Problems with Samsung 740N monitors

[PlainBill]

Re: Problems with Samsung 740N monitors

Hi i am new here, i have a samsung 740n which it just turn off blank screen but power light is blue i have been doing some reading on here about the monitor. so i open it up to find the 3 25v 820uf were bulging have replace them and also replace the 3a fuse. but the problem now is when i turn the monitor on i only see can just see the image it is very faint like if the backlight is not get much power. i have check the 8 resistors between the 2 transformers and four of them have difference values could this cause the backlight not working properly

http://wiki.xtronics.com/index.php/Resistor_Codes

PlainBill

[llpilch]

Re: Problems with Samsung 740N monitors

Greetings from Brazil! Sorry about my bad english..

I have a Samsung 940B and the background lights were not working, but the LCD image is ok!

I found the 3 820uF 25V bulged capacitors and changed it for 3 1000uF 25V, but I think I'll change one of them and put a 470uF 25V.

I found the 3A fuse, it was blown and I put a normal 3A fuse (with wires out of the board), is there a problem here?


The monitor worked well by 2 hours, and then the lamps started to turn off and on by a few seconds, I turned the monitor off and when I turned it on, the lamps were not working anymore, neither flashing. The 3A fuse was blown again, and now I don't know what is the problem, what components may be the cause of the fuse failure...
Can be the MOSFET 4502C?

[PlainBill]

Re: Problems with Samsung 740N monitors

Greetings from Brazil! Sorry about my bad english..

Let me assure you, your English is much better than my Portuguese!

I have a Samsung 940B and the background lights were not working, but the LCD image is ok!

I found the 3 820uF 25V bulged capacitors and changed it for 3 1000uF 25V, but I think I'll change one of them and put a 470uF 25V.

That may not be necessary IF all caps are in parallel.

I found the 3A fuse, it was blown and I put a normal 3A fuse (with wires out of the board), is there a problem here?

No. I've done it myself when necessary. It is much better to use ANY kind of a fuse than to use a jumper wire.

The monitor worked well by 2 hours, and then the lamps started to turn off and on by a few seconds, I turned the monitor off and when I turned it on, the lamps were not working anymore, neither flashing. The 3A fuse was blown again, and now I don't know what is the problem, what components may be the cause of the fuse failure...
Can be the MOSFET 4502C?

It COULD be a mosfet, but I'd suspect a bad solder joint, in particular on one of the transformers.

PlainBill

[llpilch]

Re: Problems with Samsung 740N monitors

Let me assure you, your English is much better than my Portuguese!

HAHAHA! It's easier to me learn english than you learn portuguese! Portuguese is a very difficult language to learn, even for us! Too much little rules...

That may not be necessary IF all caps are in parallel.

ok, but I have a new 470uF cap, I'll change it just to get the capacitance value next to original value. Or is it better to maintain a capacitance greater than original?

No. I've done it myself when necessary. It is much better to use ANY kind of a fuse than to use a jumper wire.

Great! I can not found a fuse like the original here in my city (Curitiba). Maybe in São Paulo I can found it, but it's more than 400km from me!

It COULD be a mosfet, but I'd suspect a bad solder joint, in particular on one of the transformers.

PlainBill

ok, I'll redo all the solders in the board and then change de fuse and try again..

Thanks!

Leandro

[uke]

Re: Problems with Samsung 740N monitors

Hello all!

I got in my hands two SyncMaster 940b's. I changed three 820uF caps to 1000uF+470uF. Also the pico fuse and two mosfets (AF4502C) was burned. I found replacing mosfets from On semiconductor; MMDF2C03HD and soldered those to replace.

Monitors are working fine now, thanks to the forum!


-uke

[llpilch]

Re: Problems with Samsung 740N monitors

Hello all!

I got in my hands two SyncMaster 940b's. I changed three 820uF caps to 1000uF+470uF. Also the pico fuse and two mosfets (AF4502C) was burned. I found replacing mosfets from On semiconductor; MMDF2C03HD and soldered those to replace.

