Problems with Samsung 740N monitors

[newbie1]

Re: Problems with Samsung 740N monitors

Greetings from Asia,..

as the name goes a newbie here..

First of all, this is a Fantastic Forum..

just to thanks all who contributed to this thread as i managed to revive a dead samsung 740N, by changing the 3 caps+ 1 pico fuse (despite the difficulty in finding it).. u guys help me locate it..

eerrr, but problem is i bought an oversized cap & the case wouldn't shut.. oh well...

many thanks to the veterans & masters here...

[EGuevarae]

Re: Problems with Samsung 740N monitors

Greetings from Asia,..

as the name goes a newbie here..

First of all, this is a Fantastic Forum..

just to thanks all who contributed to this thread as i managed to revive a dead samsung 740N, by changing the 3 caps+ 1 pico fuse (despite the difficulty in finding it).. u guys help me locate it..

eerrr, but problem is i bought an oversized cap & the case wouldn't shut.. oh well...

many thanks to the veterans & masters here...

At least you know that with the proper size caps, your unit will be like new....congratulations...

[retiredcaps]

Re: Problems with Samsung 740N monitors

In one thread, PlainBill suggests a ballpeen hammer! See

https://www.badcaps.net/forum/showpo...3&postcount=62

There was a 2 page discussion on order the right diameter (for tight squeezes and close quarter) and height.

eerrr, but problem is i bought an oversized cap & the case wouldn't shut.. oh well...

[nanashi]

Re: Problems with Samsung 740N monitors

Hi everyone. Here is my story.
I got this samsung 721n, which happened to have a psu discussed here.

At first it has no image, no lamp power, and i don't remember the state of the power led.
I disassembled it, there were 3 bulged (two 820uf/25v and one 330uf/25v) CapXon capacitors.
I replaced all the caps on the board (with exception 820uf with 1000uf). How stupid of me, I haven't done any measurements before i did the replacement. Then I connected it all together and plug AC in. At first it was in off state, after pushing power button there were no image or lamp light, power led begun flashing about 2Hz and some ticking noise along. Now i done some measurements of 5v and 13v lines, which were 0.06v and ~2v (i don't remember), then removed AC plug.

Next thing I done is completely stupid. I disconnected cable that connects IP board and scaler board, and connected AC plug. Something started burning rapidly, smoke coming from below the IP board.
I put everything apart, on the solder side of the board i saw just burned U205 component with SP8M5 marking on it.

It's the first time i gone that far in repairing, I desperately need your advice before i burn something else and throw that lcd in trash (new IP board is just too expensive here).

[PlainBill]

Re: Problems with Samsung 740N monitors

Hi everyone. Here is my story.
I got this samsung 721n, which happened to have a psu discussed here.

At first it has no image, no lamp power, and i don't remember the state of the power led.
I disassembled it, there were 3 bulged (two 820uf/25v and one 330uf/25v) CapXon capacitors.
I replaced all the caps on the board (with exception 820uf with 1000uf). How stupid of me, I haven't done any measurements before i did the replacement. Then I connected it all together and plug AC in. At first it was in off state, after pushing power button there were no image or lamp light, power led begun flashing about 2Hz and some ticking noise along. Now i done some measurements of 5v and 13v lines, which were 0.06v and ~2v (i don't remember), then removed AC plug.

Next thing I done is completely stupid. I disconnected cable that connects IP board and scaler board, and connected AC plug. Something started burning rapidly, smoke coming from below the IP board.
I put everything apart, on the solder side of the board i saw just burned U205 component with SP8M5 marking on it.

It's the first time i gone that far in repairing, I desperately need your advice before i burn something else and throw that lcd in trash (new IP board is just too expensive here).

Here's the likely scenario: As the caps deteriorated the components were subjected to increased voltage spikes. Eventually U205 shorted, overloading the power supply.

After you replaced the caps the shorted U205 still overloaded the power supply. When you tried powering it up with the logic card removed the increased current caused U205 to flame out.

You should be able to readily find replacements for the SP8M5. It's a Rohm part, and the datasheet is readily available. I've seen equivalent parts from other vendors.

