Problems with Samsung 740N monitors

[budm]

Re: Problems with Samsung 740N monitors

You can make two tester lamps like these:
https://www.badcaps.net/forum/showthread.php?t=19987

http://s807.photobucket.com/albums/y...klight%20lamp/

http://s807.photobucket.com/albums/y...klight%20lamp/

[sv_tn]

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
start at post 19

Thank for your answer. I will check CCFL bulbs as budm's method (I don't have any spare bulb). If it fine. I will cleaning flux on the board and take a good picture to show you. 0.am right now in my location righ now, so I can't do it. I will doing on tomorrow.

[sv_tn]

Re: Problems with Samsung 740N monitors

I checked CCFL already, it's working fine. Problem on Inverter board but I don't know where? (may be 4502C IC died but I don't know how can I check it). I just heard a low noise of inverter board but I don't know it's normal or not.
Some pictures I just taken after I cleaned resin on the board.
Looking for answer. Thanks!
Sorry for my English.

[selldoor]

Re: Problems with Samsung 740N monitors

I checked CCFL already, it's working fine. Problem on Inverter board but I don't know where? (may be 4502C IC died but I don't know how can I check it). I just heard a low noise of inverter board but I don't know it's normal or not.
Some pictures I just taken after I cleaned resin on the board.
Looking for answer. Thanks!
Sorry for my English.

That is much better.
Test for mosfets is in the guide I posted earlier - I have extracted the part.


2) Shorted Transistors/MOSFETs

On some LCD brands (example: Benq) the transistors (C5707) are sometimes shorted. Transistors are usually marked with a "Q" designation.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for shorted transistors "in circuit" (with power off and unplugged)

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any reading is less than 30 ohms you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

Note: a shorted transistor would likely cause a very brief flash of the backlight or no backlight.

Mosfets

If you have a manual range multimeter, set it to 200 ohms (two hundred). This is for 3 pin ICs only. You can test a mosfet (Q, IC designation) "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.

Here is a case study.

https://www.badcaps.net/forum/showthread.php?t=17659


Your English is fine

[sv_tn]

Re: Problems with Samsung 740N monitors

That is much better.
Test for mosfets is in the guide I posted earlier - I have extracted the part.


2) Shorted Transistors/MOSFETs

On some LCD brands (example: Benq) the transistors (C5707) are sometimes shorted. Transistors are usually marked with a "Q" designation.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for shorted transistors "in circuit" (with power off and unplugged)

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any reading is less than 30 ohms you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

Note: a shorted transistor would likely cause a very brief flash of the backlight or no backlight.

Mosfets

If you have a manual range multimeter, set it to 200 ohms (two hundred). This is for 3 pin ICs only. You can test a mosfet (Q, IC designation) "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.

Here is a case study.

https://www.badcaps.net/forum/showthread.php?t=17659


Your English is fine

Thanks for your prompt reply. I checked 4502C as your instruction. S2-D2 = 0.5 Ohm. So I removed that ICs (both of them). Now I'm wating the new IC from supplier. I purchased this morning. Hope the problem will fix in this time.
I'll return and notice the result to you.

[sv_tn]

Re: Problems with Samsung 740N monitors

Hi.
I replaced 4502C by 4606 ICs. The monitor doesn't flash but the CCFL still off. The voltage on 13V pin = 17.4V and 5V in Vcc pin of BLT3195G IC now. What's the matter here? Where do I can checking now?

[selldoor]

Re: Problems with Samsung 740N monitors

I assume that you have checked that a 4606 is a good substitute for the 4502C.
Did you test the new ones before putting them in.
Can you check them again - and also recheck the old ones now they are out.
Check your soldering and that you have put the chips in the correct way round.

Looking back you say you tested the ccfls - can you just say exactly what you did?

[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?

[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.

[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

[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

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

[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

[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!

[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.

[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

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

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

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…

[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?