Problems with Samsung 740N monitors

[moondoggie]

Re: Problems with Samsung 740N monitors

i think this is the best i can do...

[selldoor]

Re: Problems with Samsung 740N monitors

They are good compared to some we get to see (or not see)
If you have still got it in bits I have had another look at the PS base and the soldering on one of the diodes D107 looks a bit brown at one end. The board also looks a bit browner in that area so perhaps you can reflow all 4 diodes. Also one of the caps you have replaced the lower one in the picture has a bit of solder between the legs - can you clean it off and also the right leg in the picture the solder doesnt look just right if you can check it ( May just be shadow )

[selldoor]

Re: Problems with Samsung 740N monitors

OK 4525GEH is a mosfet- data sheet is attached.
In the 2 seconds to black guide there is a test for mosfets.
Ohms test so doesnt need to be connected or swithced on.

[moondoggie]

Re: Problems with Samsung 740N monitors

the whole area around those diodes is yellowish...like some glue or some lacker (dunno the word for it) spilled or smthn. will clean it up tomorrow and resolder the diodes just in case. it doesnt look like it burnt, but rather it heated up pretty well, so something leaked.

about the solder between the legs, its not a solder but a thin scratch line where my screwdriver slipped :P don't ask its clean no touching between the contacts.

its not a shadow but a rather stupid light play. that pin is somewhat blueish from my tweezers that i used to shorten the contact from the capacitor. and its giving the impression of a loose contact...

will take another pic in the morning at work, when its all done

ty for the great help mate

[selldoor]

Re: Problems with Samsung 740N monitors

OK You replaced two caps - if they were leaking it could be electrolyte stain. While on the subject - Why did you change the two caps - bulged /leaking or what.
I suppose I am asking why didnt you change the other biggish cap next to them and also
do you have a source where you can you get the other caps ( But not The BIG Cap) on that board to replace if neccessary. I have been following another thread where member is in Serbia and it seems a bit of a problem. Speak again tomorrow.

[moondoggie]

Re: Problems with Samsung 740N monitors

both were bulged, and one was leaking, well... yeah, leaking...

i can get all the capacitors i need, the firm has them :P, and a local tv shop can get me anything i mostly need. i didnt change the other caps as they looked ok, but i can test them if u like on the cap meter at work.

on the first note, im not getting ANY reading of the AP4525GEH. nothing on any combination of pins.On taking a very close look, it looks like it was heated too much and died. i will check that too under a magnifier tomorrow.

ty again.
till tomorrow
md

[moondoggie]

Re: Problems with Samsung 740N monitors

Well,we have a part to blame... the ap4525 mosfet died.
I have resoldered the diodes and checked all caps on the capmeter.just in case.
Will try to find it here localy,if not gotta order it.

Will write when i solder the new one.
Ty for the great help
Md

[selldoor]

Re: Problems with Samsung 740N monitors

If you cant find one there are alternatives - see here
http://www.edaboard.com/thread253214.html
There are probably others as well

[moondoggie]

Re: Problems with Samsung 740N monitors

well...it works. kinda.
After changing the MOS, it started normal, and worked for 1h, than just went black. On&Off and its working again. While typing this it did it two more times.
(the signal on the button stays blue and ON, like it has signal and everything, but the display is black... its def not a lamp as there is no picture in background)

Its producing a strange little squeeek noise, like something is shortcircuted or smthn..cant describe. but only at startup...

[selldoor]

Re: Problems with Samsung 740N monitors

Good so far then. Sounds like bad joints
Did you resolder the diodes? next would be to go over the board with
a magnifying glass and look for possible bad joints and resolder them. Especially on the transformers - probably do them anyway.
Try wriggling components and looking for movement on the underside of the board.
See of you can pinpoint where the sound comes from I know you say you tested the caps but it could still be one that is on its way out.