Monitors are working fine now, thanks to the forum!


-uke

did you change the 2 AF4502C by 2 MMDF2C03HD?

I can't found the AF4502C mosfet here, if i found the MMDF2C03HD can I put it n the place of the mosfet?

[stevelay]

Re: Problems with Samsung 740N monitors

hi all,
im new here, facing a prob with 740N monitor.
I restarted my comp and the monitor stops displaying images and nothing shown on the screen.
The blue light is still on, still working fine but nothing on screen.
I read through all the posts here, found that some of u facing the same problems.
However, before I start, can someone kindly teach me or take some pics to tell me how to open up the LCD monitor for 740N? I've been trying to find out but couldn't.
Would be grateful if someone would provide tutorial on that.
Thanks alot!

[stevelay]

Re: Problems with Samsung 740N monitors

managed to open it finally after so many attempts.
Im gonna try out the steps in previous posts.
Hope mine works.

[uke]

Re: Problems with Samsung 740N monitors

did you change the 2 AF4502C by 2 MMDF2C03HD?

I can't found the AF4502C mosfet here, if i found the MMDF2C03HD can I put it n the place of the mosfet?

Yes, I did change both of them. Couldn't find AF4502C's here either.

[mmsoft]

Re: Problems with Samsung 740N monitors

Hi to all.
I'm new here, and new to LCD monitor repair too. Made the repair to two 19" LCD Samsung 940 (very easy ) - replacing just the three capacitors 820x25V with 1000x25V on the first and same plus pico fuse 3A that goes to Inverter part on the second monitor.
But now I'm stuck on the third ( 17" 740N) having same sympthom like the other member on this post:
- No backlit, 5V is OK, but instead of 13V on inverter input I have 18,5V with soft Power switch ON, and 15V with switch turrned OFF.
What do You suggest?

Thank's

[johnny257]

Re: Problems with Samsung 740N monitors

Quote:
Originally Posted by llpilch
did you change the 2 AF4502C by 2 MMDF2C03HD?

I can't found the AF4502C mosfet here, if i found the MMDF2C03HD can I put it n the place of the mosfet?


Yes, I did change both of them. Couldn't find AF4502C's here either.
uke is offline Report Bad Post Reply With Quote


is there another substitute for AF4502C aside from MMDF2C03HD? i can't find them both here either! T_T thx a more power guyz!

[PlainBill]

Re: Problems with Samsung 740N monitors

Hi to all.
I'm new here, and new to LCD monitor repair too. Made the repair to two 19" LCD Samsung 940 (very easy ) - replacing just the three capacitors 820x25V with 1000x25V on the first and same plus pico fuse 3A that goes to Inverter part on the second monitor.
But now I'm stuck on the third ( 17" 740N) having same sympthom like the other member on this post:
- No backlit, 5V is OK, but instead of 13V on inverter input I have 18,5V with soft Power switch ON, and 15V with switch turrned OFF.
What do You suggest?

Thank's

Symptoms of an open fuse in the supply line to the inverter. If you have problems identifying it, read this FAQ, then post pictures (top and bottom) of the inverter. (If there already are pictures in this thread, just reply with the post number.)

Explanation: The power supply design used in most monitors produces 5V and 12V. The 5V supply is regulated quite closely, the 12V supply is designed such that it is correct when both supplies are loaded normally. The 5V supply is loaded most heavily when the LCD panel is on and displaying a signal. This is also the time when the 12V supply is loaded most heavily.

PlainBill

[Drools]

Re: Problems with Samsung 740N monitors

Can I replace the 820uf caps with all 1000uf caps or do I have to try and get as close to the 2460 total these caps are producing?

Thanks

[PlainBill]

Re: Problems with Samsung 740N monitors

Can I replace the 820uf caps with all 1000uf caps or do I have to try and get as close to the 2460 total these caps are producing?

