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Old 03-07-2009, 06:37 AM   #41
mladen82
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by Sherlock
I am new to this site and hoping there are a few of you out there that can help me get to the bottom of a problem with a number of Samsung 740N monitors that have been failing after about 18 months of continuous operation. The symptom is no picture and bulging capacitors on the power supply/inverter board. This is a typical flat panel power board where the backlight inverter and video controller power are integrated onto the same circuit board. The monitor is repaired by replacing the capacitors and replacing a fuse that supplies power to the backlight inverter circuit.

Samsung has admitted to numerous failures of this type and is claiming that the failure is due to bad capacitors made by CapXon.

However, I don't want to readily accept that this is the root cause of the problem.

I am surprised that a company like Samsung would be using "bad" capacitors or that CapXon is still shipping capacitors that don't meet their specifications. It seems that the problem may not be as simple as having just used cheap capacitors. I am wondering if the problem may be a marginal power supply design overall and that the CapXon capacitors are just the first component to go. Or that the fuse may have failed first and caused the capacitors to fail. Is there any advice someone could give to get to the bottom of this mystery?

Thanks for reading to this point.
I will do some test with this board and i will post photos and measurements to see why fuse is open, because that is main mystery.
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Old 03-07-2009, 08:12 AM   #42
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by eguevarae
Post a pic of your board , of the area with the suspected bad component.
I have attached a photo marking two suspicious component 1.TH101 ,2.C102
TH101 under that white stuff is burnt and cracked cant even see values and the C102 gets too hot i cant do much as in a sec when you plug in the board fuse get blown.anyone came across this before??Thanks guys
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Old 03-07-2009, 08:37 AM   #43
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by gveranda
I have attached a photo marking two suspicious component 1.TH101 ,2.C102
TH101 under that white stuff is burnt and cracked cant even see values and the C102 gets too hot i cant do much as in a sec when you plug in the board fuse get blown.anyone came across this before??Thanks guys
and i just found out rectifier D101 is fully shorted
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Old 03-07-2009, 10:35 AM   #44
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by gveranda
Thanks PlainBIll Just checked the board carefully and found that green colour part in between cylindrical fuse and blue capacitor has burnt(just below the yellow cubic) I really appreciate if someone can name them for me and get the value of that green thing looks like a resistor to me.Thank you guys.
You said the green thing looks like a reisistor, it should look like a dab on two legs, TH101 is thermistor C 102 is a cap,we knew what part of the board you were talking about, but needed pictures of your actual board, if you can post them,please,if not no problem.

its advisable to check that resistor by that diode you mention
edit sorry I meant resistor by D102

Last edited by Bobdee; 03-07-2009 at 10:44 AM..
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Old 03-07-2009, 03:09 PM   #45
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by gveranda
and i just found out rectifier D101 is fully shorted
Got it in two! That is (probably) a bridge rectifier. Shorted, that will certainly blow the fuse. Now the question is - is anything else shorted?

For the record, I hate working on switching power supplies. By the time you've found the transformer had a shorted turn which overloaded the switching transistor, which shorted, shorting the bridge rectifier, which blew the fuse.... Well, you get the idea.

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Old 03-07-2009, 10:13 PM   #46
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by PlainBill
Got it in two! That is (probably) a bridge rectifier. Shorted, that will certainly blow the fuse. Now the question is - is anything else shorted?

For the record, I hate working on switching power supplies. By the time you've found the transformer had a shorted turn which overloaded the switching transistor, which shorted, shorting the bridge rectifier, which blew the fuse.... Well, you get the idea.

