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Old 03-24-2008, 04:19 PM   #1
Sherlock
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Default Problems with Samsung 740N monitors

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.
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Old 03-24-2008, 04:42 PM   #2
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by Sherlock
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.
Welcome Sherlock.
Bad psu design can be sometimes traced and recognized on schematics or on photos; however bad caps will fail even on good designs as well (e.g. Antec psu) simply because their quality is inferior and/or suffer from manifacturing defect (bad electrolyte formula or bad Al supplies).
Posting a few pictures may help Davmax and Kc8's diagnosis.

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Old 03-25-2008, 08:08 AM   #3
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Default Re: Problems with Samsung 740N monitors

I took your suggestion and am posting photos of two different power supplies that are from 740N monitors. The encircled caps are all bulging. I know on the second one that replacing the caps and the fuse restores operation.
Attached Files
File Type: doc 740N Power Boards.doc (575.5 KB, 1924 views)
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Old 03-25-2008, 07:41 PM   #4
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by Sherlock
I took your suggestion and am posting photos of two different power supplies that are from 740N monitors. The encircled caps are all bulging. I know on the second one that replacing the caps and the fuse restores operation.
Since your photo are quite big and putting them into a doc file is bad idea, I attach them to this post.
In pic1 the failed caps are the black ones near the transformer: unfortunately is too out of focus to recognize them.
In pic2 bulged caps seem United Chemicon ones (from the cross vent and that brown jacket): perhaps KZG?
Try posting more detailed pictures and their signatures too: from top view I can guess caps only from vents and these are common to many companies.

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Attached Images
File Type: jpg 740N-1.jpg (178.8 KB, 1640 views)
File Type: jpg 740N-2.jpg (849.4 KB, 1469 views)

Last edited by zandrax; 03-25-2008 at 07:46 PM..
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Old 03-25-2008, 08:59 PM   #5
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Default Re: Problems with Samsung 740N monitors

that looks like typical topswitch (or alike) design, and there really is not much that designer can do with todays silicon but pick best surrounding components, which samsung didn't do in this case.

so it's a good design with bad caps.
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Old 03-25-2008, 10:55 PM   #6
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Default Re: Problems with Samsung 740N monitors

those are samxon gl.
known junk.
they are a replace on sight part.
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Old 03-26-2008, 08:52 AM   #7
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Default Re: Problems with Samsung 740N monitors

Thank you for your comments. Just to clarify, the failed caps are CapXon. On the first photo, the failed caps are the three nearest the upper heatsink and they are CapXon KF 10v 1000uF. On the second photo, it's the three caps to the right of the upper heatsink. They are also CapXon but GLs 25v 820 uF. There are some SamXon to the right of those but they appear to be fine.

So with that added information, are your conclusions the same?

If so, why does a company like Samsung use caps from a known bad supplier? Maybe this doesn't surprise you guys but it is going to turn out to be a big warranty issue for Samsung. It seems that a company of their size would not slip up like this. Your thoughts on this subject are welcome.
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Old 03-26-2008, 12:03 PM   #8
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by i4004
that looks like typical topswitch (or alike) design.

so it's a good design with bad caps.
Yup my Samsung inverter uses the Topswitch PWM/FET IC too and was loaded with Crapxon as well they weren't bulging but I took preemptive measures and replaced them with Panasonic FCs anyways.

The Topswitch is a pretty good design it makes a reliable compact switcher with very few external components. As a matter of fact I have never had a single one of my Topswitch based switchers fail.

I have seen pics and reports of them getting destroyed by overvoltage conditions and lightning strikes but that is to be expected and is not a reliability issue.

I must admit it was real confusing the first time I saw one.

I'm saying to myself hey wheres the PWM controller IC ?

As for Samsung using these caps Capxon in my eyes is a crapshoot sometimes they will hold up and perform well and other times they will go south.

I guess Samsung is willing to take that gamble or is hoping that customers will junk their monitors and buy new ones.

Good old built in obsolescence strikes again.
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Last edited by Krankshaft; 03-26-2008 at 12:10 PM..
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Old 06-13-2008, 06:34 PM   #9
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by kc8adu
those are samxon gl.
known junk.
they are a replace on sight part.
Hey KC8!

Sorry for bringing this thread to the top...

I thought Samxon were supposed to be good? They are what Big Pope on here sells aren't they? I have a late Intel 965 board full of them, please don't tell me it's going to turn out to be crap!
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Old 06-13-2008, 07:24 PM   #10
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Default Re: Problems with Samsung 740N monitors

gf,gl,and a few other series of samxon are known bad.
seems the samxon and capxon are from the same mfr.
every aspect of the ones i compared were identical.
one of those mfr's that will stamp whatever you want on an item if you buy enough.
i have used fc,fm,and even mbz on these units over the last few years with no comebacks.
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Old 06-14-2008, 07:06 PM   #11
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Default Re: Problems with Samsung 740N monitors

the neat thing about this simple PSU is nearly all the stuff is right in one power hybrid IC and it is RARELY a failure in this case. It's the caps that is ailing us.

Cheers, Wizard
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Old 06-27-2008, 09:09 PM   #12
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Default Re: Problems with Samsung 740N monitors

@sherlock or to others


i have a samsung 740n lcd,

1st - the problem was no display produce, but there is power on the lcd

2nd - i replaced this three bloated capacitors as per instruction here


3rd - my lcd has display now but no lights, do i also have to replace the fuse?
if i have to replace it, where is the fuse located?
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Old 06-28-2008, 05:16 AM   #13
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Default Re: Problems with Samsung 740N monitors

found the 3A fuse, but it seems its so hard to look for one, do you think masters it's really the fuse?
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Old 06-28-2008, 08:18 AM   #14
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Default Re: Problems with Samsung 740N monitors

looking at the pic i'd say those inverter-transformers are toasted...

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Old 06-28-2008, 09:57 AM   #15
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Default Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by Scenic
looking at the pic i'd say those inverter-transformers are toasted...

I think both transformers are fine... the black color is due to the part number printing. Please post a picture of solder side. The switching transistors are probably SMD ... and they could have gone bad.

Last edited by Brian C; 06-28-2008 at 10:00 AM..
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Old 06-28-2008, 10:38 AM   #16
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Default Re: Problems with Samsung 740N monitors

well... okay.. the upper one looks fine... but what about the bottom one? the coils look much darker than they should be (right side ; input)
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Old 06-29-2008, 02:34 AM   #17
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Default Re: Problems with Samsung 740N monitors

@sir Scenic
here is the pic of the solder side

transformers are all fine its just the part number that it's makin it look bad

Last edited by kharr; 06-29-2008 at 02:36 AM..
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Old 06-30-2008, 10:49 PM   #18
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Default Re: Problems with Samsung 740N monitors

finally solve the problem i replaced 3 capacitors and the the said 3A pico fuse

it's now working

thanks to all who helped
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Old 07-30-2008, 09:39 AM   #19
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Default Re: Problems with Samsung 740N monitors

masters, had another one of this problem, i replaced the three caps and the fuse, the lcd has display but still no light replaced an inverter from other lcd the lcd and the lcd worked fine, it means the inverter board is still not fixed by replacing the three caps and fuse, what seems to be the problem?
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Old 11-04-2008, 09:05 PM   #20
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Default Re: Problems with Samsung 740N monitors

Hey man i have same monitor with same problems, where i can get that 3 caps and how much they will cost ?
Can i replace them by REUSED caps from another PC parts ?
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