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Old 01-10-2013, 02:36 PM   #1
Beowulf
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Default 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) :






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
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Old 01-10-2013, 04:13 PM   #2
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Default 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?
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Old 01-10-2013, 06:29 PM   #3
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Default Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Can I see the pictures of the whole back side of the TV?
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Old 01-10-2013, 06:45 PM   #4
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Default 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.
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Old 01-10-2013, 06:52 PM   #5
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Default Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Sure here are some pictures guys.




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Old 01-10-2013, 06:57 PM   #6
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Default 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.
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Old 01-10-2013, 07:19 PM   #7
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Default 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.
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Old 01-10-2013, 08:31 PM   #8
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Default 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.
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Old 01-10-2013, 09:23 PM   #9
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Default 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.
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Old 01-10-2013, 09:24 PM   #10
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Default Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

So what did you use for that shorted out surface mounted Diode?
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Old 01-10-2013, 09:25 PM   #11
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Default Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Quote:
Originally Posted by Mr Bill View Post
*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 :-)
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Old 01-10-2013, 09:37 PM   #12
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Default Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Mouser has them:
http://www.mouser.com/ProductDetail/...FyJ38oLJoTE%3d
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Old 01-10-2013, 09:47 PM   #13
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Default Re: Sylvania lc320ss2 LCD TV keeps blowing fuse


I never even considered mouser. I did check digikey and they didn't have them so i went to ebay.

Thanks Bud
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Old 01-10-2013, 10:00 PM   #14
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Default 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
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Old 01-10-2013, 10:12 PM   #15
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Default 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.

Last edited by Beowulf; 01-10-2013 at 10:16 PM..
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Old 01-10-2013, 10:26 PM   #16
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Default 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?
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Old 01-10-2013, 10:40 PM   #17
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Default 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.
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Old 01-10-2013, 10:50 PM   #18
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Default 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.

Last edited by budm; 01-10-2013 at 10:55 PM..
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Old 01-10-2013, 11:13 PM   #19
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Default 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.

Last edited by budm; 01-10-2013 at 11:24 PM..
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Old 01-10-2013, 11:29 PM   #20
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Default 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
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