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Get a 940T PSU and forget about this ....
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Old 03-08-2009, 12:48 AM   #47
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by eguevarae
Get a 940T PSU and forget about this ....
Can i buy them on ebay or something,i just had a look but couldnt find anything.can anyone please tell me the value of that green thermistor(TH101)? i found the rectifier (D2SB60) they reckon two weeks for overseas delivery, is there any matching one out there i can use instead of this Thank you all
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Old 03-08-2009, 03:38 AM   #48
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by gveranda
Can i buy them on ebay or something,i just had a look but couldnt find anything.can anyone please tell me the value of that green thermistor(TH101)? i found the rectifier (D2SB60) they reckon two weeks for overseas delivery, is there any matching one out there i can use instead of this Thank you all
PM me your location so I can quote price & estimate an ETA. Also, how much are you willing to spend on a new PSU? Maybe I can get one, but I need more info (I just bid on a 940T on eBay)
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Old 03-08-2009, 12:00 PM   #49
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by Bobdee
You said the green thing looks like a reisistor, it should look like a dab on two legs, TH101 is thermistor C 102 is a cap,we knew what part of the board you were talking about, but needed pictures of your actual board, if you can post them,please,if not no problem.

its advisable to check that resistor by that diode you mention
edit sorry I meant resistor by D102
I have attached the photos I have taken out the rectifier and the thermistor to see whether they are working.
Attached Images
File Type: jpg DSCF0833.JPG (224.8 KB, 763 views)
File Type: jpg DSCF0835.JPG (210.1 KB, 653 views)
File Type: jpg DSCF0836.JPG (187.7 KB, 788 views)
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Old 03-08-2009, 03:56 PM   #50
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Default Re: Problems with Samsung 740N monitors

first of all sorry for my bad english
here are some photos and measurements i did with bad 740N psu board
i only replaced bad fuse 3A on secondary side +13V line with new one - slow ceramic 3,15A as you will see on photos, but bad bulged caps are there
my scope is to force to break new fuse, to make situation where fuse will broke!!! to track down problem why original is broken?
one of main questions is what first was happened, fuse broken or bulged caps? and why caps are bulged - overvoltage, high temp, over ripple current, or simply bad quality of caps
for heavy loads on secondary outputs,+5V and +13V, i will use 4 (2x2) halogen lamps 115V/600W, they will suck very high current but will not cause over current protection state
you will see all on photos
also, first you better to check demo application schematic for Fairchild FSDM0565R smps controller to understand topology of circuit as similar to our 740N board, but our board have 3 caps on +13V and fuse 3A after them
here is datasheet where you can find principle schematic
http://www.datasheetcatalog.org/data...ylg1g7uc7y.pdf

up side of board


solder side


manufacturer specifications


1. case: open out circuits +5V and +13V respectively, as you will see +13V is not actually +13V, it is only some 8,5V because of bulged caps



2.case: open +5V output (red), +13V (yellow) loaded with one halogen lamp - i taking measure after fuse 3,15A!!!



3.case: loaded +5V with 2 halogen lamps, +13V open circuit (only small current from connected inverter circuit)




4.case: loaded +5V with 2 lamps, loaded +13V first with one, second with 2 lamps, i measure voltages and currents also, have on mind i taking measure after fuse 3,15A!!!








5.case: shorted outputs, only measured current on +13V, because of shorted outputs over current protection is in action, and then you can se that current is high in some moments in other small, it is not continuous only peak value in very small moments. voltages are zero off course, but there is huge energy inside primary circuit





From this tests we can clearly say that original fuse 3A was not broken because of bad bulged caps, we can say with 99,9% sure that fuse was first broken before caps are bulged. Why fuse was broken? There may be many aspects and answers, overvoltage, high temperature, inverter circuit suck too much current when CCFl lamps are on, or even original 3A fuse is badly manufactured.

Second question is why caps are bulged? Well, if fuse is broken and we have loaded +5V with some 1.5-2A as you can see in 3. case, +13V goes high up to 20V and if caps are bad quality with more than 2000 working hours we have simple bulging

Please be aware this situations maybe not real happened in past with this 740N boards, but this can help in understanding better situation.

