Problems with Samsung 740N monitors

[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

[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

[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

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 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.

[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 )

[moondoggie]

Re: Problems with Samsung 740N monitors

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

[moondoggie]

Re: Problems with Samsung 740N monitors

4525GEH
716C85

if im not mistaken, its rather tiny
by main board u i reckon u think the other small one?
what voltages should i measure?

[selldoor]

Re: Problems with Samsung 740N monitors

Great - that more like it - On th eback of the power board between where I marked S2 and S4 there is a component with 4and half legs can you make out a part number on it?

Can you try for pictures of the main board front & back.

Then probably connect it up and do so voltage tests at the connectors.

[moondoggie]

Re: Problems with Samsung 740N monitors

righto. You're right.
the top equals to 1.031 and the bottom 1.029

p.s. soz mate, no part numbers on it, nothing identifiable as far as i can see.

[selldoor]

Re: Problems with Samsung 740N monitors

I am now wondering if it is wound differently to what I have seen before.
Try measuring between the top two left hand side pins and the bottom two left hand side pins. Is there a part number on it anywhere perhaps I can find a data sheet.

[moondoggie]

Re: Problems with Samsung 740N monitors

the bulbs give a normal light in those two seconds. i have tried it numerous times, and all 4 seem to be the same. not one is "half lit" so to say.

i can disolder the transformer tomorrow at work and test it. will check for breaks.
if it's close to the joint, i can repair it. it wont make much of a difference as it wont be actually shortened.

ty for your help mate. will try tomorrow and see what goes on

[selldoor]

Re: Problems with Samsung 740N monitors

Well, you can try again tomorrow - the thing is that if the back light flashes on
then at least one secondary must be ok so hopefully both are. But even if they are I am not sure what to test next- I would test the bulbs but that can only really be done by substitution either by using a spare bulb/panel or trying this
panel/bulb in another working monitor. I guess we assume the bulbs are good and do some voltage checks. If the transformer doesnt appear good
it is usual to take it out carefully and re measure out of circuit also if it is open
if the break is just where the very thin wire joins the pin it may be possible to repair, but have never seen it done.

[moondoggie]

Re: Problems with Samsung 740N monitors

definitely a no go..tried upto 200k Ohms, OLed.
its not lacquered, or coated.

[selldoor]

Re: Problems with Samsung 740N monitors

You may need to scratch at the solder if it has been lacquered or even just flux coat.
Your meter should work - try on 20000 ohms ?

[moondoggie]

Re: Problems with Samsung 740N monitors

yep, those are the ones. My cartography is good it seems, and so is yours

will test the board again at work tomorrow, with a Fluke 84 true rms, as im limited to a chinese piece of ...

[selldoor]

Re: Problems with Samsung 740N monitors

Hmm unusual for BOTH secondaries to be out.

Here are the pins to measure on the back (I Think)
not very good at back to front geography
If they are try again.

[moondoggie]

Re: Problems with Samsung 740N monitors

if im doing it right, and i reckon i am ( o0 ) i get OL (1 on left side of multimeter) on 2K Ohms.

it's soldered to the board, and if i try to measure it, nothing happens. tested the fuse to check the MM, and it shows .000, same goes when i connect the two testpins together.


(no spare parts sadly)

[selldoor]

Re: Problems with Samsung 740N monitors

Yes pics are a bit blurry for troubleshooting however it is usually big bits that go wrong on these.

As it is there canwe start with the inverter transformer - big rectangular part
Can you test as follows@ No power to board, set meter on ohms 2000 and
measure S1 to S2 and S3 to S4 as shown in attachment
Post results in ohms.

Have you any spare ccfl lamps or a panel (even a broken one)

[moondoggie]

Re: Problems with Samsung 740N monitors

hello, and thnx for the welcome.
It's a definite yes for 2 seconds to black.
i'm reading the guide now.
I do have a multimeter.

I'm upping the pic's as i have not seen this ver of psu board here yet (still browsing the thread tho)

i have changed the 2 blue ones that are in the pic
alas the pics are not the greatest, as i only have access to my galaxy s camera...