Problems with Samsung 740N monitors

[raokd]

Re: Problems with Samsung 740N monitors

Today I re-soldered all the remaining joints. Now when I did a cold start (after all the soldering)...the backlights remained on for an hour or so.

But again the flickering resumed. Although now i notice the flickering is less frequent.

Am attaching new pics of the solder side of the board.

Next I plan to physically check the lamps.

Please advise if any other checks need to be done to reach the core issue.

Thanks

[Behemot]

Re: Problems with Samsung 740N monitors

Yeah, that's the good one (: Wish you luck

[raokd]

Re: Problems with Samsung 740N monitors

Hello Behemot,

The solder that i am using is TIn-Lead (60-40) composite. Hope thats good enough.

Have resumed now...will finish re-soldering the remaining joints and then post my board's pics.

Thanks...

[Behemot]

Re: Problems with Samsung 740N monitors

Taking your advice I resoldered many points....especially since my monitor was manufactured in Feb 2006. The monitor turned on for atleast half hour without flickering. But soon the symptoms came again.

It's probably lead-free already. Try adding slightly more leaded solder, may help. Lead-free sodler is shit and has many known problems, including bad thermomechanical characteristics, resulting in often cracking.

You may want to read What is RoHS? in your free time. I have added nice document from NASA couple days ago.

[raokd]

Re: Problems with Samsung 740N monitors

Yes it was taken at different times....normally fluctuates 3-4V....for example during 1 reading 319V-323V...

[selldoor]

Re: Problems with Samsung 740N monitors

The voltage on the large DC cap comes between 315-330V...is this normal?

Perhaps should not fluctuate that much - or was that testing at different times.
It should be 1.4 * your mains voltage ie 322 vDC

[raokd]

Re: Problems with Samsung 740N monitors

@Behemot:

The drain voltage when the Lamps were on was shuttling between 11V-14V. Not sure if my multimeter was jumpy, or the voltage itself.

When I removed both the lamps completely, the voltage on 820uF remained steady at 13V.

Taking your advice I resoldered many points....especially since my monitor was manufactured in Feb 2006. The monitor turned on for atleast half hour without flickering. But soon the symptoms came again.

So am going to resolder, rest of the points and then give it a try again...although visually, the joints look fine. Will repost the pics of my board after all the soldering...

@budm
Thanks for the link...I dont have test lamps...will try to engineer based on your guidelines...also am trying to get back an old monitor that I had given in for sell off...

The voltage on the large DC cap comes between 315-330V...is this normal?

Will be taking a small break for 2 days...so will come back to follow your guidelines...pls be there when i come back

Thanks for your advice

[budm]

Re: Problems with Samsung 740N monitors

Do you have test lamps? if not, you should make two of these. Plug the pair of test lamps on the top pair connector and see if the backlights will stay on, if they do, then plug the original lamps back in place, then use the two test lamps on the bottom set of the lamp connector to see if the lamps will stay on, if it does not, then you do have inverter problem. Make sure the moinitor is plugged into running PC otherwise it will go into standby mode.
https://www.badcaps.net/forum/showth...cfl+test+lamps

It only take one bad lamp to cause problem.
http://s807.photobucket.com/albums/y...klight%20lamp/

And what DC voltage do you get on that large DC filter cap on the primary AC side?

[Behemot]

Re: Problems with Samsung 740N monitors

Seems that CCFLs are fine, than the problem definitelly lays somewhere in the inverter.

Yes, the drains should provide high-frequency input to the transformers so you should use AC. The question if whether the multimeter is fast enough to read it…

[raokd]

Re: Problems with Samsung 740N monitors

Thanks Behemot for the speedy reply.

The solder joints are looking strange in the photos...due to the flash and resolution. Let me take a better picture in the daylight tomorrow.

With only 1 CCFL connected (either of them)...they were still blinking.

I will check again the voltage on the 820uF caps, without the CCFLs and post in my reply tomorrow.

By the transistors drain, do you mean drain pins on the 4502C? If yes please let know if the multimeter should be set on AC / DC ?

Thanks once again.

[Behemot]

Re: Problems with Samsung 740N monitors

Some of them solder joints somewhat strange, look for posible cracked/cold ones.

Does that problem appear while backlight disconencted? Try having only one CCFL connected at the time (and check both), sometimes it goes bad as well.

Is there any voltage on the transistors drain?

[raokd]

Re: Problems with Samsung 740N monitors

Hi there,

I have a Samsung 740N for which I had faced a problem with backlight in Dec2011. Changing the 820uF caps and the pico fuse had solved the problem them.