I will expect your opinions freely welcome any sugestions.
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Old 03-08-2009, 04:03 PM   #51
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Question Re: Problems with Samsung 740N monitors

Well, let me see.... We need help here.
I did some research about the D2SB60. It is a GP Bridge rectifier, 600V 1.5A. Digikey did not carry it (or I didn't find it). But I found one that I think it can replace it:
Datasheet
Original Component datasheet

For the mysterious TH101, I took a Samsung 172N, a Samsung 203B and a Sceptre Komodo II PSU/Inverter boards. In that order, they have TH101:SCK-053, TH110:SCK-2R55A, and a unmarked SCK-103 (for the Sceptre).
You can find the specs in the attached datasheet. The first on page 4 and the last two on page 5. I highlighted them.
With this data can someone suggest a replacement for the TH101 burnt that gveranda has?
I still have to take apart the 940T I have, but I won't be able to do so until late night.
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Old 03-08-2009, 04:16 PM   #52
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by gveranda
I have attached the photos I have taken out the rectifier and the thermistor to see whether they are working.
It's very likely you have another bad component. The thermistor is present to reduce the current surge when the power comes on. The other component is a bridge rectifier (yeah, I'm sure you knew that). At this point I'd be suspicious of IC101 - the regulator.

I have used two techniques to troubleshoot these, both with limited success. The first requires a variable DC power supply, preferably one with current limiting. Hook it across the main filter cap and increase the voltage slowly while monitoring the current. You should be able to run the voltage up to 25 volts or more without the regulator IC starting up (when that happens the current will go way up).

The second is to use a variac (a variable ac transformer) to slowly increase the voltage to the power supply (it's a good idea to monitor the current, too). As you do so, look for something getting hot.

Another thing I do is identify any ICs and look up the data sheet. These often include several schematics for typical applications. With that information you can figure out what is actually going on in the circuit. With luck you will reach the point where you can replace 2 or three parts and repair the supply quickly. One problem with this. If a common failure is due to a bad part (marginal caps) you can do a lot of repairs with confidence they won't be coming back to haunt you. If the original parts were under rated, or the supply has too little ventilation, it requires some redesigning.

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Old 03-08-2009, 04:20 PM   #53
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by mladen82
first of all sorry for my bad english
here are some photos and measurements i did with bad 740N psu board
i only replaced bad fuse 3A on secondary side +13V line with new one - slow ceramic 3,15A as you will see on photos, but bad bulged caps are there
my scope is to force to break new fuse, to make situation where fuse will broke!!! to track down problem why original is broken?
one of main questions is what first was happened, fuse broken or bulged caps? and why caps are bulged - overvoltage, high temp, over ripple current, or simply bad quality of caps
for heavy loads on secondary outputs,+5V and +13V, i will use 4 (2x2) halogen lamps 115V/600W, they will suck very high current but will not cause over current protection state
you will see all on photos
also, first you better to check demo application schematic for Fairchild FSDM0565R smps controller to understand topology of circuit as similar to our 740N board, but our board have 3 caps on +13V and fuse 3A after them

From this tests we can clearly say that original fuse 3A was not broken because of bad bulged caps, we can say with 99,9% sure that fuse was first broken before caps are bulged. Why fuse was broken? There may be many aspects and answers, overvoltage, high temperature, inverter circuit suck too much current when CCFl lamps are on, or even original 3A fuse is badly manufactured.

Second question is why caps are bulged? Well, if fuse is broken and we have loaded +5V with some 1.5-2A as you can see in 3. case, +13V goes high up to 20V and if caps are bad quality with more than 2000 working hours we have simple bulging

Please be aware this situations maybe not real happened in past with this 740N boards, but this can help in understanding better situation.

I will expect your opinions freely welcome any sugestions.

First, the caps bulged because CapXon just plain sucks.
Second, that fuse is the one that protects/isolates the inverter portion of your board. There might be a problem on the inverter side if, after you recap, the fuse keeps blowing.
Third : bad caps that cause variable voltage going out can mess something on the inverter side, causing the fuse to blow (that's what it is there for ...). You may have bad CCFLs connected to the inverter also.
I doubt fuse blew before the bulging caps, but as I can not give any proof, I'll leave it as only "I doubt"

A big observation here is that YOU CAN NOT DO A RELIABLE TROUBLESHOOTING OF THE PROBLEM WITH BULGED CAPS PRESENT. You said you only replaced the fuse. You must replace ALL THOSE CRAP CAPS.

