Problems with Samsung 740N monitors

[Venomus28]

Re: Problems with Samsung 740N monitors

Change only the bulged ones, fire it up and see how it goes
My guess is a working monitor

[jondoe]

Re: Problems with Samsung 740N monitors

Yep, those purple ones don't pass the flat top visual inspection, get those changed with some good ones. Change the blue ones while you're at it.

[capmo]

Re: Problems with Samsung 740N monitors

Open, take pictures, share with the whole class.

If you can use a soldering iron without stabbing yourself in the eye or setting fire to yourself, you should be fine to do the caps. Take your time.

Thanks for the vote of confidence.

Attached are some images.

i think that the caps are bad, open it up and see for any bulged one

Was my first guess, but not sure.

[Venomus28]

Re: Problems with Samsung 740N monitors

i think that the caps are bad, open it up and see for any bulged one

[jondoe]

Re: Problems with Samsung 740N monitors

Open, take pictures, share with the whole class.

If you can use a soldering iron without stabbing yourself in the eye or setting fire to yourself, you should be fine to do the caps. Take your time.

[capmo]

Re: Problems with Samsung 740N monitors

Hello everyone, My 740N gets the power but doesn't stay ON for long. I turns ON for a fraction of a second and then OFF and that keeps happening. The power LED(on the button) keeps blinking while the display is OFF.
The display does function correctly as when it does turn ON for that fraction I can see everything working fine.

Before this happened, from past few weeks/month ago it used to take 5-8mins for the display to turn ON. I should have tried to fix it that time.

When under warranty few years back I think I had a similar problem not sure but that time the Samsung guys had replaced 2 CapXon capacitor(2x1000uF 10v)So I am guessing the rest are the terrible CapXon and might be the problem?

What is the first step to know what I need to replace? I haven't opened mine yet and checked for bulged capacitors, but I have already bought all the required capacitors from local shops. (from the reference/manual/ebay listings available online)

My soldering experience is minimal and I have never done any capacitor-PCB soldering that was this critical, hope it goes fine.

Thank you for the help.

[Venomus28]

Re: Problems with Samsung 740N monitors

Did a resolder and all works perfectly now.
Thanks

[Venomus28]

Re: Problems with Samsung 740N monitors

After changing both mosfets the monitor turned on and worked right for about 10 mins then it the back light turned off again
What could it be ?

[jondoe]

Re: Problems with Samsung 740N monitors

The monitor is reassembled now being tested, so I can't take a picture, sorry.

[Venomus28]

Re: Problems with Samsung 740N monitors

Yes i know, can you put up a pic ?

[jondoe]

Re: Problems with Samsung 740N monitors

How easy it is will depend on your own experience and the tools you have. Removing them was probably harder than replacing them, the glue holding them in place didn't want to let go. The heat sink also made it harder work than it needed to be, removal of that before hand might have made it easier.

I used a little solder wick to clean the pads up before placing the new ones on the board, they aren't perfectly straight, but they are all working. I do have a cheap hot air station, it's quality is questionable so I managed without it!

[Venomus28]

Re: Problems with Samsung 740N monitors

Was it hard to solder the new mosfets ?
I have had 2 ,Samsung 940T and on the 1st one (im using now) i changed 3 caps and one pico fuse
and at the other one i have changed 3 caps and the pico fuse and i am waiting for the mosfets to get delivered so i can change them too

[jondoe]

Re: Problems with Samsung 740N monitors

In all, I changed 3 of the large capacitors, 1 pico fuse, 2 x MOSFETs (replaced with IRF7319PBF) and a zener diode (BZX84C7V5LT1G) although I didn't need to change the Zener, I just destroyed when removing it to test.

It's still working fine today, i turned it off over night to cool then powered it back up this morning.

[Venomus28]

Re: Problems with Samsung 740N monitors

Did you changed the 2 mosfets too ?

[jondoe]

Re: Problems with Samsung 740N monitors

So far, so good. I left it turned on all day yesterday and last night and she's still running. Thanks to everyone who contributed in this thread, past and present

[jondoe]

Re: Problems with Samsung 740N monitors

That was a little harder than I imagined, but the MOSFETs are in place, the new zener and a fresh fuse. The heat sink on the back of the MOSFETs common ground makes soldering a pain in the butt, removal before hand might have been easier.

I destroyed one pad while removing one of them, but managed to bend the leg a little and scrape some trace so it could take a tiny bit of solder make the connection.

I won't post a picture of the work, I'm too embarrassed

It fired up first time, back lights working, etc, so I'm leaving it on the bench to see if it pops a fuse again.

[jondoe]

Re: Problems with Samsung 740N monitors

I used a dirt cheap plastic jewelers loupe and my camera phone for the picture, they come out surprisingly well

[Behemot]

Re: Problems with Samsung 740N monitors

Yeah they usually glue them to hold in place during wave soldering.

Nice work with the magnifying glass BTW, I am surprised you managed to focus it

[jondoe]

Re: Problems with Samsung 740N monitors

Well, that was a little more difficult to remove than I had hoped, she was glued and didn't want play.

I've tested it off the board with my DMM

  |
[ZD201]
|   |
0.7   OL

One leg doesn't appear to work at all, which according to the datasheet I think is correct (at least it is with the replacement ordered). So I assume this was working fine.

I'll replace it with the new one as the old one looks a little worse for being pulled out with my ham fists

[jondoe]

Re: Problems with Samsung 740N monitors

A very valid point, that did cross my mind. I could always change the mosfets first before the zener and test