This time I have the same problem as described by many others in this thread. The backlight blinks (there is no pattern..and now it prefers to stay off longer).

Checked the secondary resistance of the inverters and found the resistance on both the windings to be 1338 ohms. I understand from a previous post that this is correct.

Checked the pins of 4502C MOSFET and none were found shorted as per instructions on https://www.badcaps.net/forum/showthread.php?t=10419

Voltage across the 820uF is 18.1V when the backlight turns off and is 13.5V when the backlight turns on momentarily.

Request the veterans / someone who faced something similar to help me with further checks or diagnostics.

Attaching pics of my board below.

[Behemot]

Re: Problems with Samsung 740N monitors

Typical problems for 5 seconds to dead are with invertor. Check all fuses in there, also transistors on it.

[saman_asadi]

Re: Problems with Samsung 740N monitors

hi every 1
my story is almost the same.after turning on my lcd it was dark i noticed that its just the backlight. opened it notice the caps but didnt change them and just changed the fuse turned lcd on ,the back light takes back but suddenly starts to blink with a noise and smoke, i turn it off change the crappy caps( i didnt find out where the smoke came from).test it again , this time it turned on , no backlight , but (in the light) i could see the lcd turn white, but after 5sec the power led turned off and turned on again and the same story ,and it repeats for ever.
i dont know if its still from the psu are the main board!!!!
hope its the psu.i appreciate if any1 could help me out with this!

[e0ne199]

Re: Problems with Samsung 740N monitors

hello anyone,
i have exactly same problem with what is being discussed here. Btw i had replaced that 3 troubled caps (820uF/25V) with the caps that had same values and 3A fuse, but my LCD was still failed to work....after i replaced them all, i tried to connect it to my PC and turned it on, the LED on the power button worked correctly, but when the LCD started to display, it could only display blinking image for a moment, and after that, it burnt the 3A fuse again....
do you know what component is actually troubled for my case? thx for the reply

[madfreak]

Re: Problems with Samsung 740N monitors

JamiCon like others got better and really bad models of capacitors, but just think what You will prefer, Chong or Jamicon . Replacing those smd is not an easy task, i agree, you just have to be patient, and you have to use really small tip in soldering iron, or just use hot air, it takes 5 min. to replace them .

Cheers

[Behemot]

Re: Problems with Samsung 740N monitors

Replacing those SMD craps is easier said than done…

Also note that Jamicon is crapiest crap which should NOT be used ever, the more in display where it is stressed like hell.

[madfreak]

Re: Problems with Samsung 740N monitors

Possible the best repair solution for 940N, and 740N maybe if components are the same:

This will fix no backlight but power on.
Backlght flashes in specific time periods ( for ex. 3-7 sec).

Replace all capacitors to some kind of LOW ESR from JamiCon, Rubycon, Panasonic, i etc : small cost (really small). But hey if You do not have such a great access to electronic parts market, then any kind of brand should be OK, just be careful in parameters of those capacitors.

Replace the 3A fuse ( its ceramic some kind of material with wire), in 940 its near 820uf/25V capacitors and its green) to any kind of fuse that is at least 3A, it may be 3,15A (glass) 3,5A, and maybe 4A, but don't get any further .

Check output resistance of transormers (near dvi, and vga ports), in both the difference in resistance should be less or close to 3%. If it's more, then You should really carefully check the transormers for possible damage of one of them.

Check both 4502C MOSFET TRANSISTORS (8PIN SMD) ( 3+4 PIN (on beeper function in multimeter)), they are on printed side of the PCB, if they are shorted, then replace them with the same new transistors or replacements like IRF 7389 or SP8M3.

The monitor should work like the new one if the lamps are fine .

Cheers

Thomas

[selldoor]

Re: Problems with Samsung 740N monitors

Hey all, anybody here?
I noticed the 820 uF caps are series, so do u think it'll work if i take out all three 820uF & replace it with one 2400uF or 2500uF with the same 25v or 50v cap will do the job?

I am sure its been asked before and the answer is not recommended as the difference in esr and ripple may put pressure on other components.
probably ok for a short term fix to see if the set powers up.

[Sennheiser]

Re: Problems with Samsung 740N monitors

Hey all, anybody here?
I noticed the 820 uF caps are series, so do u think it'll work if i take out all three 820uF & replace it with one 2400uF or 2500uF with the same 25v or 50v cap will do the job?