If you can, please post the part number/marking of TH101, located near the AC input of the board.

And welcome to the forums

BTW, Do you have any problem with the 740N besides the blown fuse? You didn't mention it.
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Old 03-08-2009, 04:42 PM   #54
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by eguevarae
Well, let me see.... We need help here.
I did some research about the D2SB60. It is a GP Bridge rectifier, 600V 1.5A. Digikey did not carry it (or I didn't find it). But I found one that I think it can replace it:
Datasheet
Original Component datasheet

For the mysterious TH101, I took a Samsung 172N, a Samsung 203B and a Sceptre Komodo II PSU/Inverter boards. In that order, they have TH101:SCK-053, TH110:SCK-2R55A, and a unmarked SCK-103 (for the Sceptre).
You can find the specs in the attached datasheet. The first on page 4 and the last two on page 5. I highlighted them.
With this data can someone suggest a replacement for the TH101 burnt that gveranda has?
I still have to take apart the 940T I have, but I won't be able to do so until late night.
The bridge rectifier is a 'jellybean' part (made by dozens of manufacturers, all essentially identical). Replace it with any 600V bridge rectifier. Off the top of my head, 1.5A seems a little small; I'd substitute a 2 or 3A part.

A SCK-2R55A is a surge suppression thermistor rated at 5A, cold resistance 2.5 ohm. The SCK-053 is rated at only 3A, 5 ohm cold resistance. The SCK-103 is also rated at 3A, but with a 10 ohm cold resistance. Again, these are 'jelly bean' parts. Digikey carries the MF72-005D9 which should be a drop-in replacement for the SCK-053; they have over 6000 in stock at less than 50 cents each.

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Old 03-08-2009, 04:43 PM   #55
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by eguevarae

First, the caps bulged because CapXon just plain sucks.
Second, that fuse is the one that protects/isolates the inverter portion of your board. There might be a problem on the inverter side if, after you recap, the fuse keeps blowing.
Third : bad caps that cause variable voltage going out can mess something on the inverter side, causing the fuse to blow (that's what it is there for ...). You may have bad CCFLs connected to the inverter also.
I doubt fuse blew before the bulging caps, but as I can not give any proof, I'll leave it as only "I doubt"

A big observation here is that YOU CAN NOT DO A RELIABLE TROUBLESHOOTING OF THE PROBLEM WITH BULGED CAPS PRESENT. You said you only replaced the fuse. You must replace ALL THOSE CRAP CAPS.

If you can, please post the part number/marking of TH101, located near the AC input of the board.

And welcome to the forums

BTW, Do you have any problem with the 740N besides the blown fuse? You didn't mention it.
No no, inverter portion is ok, i purposely leaved bad caps and replaced fuse so i can see will be fuse blow again
but that doesn't happened even with heavy loads as you can see, so fuse not blown because of bad caps!!!
scenario: bad caps cause blown fuse is not possible in my opinion, evidence is on photos

i will do test also with new caps and we will see what happen


TH101 have label: SCK 053
you can use this pdf
http://www.voltts.com.br/produtos/tks-termistor-ntc.pdf

in my opinion, ntc thermistors are bad protection against surge, varistors are better protection

Thanks for welcome!

