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-   -   Sylvania lc320ss2 LCD TV keeps blowing fuse (http://www.badcaps.net/forum/showthread.php?t=25477)

Beowulf 01-10-2013 02:36 PM

Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Hi guys,
sorry this is going to be a long post and I'll include pictures. I just want to give out the best info to increase my chances of getting the right advise :)

Here are the pictures from the MPW (P/N BA17F8F0102 2) :

http://i976.photobucket.com/albums/a...01022-Copy.jpg

http://i976.photobucket.com/albums/a...omed2-Copy.jpg


The first thing is that this TV original was dead. When first opened, my friend made the terrible mistake, trying to pull a fast one, and replaced the fuse with a higher rated one :( which of course made everything worse.

Well when I got it I saw the replaced fuse all blown and black.

I'm learning this stuff so I was getting some little help from other forums.

Things I found, I've only focused on the primary side:
Defective two large cement resistors.
Defective mosfet.
Defective current sense resistor.
Replaced opto isolator chip as I was advised.
Also found a shorted zener (currently waiting for the replacement).

The thing is that still I have not found the root cause which seems to be a another shorted component.

-Before I replaced the above components the tv was dead. Now at least I can get stand-by, with a lightbulb attached across the fuse, if not lightbulb attached it'd blow the fuse and FET again as soon as I plug it in.

-I'm not getting the 150v on the filter cap.
The bridge rectifiers four diodes read good when checked out of the circuit.

-I did a test with a blue ring tester on the transformer and all the lights go off, meaning that there is still a short somewhere.

-The transformer reads good outside.

Something weird I hope it'll make seance to someone. when I was soldering the transformer back, I soldered the secondary joints first and took a reading with the primary side joints still desoldered and I still got the shorted, reading from the blue ring tester (could this mean that my problem is on the secondary side or that the little contact from the primary joints, even though still unsoldered were enough to indicate a short on the primary side?)


ALSO LET ME POINT OUT THAT THE ABOVE TEST PROCEDURES I'VE BEEN DOING WHILE HAVING THE SHORTED ZENER DIODE OUT OF THE CIRCUIT.

Any service manual would be welcomed too. These are some equivalent model numbers:

Emerson LC320EM2
Emerson LC320SS2
Magnavox 32MF301B/F7
Philips 32MF301B/F7
Philips lc320ss2

Thanks

Redion 01-10-2013 04:13 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Hi Beowulf

Very good post you got there :-)
Could you also attach pics from the bottom of the board?

budm 01-10-2013 06:29 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Can I see the pictures of the whole back side of the TV?

budm 01-10-2013 06:45 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
On the bottom side of the board where the power transformer is, on the secondary side where you have circled bunch of through hole Diodes, you also have a bunch of SMD Diodes connected to the transformer: I can see some of them, D640, D642, D660, D649, etc. Please check them out also.

Beowulf 01-10-2013 06:52 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Sure here are some pictures guys.
http://i976.photobucket.com/albums/a...pse14dc25b.jpg

http://i976.photobucket.com/albums/a...ps974ac269.jpg

http://i976.photobucket.com/albums/a...ps23393eec.jpg

budm 01-10-2013 06:57 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
It is a simple two transistors switching power supply, any short circuit in the secondary side will cause primary to draw lots of current, and since the circuits has no current sensing circuits, then there is nothing to stop it from blowing up in over current mode, it does have voltage regulation.

tom66 01-10-2013 07:19 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Although it has voltage regulation, it has no over voltage protection in case the oscillator becomes too resonant (the second transistor won't always be able to turn it off.) It is easily able to pump tens of amps into a short or open circuit, leading to massive high output voltages. We are seeing a lot more of these TVs fail but they are a Funai design so made as absolutely cheap as possible.

Beowulf 01-10-2013 08:31 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Thank you guys, you know what you're doing. I dont understand much and I'll take my time to study.
Well, indeed I found a shorted diode (d640), I checked the other four too, as budm advised, and they are alright. It took care of the shorted reading I was getting with the blue ring tester.

Now I dont know what the next step is cause the tv is still not behaving how I think it should.
For example when I plug it in, with the lightbulb across the fuse it still dont power on and also dont give me the 150 or so volts on the filter cap. Dont know if after replacing the zener (which is not soldered to its location) with the new one i'm waiting for and installing the replacement one for d640 it'll make more seance.

Mr Bill 01-10-2013 09:23 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
*Sorta off topic*
Where did you source the 5W2R2 cement resistors? I had to borrow 2 from a spare board to finish a project and i need to replace them. Ebay is the only place i can find them (last time i checked) but you have to get 6 and i only need 2.

budm 01-10-2013 09:24 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
So what did you use for that shorted out surface mounted Diode?

