an active-high "power on" input --- an ATX PC PSU with 12V and 5V will work just as well with an extra inverter on the power-on signal.

I've converted more than one LCD with a bad power supply to use a regular PC PSU. 13V is likely for the backlight inverter which will be fine with 12V, and 5.1V is close enough to 5V to not matter --- it's the digital supply, and there will be LDOs on the main board to lower that too.

[JPicasso]

Okay.  After much work, prying the mosfet off the heatsink was just a matter of getting a good bite before I pryed it.  And I'm beginner lucky with the solder wick, I managed to replace the mosfet and the IC.

But when I plug it in, I blow the fuse again, AND the mosfet is now showing a closed connection between the second and third lead again.  DAMN.

Would someone be so kind as to check my part replacements?

Mostfet original was 2SK3532  600V/9A TO220F
I replaced with mouser # 726-IPP60R280CFD7XKS   link: https://www.mouser.com/ProductDetail/Infineon-Technologies/IPP60R280CFD7XKSA1?qs=sGAEpiMZZMshyDBzk1%2fWi%2fD7Em5shE8qwJaYgRZbrrIvr96nedti4Q%3d%3d

And I replaced the ic ICE2QS01 with mouser # 726-ICE2QS03XKLA1  link: https://www.mouser.com/ProductDetail/Infineon-Technologies/ICE2QS03XKLA1?qs=%2fha2pyFadujK5gY6BJfyHGCv8xqyNdcgLisQCbfN%2f6BKOZmJX%252bzd%2fA%3d%3d

I'm at a loss at this point.  Might have to bite the bullet and get a new TV.

amyk:  I do have an old power supply sitting right here.  I could use it, but I'm not sure how to hook up what to what. 

[Armadillo]

For shorts, after you replaced the faulty components,
cursory measure ohms before you conclude ready for test.
And use the light bulb in series method. 

Edit:
TO-220-3 is different from the TO-220F Package [insulated].
The STF7N90K5 recommended to you is a TO-220FP package.   

[Armadillo]

Come to think of it;

The 2SK3532 you bought "may" still survive. Take it out and measure it.

If survived; you have 3 options;

1.0 Buy insulating kits comprising mica + sleeve. But considering this 1 cents thing will be expensive when you buy it and also possible mistake in installing it, I would therefore not recommend.

2.0 Buy STF7N90K5. Check other components before concluding all purchase.

3.0 Buy a new TV and stop further risks.