[moondoggie]

Re: Problems with Samsung 740N monitors

the 3x 820uF gave 821, 803, and 828uF...the others were around the same as stated capacity.

i will look for loose contacts tomorrow at work with a mag glass, and resolder everything i see....
i did the diodes earlier, but will do it once more...

ty for pinpoints, will check it all out

[selldoor]

Re: Problems with Samsung 740N monitors

Ok if you dont find anything or it doesnt fix it I would consider replacing all the caps.
If two had gone as far as bursting, others of same make and series will not be far behind.

[moondoggie]

Re: Problems with Samsung 740N monitors

probs are, these two are the only of the kind, and the rest are diff
well, we shall see tomorrow,if nothing, i'll change em all

[sv_tn]

Re: Problems with Samsung 740N monitors

I replaced 3 caps 820u by 2x1000u + 1x470u and 3A pico fuse by a small copper wire (I cannot found it in my location). My monitor don't working. Power led on but it off every 3 second, the panel flash after power led off (off time about 0.5 second). I check the board but it in good condition (I think may be 4502C burned but it look good). The voltage on 13V pin oscilated from 14.5-18.3V, in on/off pin about 0.3-2.5V. Oscilating same with power led ligh time. No picture on display. I remove fuse (copper wire) and check again. The power led don't flash, on/off pin about 2.5V but 13V pin is 18.3V. The picture shown on display, (no back ligh of course). What is this problem here? May the 4502C burned?

[selldoor]

Re: Problems with Samsung 740N monitors

Hi - please update your profile with country and mains voltage.
It could be that BOTH 4502c need replacing or it could be a cold joint somewhere.
Any sort of fuse with the correct rating would be better than wire - if you have read this thread you will see others have caused more damage doing that.
There is also someone who had the same 18.x voltage problem.

We can help a lot more if you please post good clear pictures of each board, front and back and close up of connectors, (max resolution 2000x2000 and 2MB) 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.

Examples of what is needed
https://www.badcaps.net/forum/attach...1&d=1290283049

https://www.badcaps.net/forum/attach...7&d=1280167246

[Rtech]

Re: Problems with Samsung 740N monitors

I replaced 3 caps 820u by 2x1000u + 1x470u and 3A pico fuse by a small copper wire (I cannot found it in my location). My monitor don't working. Power led on but it off every 3 second, the panel flash after power led off (off time about 0.5 second). I check the board but it in good condition (I think may be 4502C burned but it look good). The voltage on 13V pin oscilated from 14.5-18.3V, in on/off pin about 0.3-2.5V. Oscilating same with power led ligh time. No picture on display. I remove fuse (copper wire) and check again. The power led don't flash, on/off pin about 2.5V but 13V pin is 18.3V. The picture shown on display, (no back ligh of course). What is this problem here? May the 4502C burned?

Perhaps it is my eyes,although I doubt it,but looking at a PSU photo shown on an earlier Post, I cannot even see a 3A pico fuse.So if your Board is not the same, then suggest you add the photos , the same way and the same perspective as that Poster did,and then you may get some more practical help

[selldoor]

Re: Problems with Samsung 740N monitors

Pico Fuse - On the 740N- Some boards have one some dont- some do but its covered in glue.

[sv_tn]

Re: Problems with Samsung 740N monitors

Hi, Thank for all reply.
I just uploaded some pictures of the board.
R of transformer = 1.3k (both of them)
Look forward to hearing from...

[selldoor]

Re: Problems with Samsung 740N monitors

Hi - is the Dark brown area on the bottom or the board where you have replaced caps?
Is the brown burnt or is it just flux?
If it is flux can you clean it off with something like an old toothbrush and some
isopropyl alcohol so we can see the traces.

[selldoor]

Re: Problems with Samsung 740N monitors

Was going to ask you to check inverters but just spotted you already did.

It may be a faulty backlight or wiring ,do you have any spare CCFL bulbs.

Did you read this:https://www.badcaps.net/forum/showthread.php?t=10419
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