Redion 01-10-2013 09:25 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Quote:

Originally Posted by Mr Bill (Post 301577)
*Sorta off topic*
Where did you source the 5W2R2 cement resistors? I had to borrow 2 from a spare board to finish a project and i need to replace them. Ebay is the only place i can find them (last time i checked) but you have to get 6 and i only need 2.

Well some times 6 will cost less than 2 from local stores like Sayal. The bigger the better :-)

budm 01-10-2013 09:37 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Mouser has them:
http://www.mouser.com/ProductDetail/...FyJ38oLJoTE%3d

Mr Bill 01-10-2013 09:47 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
:duh:
I never even considered mouser. I did check digikey and they didn't have them so i went to ebay.

Thanks Bud

budm 01-10-2013 10:00 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Might be better to get from Digikey since they have USPS shipping charge.
http://www.digikey.com/product-detai....2W-5-ND/18639

Beowulf 01-10-2013 10:12 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Oh i took them two cement resistors from a spare board I had from a similar model (phillips).

budm, I have not replaced the smt diode, I just desoldered it and checked the readings with the blue ring tester multiple times with the diode on and with it off and I saw how it'd lit all the LEDs (good reading) when the diode was desoldered, and it'd turn all LEDs off (meaning a short reading) when the diode was soldered.

budm 01-10-2013 10:26 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
I never use ringer to test the Diode so i have no idea what the result will be. Do you have regular Ohm meter to test the diode, you do not need to use diode mode since we are looking for short/low resistance reading on both direction of bad diodes?

Beowulf 01-10-2013 10:40 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Oh no I was checking the diode with a DMM, I'm still waiting on my analog meter i ordered.
The diode is shorted for good, it reads and beeps on both directions while the other four read good.
I was using the blue ring tester trick I learned from jestine yong's smps ebook.
So what I was doing is checking the reading on the power transformer's primary side junctions to check for shorts in the circuit.

budm 01-10-2013 10:50 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
OK, I see. So at this point we will not know until you get the new Diode then. I will not try to run it until you get the replacement parts installed. Then we can look to find out why you are not getting the 160V on that large filter cap since it is fed by the bridge rectifier. It is not really reliable to ringing test the transformer that is still connected to the circuit since the windings are still connected to the diodes, caps, and load which will damp down the ringing when the ringer send the pulse to the transformer and it looks for ringing, that why it is called ringer, so it may show as bad transformer. I use my Sencore LC-75 for ringing test the transformer out off the board.

budm 01-10-2013 11:13 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
By the way, that Zener diode D610 in the primary side that you have circled as shorted out Zener, I look at the board layout, that Diode is in parallel with the Source resistor R608 (0.68 Ohms?) of the MOSFET (mounted on the heat sink), that is why it will show as shorted out if you measure it on the board. If it is tested short out off the board, then that means the MOSFET is shorted out.

Beowulf 01-10-2013 11:29 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Yeah I removed the transformer before and it reads good off the board.
The zener I checked off the board and it's shorted for good too.
The mosfet was replaced too before. In fact i blew it a second time when after first replacing some defective parts I went and plugged the tv without the light bulb across the fuse trick, It blew the fuse and the FET :D

budm 01-10-2013 11:41 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
That is the typical failure of that Zener when the MOSFET shorted out, I am surprised that the Source resistor did not burn open also. I am still trying to locate the diagram for this board, no luck yet.

Beowulf 01-11-2013 10:28 AM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
I appreciate the effort budm.
Yeah R608 was indeed defective with high resistance. It was one of the components I initially replaced. I was calling it "current sense resistor".

Beowulf 01-11-2013 04:02 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Hi,
need a little help choosing a replacement diode. The shorted smt diode is a b5 sk2b, here is the datasheet
http://pdf1.alldatasheet.com/datashe...EKEC/SK2B.html

I'm looking at digikey and I'm not sure on what category this one falls to start narrowing down the results.

http://www.digikey.com/scripts/dksea...words=Schottky

Beowulf 01-13-2013 11:35 AM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Can anybody give me a minute of their precious time to point me in the right direction here. I promise I'll do my homework once I'm given a little clue. I just wanna be extra careful in getting the right thing. With my beginner's knowledge is something I dont need.
thanks

selldoor 01-13-2013 12:53 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
You could try looking at this one
http://www.digikey.com/product-detai...STR-ND/2041487

Just for future info you might get more people looking at your threads if you
add pictures using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Beowulf 01-13-2013 01:13 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Great thanks, actually more than I asked for :)
So, it was under the rectifiers category.
Oh I had not idea about that, I'll try that next time.