Thanks

I'd stick with the original values. Looking at the pictures others have posted, there isn't a lot of extra space there.

PlainBill

[Drools]

Re: Problems with Samsung 740N monitors

Thanks Bill.
It is just that I have new, 2 - 1000uf 35v caps and 1 - 470uf 35 which are physically compatible with the 820 caps that were in there. Getting the exact 820 caps would require a special order from Digikey and I would rather avoid that if I can. What I was thinking is put the proper caps on the list and order them with my next big order. I could use other values as a temporary fix.
Also I should get away with replacing the little fuse green (if its blown) with a glass one?
This thread has been fantastic, I thought I would end up tossing the monitor. There is still hope!

[PlainBill]

Re: Problems with Samsung 740N monitors

Thanks Bill.
It is just that I have new, 2 - 1000uf 35v caps and 1 - 470uf 35 which are physically compatible with the 820 caps that were in there. Getting the exact 820 caps would require a special order from Digikey and I would rather avoid that if I can. What I was thinking is put the proper caps on the list and order them with my next big order. I could use other values as a temporary fix.
Also I should get away with replacing the little fuse green (if its blown) with a glass one?
This thread has been fantastic, I thought I would end up tossing the monitor. There is still hope!

Here are the steps I would take in this case.
1. Are the caps on hand rated for low ESR? If not, don't try it.
2. Are the three caps electrically connected together without any inductors, diodes, etc between them? If no, don't try it.
3. Is there physical room (both height and diameter)? If no, don't do it.
4. If you are in the US, it will cost about $2.00 for postage and take two days to get the caps to you. Is that delay and cost unreasonable?
5. Are you willing to open the case again to install the correct caps?

If you answered 'Yes' to all the above, go ahead.

PlainBill

[Drools]

Re: Problems with Samsung 740N monitors

I'm pretty sure all the CAPS are the low ESR type but I will have to verify. I have replaced lots of CAPS on motherboards and I usually have to order the correctones from DigiKey but this is a Power Supply and my first one. I live in Canada and Digikey has a 200.00$ minimum to get free shipping and I'm pretty sure it is 8.00US for shipping. Maybe I will just look at moving foreward my Digikey order of waiting for the monitor repair. How do I verify if a CAP is low ESR?

Thanks

[PlainBill]

Re: Problems with Samsung 740N monitors

I'm pretty sure all the CAPS are the low ESR type but I will have to verify. I have replaced lots of CAPS on motherboards and I usually have to order the correctones from DigiKey but this is a Power Supply and my first one. I live in Canada and Digikey has a 200.00$ minimum to get free shipping and I'm pretty sure it is 8.00US for shipping. Maybe I will just look at moving foreward my Digikey order of waiting for the monitor repair. How do I verify if a CAP is low ESR?

Thanks

Digikey has a USPS First Class Mail shipping option for orders in the USA. Without that, small orders are much less practical.

The series of the caps is on the cap (usually next to the temperature rating). The ESR rating is on the series spec sheet (on the manufacturer's web site), or you can check the FAQ 'Which caps should I buy'.

PlainBill

[Drools]

Re: Problems with Samsung 740N monitors

I think I will close the patient up until I get the correct parts...
I just want so say a very loud thanks to Bill and the site owners. It has most likely saved me a few bucks.

[sequille]

Re: Problems with Samsung 740N monitors

hello everybody!
I read this thread multiple times, even locked the tab in firefox so i can check it daily. It seems i`m having the exact same problem user "ethernet" has. Problem is he kind of gave up on trying (last post like 7 months ago) and my electronics knowledge is all but absent so i can`t answer questions and bring the same level of information like he did.
So: when i turn it on it sounds like it starts charging (like a camera flash, but not a linear pitch, more like an oscillating growing pitch) then the lamps flash (very short, like 100 ms) and i even see the image, then no sound for a short while after witch it cycles. Any help would be much appreciated
I can record the sound and search for some web host to upload it if you find it necessary.