Last edited by mladen82; 03-08-2009 at 04:46 PM..
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Old 03-08-2009, 05:51 PM   #56
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Post Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by mladen82
No no, inverter portion is ok, i purposely leaved bad caps and replaced fuse so i can see will be fuse blow again
but that doesn't happened even with heavy loads as you can see, so fuse not blown because of bad caps!!!
scenario: bad caps cause blown fuse is not possible in my opinion, evidence is on photos

i will do test also with new caps and we will see what happen


TH101 have label: SCK 053
you can use this pdf
http://www.voltts.com.br/produtos/tks-termistor-ntc.pdf

in my opinion, ntc thermistors are bad protection against surge, varistors are better protection

Thanks for welcome!
Thanks for the Thanks.
Also thanks for the value of TH101.
Did you notice the part that says "Second, that fuse is the one that protects/isolates the inverter portion of your board. There might be a problem on the inverter side if, after you recap, the fuse keeps blowing.
Third : bad caps that cause variable voltage going out can mess something on the inverter side, causing the fuse to blow (that's what it is there for ...). You may have bad CCFLs connected to the inverter also.
". The components might be working OK, but you are forgetting the CCFLs, which HAPPEN TO BE CONNECTED TO THE INVERTER SIDE OF THE BOARD, after the infamous 3.15A fuse. You are testing, but not with the CCFLs that were connected originally to the inverter, which might be the problem. As they age, they tend to draw more current to (try) to start up (or so I've heard around here), and that can cause that the fuse blows (draw over the A rating of the fuse.

On the other hand, I have seen at least 10 PSU/Inverter boards (including the Samsung 203B which I have in front of me) that the only thing they had bad (besides the blown fuse) were bad caps. There are lots of cases around here that have the same symptoms.
As I said, I doubt fuse blew before the bulge in the caps. Evidence is on tons of LCDs that keep going working or trying to do it running on bad caps until the bad caps make something else go south and then the fuse blows. Even the monitor that I am using to see as I type, had three bulging caps, one shorted MOSFET and one blown fuse.Another case, but of a same brand, 1 inch less in size, had been resolved, with the same symptoms and the same solution. In these particular cases, my guess is that the fuse blew because the short in the MOSFET. Caused by bad caps? I don't know, but bad caps tend to be on the wrong place, at the wrong time, and in wrong conditions : guilty as hell. The other member of my newly dual display subsystem, the VP201b, had two bad caps in the PSU. No fuse blown.
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Old 03-09-2009, 03:28 AM   #57
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by eguevarae
Thanks for the Thanks.
Also thanks for the value of TH101.
Did you notice the part that says "Second, that fuse is the one that protects/isolates the inverter portion of your board. There might be a problem on the inverter side if, after you recap, the fuse keeps blowing.
Third : bad caps that cause variable voltage going out can mess something on the inverter side, causing the fuse to blow (that's what it is there for ...). You may have bad CCFLs connected to the inverter also.
". The components might be working OK, but you are forgetting the CCFLs, which HAPPEN TO BE CONNECTED TO THE INVERTER SIDE OF THE BOARD, after the infamous 3.15A fuse. You are testing, but not with the CCFLs that were connected originally to the inverter, which might be the problem. As they age, they tend to draw more current to (try) to start up (or so I've heard around here), and that can cause that the fuse blows (draw over the A rating of the fuse.

On the other hand, I have seen at least 10 PSU/Inverter boards (including the Samsung 203B which I have in front of me) that the only thing they had bad (besides the blown fuse) were bad caps. There are lots of cases around here that have the same symptoms.
As I said, I doubt fuse blew before the bulge in the caps. Evidence is on tons of LCDs that keep going working or trying to do it running on bad caps until the bad caps make something else go south and then the fuse blows. Even the monitor that I am using to see as I type, had three bulging caps, one shorted MOSFET and one blown fuse.Another case, but of a same brand, 1 inch less in size, had been resolved, with the same symptoms and the same solution. In these particular cases, my guess is that the fuse blew because the short in the MOSFET. Caused by bad caps? I don't know, but bad caps tend to be on the wrong place, at the wrong time, and in wrong conditions : guilty as hell. The other member of my newly dual display subsystem, the VP201b, had two bad caps in the PSU. No fuse blown.
No problem,
i repairing lcd monitors
this 740N board is replaced with new psu/inverter board (other model - better one) that works with same monitor, so CCFL are ok and in most such cases CCFL are ok

also i have many monitors that have only bad caps in secondary side, they are bad quality but this is not always main cause of their failing

you can check this Panasonic Reliability chart for Aluminium electrolyte capacitors to see why are all possible causes of failing
http://industrial.panasonic.com/www-...ABA0000TE4.pdf