Beowulf 01-17-2013 08:35 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Update guys.
I installed the parts today and the situation is still the same.
Some new things I found out.
If I remove the power transformer, I do get the right voltage on the filter cap, plus normal behavior of the power supply. So, I should be right in saying that my problem is on the secondary side?
It's not on another board since I've test with a load and the MPW only and the same happens.

I was trying a little trick of removing one leg of each secondary output diode and testing, in order to see if it'd give me some pointers but it did not.
Only thing I can think of is looking at the smt components on the bottom of the board tomorrow.

Regards,

tom66 01-18-2013 05:33 AM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
The main transformer forms part of the oscillator, so removing it would make the PSU stop completely, wherever the problem is.

budm 01-18-2013 08:54 AM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
"still the same" meaning that it is still blowing the fuse?

Mark369 01-18-2013 10:10 AM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
why dont you use a lamp in place of fuse, that trick help me out alot.

Beowulf 01-18-2013 12:45 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Hi,
I guess yeah it's still blowing the fuse although I have been using the light bulb trick all the time to prevent it from not only blowing the fuse but also other components like the power FET as it happened before.

I've been trying different things with my limited knowledge.
Taking voltage readings from different points of the board all voltages are lower than what is labeled. Then again the tv doesnt turn on, all this happens while in standby.

Doing a resistance test technique from that smps ebook the only output line I see a wrong reading is from the negative line coming from a zener diode on the secondary side which shows a low resistance value. So I'm just taking a look at that now to see.

Also there is a test point (from a jumper) where I should be getting 24v and I'm getting nothing. Right there next to a ceramic filter. I dont have a way to test the ceramic filter right but I doesn't look bad and after removing it and testing everything continues behaving the same way.

This is a training for me, have been at it for various weeks on and on. Hoping to learn something :)

Thanks guys.

Mark369 01-18-2013 09:22 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
you should remove the fuse and get a old lamp cut the plug off and place one wire on one side of the fuse holder and the other wire on the other side of fuse holder. no fuses need to be in at the time so no blown fuses.

Beowulf 01-18-2013 10:10 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Pardon me if I was not clear.

I'm using the light bulb trick ever since I learned about it.

My point is that the tv is still behaving like it still has another shorted component, when I plug it in (with the light bulb across the fuse). Would love a service manual for this one :D

budm 01-18-2013 10:13 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
OK, so the lamp lights up brightly then? I have gone through the troubleshooting MARK369 try to find the shorted out component by lifting one leg at a time, but we have the service manual for his TV, it took us over 80 threads to find the short.
http://www.badcaps.net/forum/showthr...t=24444&page=5

Mark369 01-18-2013 11:16 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
This tv is sold under the emerson name and Sylvania as well if that helps still trying to find manual for you.

Beowulf 01-18-2013 11:27 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Yes the lamp is very bright.
You know I've done a few things and dont make seance.
I tried a trick of desoldering one leg off of each of the 6 output diodes, one at a time, then tried plugging the tv in after each leg lift to see if it'd tell me in which line is the shorted component. But the same thing continues happening where the light bulb is still bright with all six attempts and the voltage on the filter cap is 2vdc. Which would make me think that my problem is on the primary side.

On the other hand if I remove the power transformer, the light bulb doesn't light up and I get my good voltage (168vdc) on the filter cap. Which makes me think my problem is on the secondary side.

The output voltages on almost or all the test points are below their values.

Another thing, if I check with a clamp ampmeter across one of the wires from the lightbulb I get .7 amps, just though I'd mentioned, and of course 2vdc on the filter cap.

Oh I need a variac so bad. Just lost one auction to a sencor pr87 which seems is the best option out there.

Beowulf 01-18-2013 11:29 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Mark369, appreciate the effort.
Yeah I got the equivalent model numbers from Sylvania, Emmerson, Phillips and Magnavox listed on the first page of the thread :)
Here they are again.

Emerson LC320EM2
Emerson LC320SS2
Magnavox 32MF301B/F7
Philips 32MF301B/F7
Philips lc320ss2

budm 01-18-2013 11:30 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
I think the short is in the primary side of the circuit. If you remove the MOSFET, will it still light up the lamp?
I also search using all those models without luck.

Beowulf 01-18-2013 11:42 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
Thanks,
I'll try that first thing tomorrow.

Mark369 01-18-2013 11:50 PM

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse
 
1 Attachment(s)
I found this to bad it won't help much.


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