PlainBill

[nanashi]

Re: Problems with Samsung 740N monitors

Here's the likely scenario: As the caps deteriorated the components were subjected to increased voltage spikes. Eventually U205 shorted, overloading the power supply.

After you replaced the caps the shorted U205 still overloaded the power supply. When you tried powering it up with the logic card removed the increased current caused U205 to flame out.

You should be able to readily find replacements for the SP8M5. It's a Rohm part, and the datasheet is readily available. I've seen equivalent parts from other vendors.

PlainBill

Thanks!

I was unable to find Rohm's, so I ordered IRF7389 and FDS8958A instead.

[TimeWalker]

Re: Problems with Samsung 740N monitors

Hey there! New to the forums.
Got this very same problem everyone's having. My PCB is the same as posted HERE
Replaced the 820 uF 25V caps with Panasonics but the monitor still won't power on, not even the image (without back light) whatsoever. Upon powering on and switching off the monitor makes hissing sounds that can be barely heard.
I tested the green fuse located by the blown caps and it seems to be dead. I'm unable to find a proper replacement and the only thing I was able to stumble upon is THIS
Is it even close to what I need? Or it's a no go? Maybe someone could suggest a place to ship it from for Europe (OK it ridiculous to ship one fuse TBH ) ??

Thanx in advance!

[nanashi]

Re: Problems with Samsung 740N monitors

PlainBill, thanks a lot!
I replaced U205 with IRF7389TRPBF and the monitor came back to life again!

[pedro]

Re: Problems with Samsung 740N monitors

Hey there! New to the forums.
Got this very same problem everyone's having. My PCB is the same as posted HERE
Replaced the 820 uF 25V caps with Panasonics but the monitor still won't power on, not even the image (without back light) whatsoever. Upon powering on and switching off the monitor makes hissing sounds that can be barely heard.

That could be from a high voltage arcing at either the CCFL terminations or from the inverter transformers, OR it could be from the main switcher. You need to localise the sound source.

I tested the green fuse located by the blown caps and it seems to be dead. I'm unable to find a proper replacement and the only thing I was able to stumble upon is THIS
Is it even close to what I need? Or it's a no go?

Noooo! That's a PTC, not a fuse.

[davidfonsbo]

Re: Problems with Samsung 740N monitors

I have 4 Syncmaster 204b's and one of them once started to blink and it took 50 seconds before it was ready. Another one syddenly went black a few days ago so I decided to open them, And what did I find?? Bulgy capacitors. I have now changed all capacitors in both of them and the blinking one is completely stable again. The other needs the F301 changed too so I am going to order a few and I will also order some more capacitors for no. 3 & 4 just to be a little proactive.

It was really nice to read this thread - It kinda confirmed what I found.

It is a great forum and I will be back :-)

[Diablo_Rojo]

Re: Problems with Samsung 740N monitors

Hello all, what a great forum and great contributors!
I am new to this so bear with me please. I have a Samsung 740n with a PWI1704SV(A) power/inverter board. The monitor works for maybe a hour sometimes then shuts down with no led light on front. I can let it cool down and it will again work for a while. Even with no pc hooked up I can turn it on get the message of no video input and the blue led stays on for a while then eventually it goes out and will not come back until a cool down. I have measured voltages of 5.1Vdc and 15.1Vdc with the lamps unplugged while the led light was both on and off. I see no bulging caps although it does have Capxon caps on the board. I saw under the impression that no led would be a power supply issue. I appreciate any input.

[retiredcaps]

Re: Problems with Samsung 740N monitors

The monitor works for maybe a hour sometimes then shuts down with no led light on front. I can let it cool down and it will again work for a while.

A few suggestions.

1) It could be a poor solder joint somewhere.

2) One or more of CCFLs are slowing dying. As the CCFL age, they draw more power. I have a LCD that has the similar symptoms with shutting down after some time and I have narrowed it down to the CCFLs.

3) See if your monitor stays on longer by lowering the brightness and contrast to 40/20. Usually most monitors are at 90/30 for the settings.

4) See if a cooling fan point at the back of the unit makes the LCD stay on longer.