Some thing i miss to say about original 3A fuse is that may be an fast reacting fuse, also current consumption for inverter part with ccfl is about 1,75-2A, can't be more for this 17'' monitor, so that current can't blow fuse even fuse is fast
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Old 03-09-2009, 04:20 AM   #58
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Default Re: Problems with Samsung 740N monitors

info about NTC thermistors used in psu's, they are not there for protection against surges
they are there because of inrush current limiting
more info
http://www.ametherm.com/inrush-curre...revention.html

thank PlainBill for force me to rethink about that

Every day we can learn something new
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Old 03-09-2009, 04:35 AM   #59
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Post Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by mladen82
No problem,
i repairing lcd monitors
this 740N board is replaced with new psu/inverter board (other model - better one) that works with same monitor, so CCFL are ok and in most such cases CCFL are ok

also i have many monitors that have only bad caps in secondary side, they are bad quality but this is not always main cause of their failing

you can check this Panasonic Reliability chart for Aluminium electrolyte capacitors to see why are all possible causes of failing
http://industrial.panasonic.com/www-...ABA0000TE4.pdf

Some thing i miss to say about original 3A fuse is that may be an fast reacting fuse, also current consumption for inverter part with ccfl is about 1,75-2A, can't be more for this 17'' monitor, so that current can't blow fuse even fuse is fast
My friend, glad you are back.
Well, you now gave some more info. Now you say that the CCFLs are working with other board, which mostly eliminates them from the equation.
As I've seen so far, there are many causes for either the blown/leaking caps or the fuse blowing.
The only recent case of a Samsung PSU/Inverter that blew a fuse in front of me (after I had changed it) is a 203B Board. It had bad lamps, they drew a lot of current, and blew the fuse. I replaced the fuse and it blew again. The screen only flashed for a moment (as when the inverter fails) and then it was off again. After the blown fuse (as a note, I had recapped it already), I replaced it by one slow blow as the one in your pics, but instead of attaching the 203B lamps, I attached a set of lamps from a 18" I have. They lit evenly and without problem or fuses blowing all over the place. Then I reattached the 203B lamps, and as soon as I turned on the PSU (this model has a switch. Very convenient...), the screen flashed, the fuse blown, and I was pissed off.
I took apart the lamps and they were loose or something (nearly broken), and the voltage arced to the reflection film on the inside of the rail that protect the lamps, and this short blew the fuse. In the end, the LCD itself was damaged, as I replaced he lamps only to find that the LCD has a vertical line in the middle. In this case, bad lamps blew the fuse.
In a not-Samsung case (cases, I might say), the problem was the use of bad quality caps, at a 85c temp rating, in a poorly ventilated area and with a design that left heat sources too close to the caps : Proview/KDS/Sylvania/MAG & others using Mirage Electronics PSUs. I have cases (I document every damn thing I do) of PSUs with bad caps, bad lamps and blown fuses. I have observed too that when the lamps are old, they take more time to fully start up, drawing much current in the process. maybe you didn't see it because, as you say, your CCFLs are good, but attach a set of pink-hued lamps, and you'll see that you easily break the 3.15A barrier. I guess that the fuse is there for cases like this among other things. And each case is different. Maybe in a controlled environment the PSU behaves well, but if the user has the unit in a poorly ventilated area working around the clock every day, the chances that something goes wrong (especially with the superb quality of the Elites & capXons found in many PSUs), ranging for intermittent failures to sudden death caused by blown fuses, are high.
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Old 03-09-2009, 04:39 AM   #60
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Thumbs up Re: Problems with Samsung 740N monitors

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