Problems with Samsung 740N monitors

[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?

[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?

[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.

[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

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.

[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.

[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/

[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
start at post 19

[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.

[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

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

[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

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

[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?

[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

[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

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

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

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

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