[EGuevarae]

Re: Problems with Samsung 740N monitors

2) One or more of CCFLs are slowing dying. As the CCFL age, they draw more power. I have a LCD that has the similar symptoms with shutting down after some time and I have narrowed it down to the CCFLs.

3) See if your monitor stays on longer by lowering the brightness and contrast to 40/20. Usually most monitors are at 90/30 for the settings.

4) See if a cooling fan point at the back of the unit makes the LCD stay on longer.

I Agree. also,is a pink/red hue present?

[Diablo_Rojo]

Re: Problems with Samsung 740N monitors

Wow! Thanks for the fast response. Would having no video input while turning on the monitor have go into stand by mode so the lamps do not stay be a way of eliminating them a one of the possible cause? When I do this the led light stays on for a while the shuts and will not come back on.

[Diablo_Rojo]

Re: Problems with Samsung 740N monitors

A few suggestions.

1) It could be a poor solder joint somewhere.

2) One or more of CCFLs are slowing dying. As the CCFL age, they draw more power. I have a LCD that has the similar symptoms with shutting down after some time and I have narrowed it down to the CCFLs.

3) See if your monitor stays on longer by lowering the brightness and contrast to 40/20. Usually most monitors are at 90/30 for the settings.

4) See if a cooling fan point at the back of the unit makes the LCD stay on longer.

Ok. looks like you were right. I pulled the lamps and they are black on the ends. Now to decide if I am skilled enough to try to replace them without breakage.

[coolkwc]

Re: Problems with Samsung 740N monitors

My friend got a dead Samsung 740N LCD, this is 1st time i open up this LCD, at 1st what i saw were 3 bulged Capxon cap, so i try some luck to search solution over internet, and surprisingly Samsung got such hot thread in bad cap forum.

After read briefly, i found out that the 3.15A pico fuse is blown, and 1 4502C IC got burnt mark on top. So i want to try the fast solution, i just short the dead fuse by using wire, and yes, the LCD come alive, but with blink light, after few attempts, the LCD blank again, and i smell some burnt IC. The 2nd 4502C IC die as well.

May i know which IC is suitable to replace those 4502C IC?

Totally lost confidence on Samsung product already.

[retiredcaps]

Re: Problems with Samsung 740N monitors

May i know which IC is suitable to replace those 4502C IC?

You must not have read through the entire thread because the answer is here ...

https://www.badcaps.net/forum/showpo...&postcount=198

[EGuevarae]

Re: Problems with Samsung 740N monitors

You must not have read through the entire thread because the answer is here ...

https://www.badcaps.net/forum/showpo...&postcount=198

Also, you did not say which caps you installed as replacements.

[PlainBill]

Re: Problems with Samsung 740N monitors

My friend got a dead Samsung 740N LCD, this is 1st time i open up this LCD, at 1st what i saw were 3 bulged Capxon cap, so i try some luck to search solution over internet, and surprisingly Samsung got such hot thread in bad cap forum.

After read briefly, i found out that the 3.15A pico fuse is blown, and 1 4502C IC got burnt mark on top. So i want to try the fast solution, i just short the dead fuse by using wire, and yes, the LCD come alive, but with blink light, after few attempts, the LCD blank again, and i smell some burnt IC. The 2nd 4502C IC die as well.

May i know which IC is suitable to replace those 4502C IC?

Totally lost confidence on Samsung product already.

Shorting a fuse and then complaining because a second IC burns up sounds like blaming someone else for your own foolishness. It is proper to expect a design will withstand normal use. Expecting a product to be designed to survive stupidity is childish.

PlainBill

[coolkwc]

Re: Problems with Samsung 740N monitors

Shorting a fuse and then complaining because a second IC burns up sounds like blaming someone else for your own foolishness. It is proper to expect a design will withstand normal use. Expecting a product to be designed to survive stupidity is childish.

PlainBill

I loss confidence on Samsung LCD mainly due to the bad cap and poor design they used, not because the design can't withstand my try, and i did realise it would burn the whole thing if i short the fuse, I just want the quick solution to proof the back light is still functioning.

As a so called veteran in forum, be patient when u post any reply, in this case, you are overreact.