Problems with Samsung 740N monitors

[indrair3]

Re: Problems with Samsung 740N monitors

Ok - you are not going to get much help by bumping a post.
You would get more help if you didnt post pictures in line and if the front of the board picture was the other way up so the numbers could be read.
A quick look through this thread doesnt show a board like this one and I have tried to find the board but failed - didnt spend a lot of time as I cant really read the model no.
Please state the model number and revision of the board and get a magnifing glass and post any numbers off the burnt component that you can decipher.

This the model number of the board :
--------
! level3 !
! LR76377 !
--------
-----------------------------------
! MODEL :PWI1904SJ(A)(B)(C) Rev 1.0 !
! INPUT :100-240V ~50-60Hz.1.0A !
! OUTPUTPN201) +5.1V---2.0A !
! +15.0V---0.3A !
! (PN1.PN2.PN4) = 650V~7.5mA !
-----------------------------------
YB751904SJ(A)005251

[Drools]

Re: Problems with Samsung 740N monitors

I pulled a bone-head move!
I have 2 x 940b 19" LCD monitors with badcaps in both. I'm experienced with electronics and replacing caps but I pulled a stupid.
My power board is the exact same as the one pictured in post #4 of this thread.
PN: BN44-00128A, I have seen it called Rev2.
I identified the caps needed or so I thought, 470uf at 25vdc times 5....
oops, they are not all 470uf there were 2 x 1000uf 10VDC that were identical in size as the 470uf ones however I removed the caps so where did the 2 x 1000uf caps belong?
Interesting enough, on the first monitor I did not catch my goof and replaced all the bad caps with 560uf @ 35VDC and the monitor works fine. I will fix my stupidity when I can identify the proper location for the caps in question.
C242
C235
C263
C241

C319 is a 470@25vdc as it was a different color so I remembered where it went.

Thanks for the help.

[retiredcaps]

Re: Problems with Samsung 740N monitors

oops, they are not all 470uf there were 2 x 1000uf 10VDC that were identical in size as the 470uf ones however I removed the caps so where did the 2 x 1000uf caps belong?

1) Take your multimeter and set it to continuity mode.

2) Put one lead on the connector CN2 where it says 13V.

3) Put the other lead on the positive leg of the capacitor hole. If it beeps, that is where the 25V caps go.

4) Put one lead on the connector CN2 where it says 5.1V.

5) Put the other lead on the positive leg of the capacitor hole. If it beeps, that is where the 10V caps go.

[Drools]

Re: Problems with Samsung 740N monitors

Thanks, I should have looked at CN2 more closely.
Interesting, I only have 1 cap that is on the +13 trace while 3 caps were on the +5 trace, yet I have 2 caps at 25VDC. They must have used a 25VDC cap on the 5VDC trace.

[mufti_GT]

Re: Problems with Samsung 740N monitors

Hi, lightning on my Samsung 940B stopped working, but thanks to this thread I managed to find and fix the problem. It was the usual stuff, the three 820uF 25V caps were bulged on top and the PICO 3A fuse was blown. I replaced caps with 2x 1000uF 25V and 1x 470uF 25V Teapo SY low ERS caps. I replaced fuse with a new one. The monitor works fine now.

Thanks again to creators of this thread

[moondoggie]

Re: Problems with Samsung 740N monitors

Hello all,
stumbled upon this thread and forum while fixing a 740N for a friend.

As usual opening it showed 2 cap's 1000uf 10VDC blown up. Changed them just now, and the monitor turns on to stanby. Glad i fixed it, i put it to my pc to test it for a few hours.
than i saw what i didnt see at first. the monitor lights up, and after 2 secs kills the lams. the picture works but dark. i shut the monitor off, turn it back on, same thing happens.

i'm going to open it again tonight, and search for that blasted 3A fuse i didnt know about, BUT... if the fuse isnt working, should the light come on for 2 secs than shut down?

any ideas on whats going on ?

ty in advance
md

p.s. i'll be scrolling thru these 15 pages slowly today, to see if i find my answer, so if its there soz for asking

p.p.s. the capacitors that i put on are 1000uF 16V instead of 10V

[selldoor]

Re: Problems with Samsung 740N monitors

Hi, Welcome to the forum
Does the power light stay on when the backlight goes off?
If so you probably have 2 seconds to black and should read this guide by retiredcaps - start at post 19.
https://www.badcaps.net/forum/showthread.php?t=10419
As you say it is not likely to be the fuse.

We can help a lot more if you please post good clear pictures of the whole chassis, and then 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

Do you have a multimeter

16v instead of 10v caps is ok

[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...

[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

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

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

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

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

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

[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

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

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

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

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

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?