Problems with Samsung 740N monitors

[EGuevarae]

Re: Problems with Samsung 740N monitors

Still not good. What is your reading on 5V while other 13V is at 16V?

Cheers, Wizard

Seeing the picture, I assume you changed the three caps to the left (of your picture). You must get out ALL the CapXon caps in there, not just the three to the left. try To stay as close as possible to the original capacitance - if they are in parallel, 2@1000 & 1@470 are OK (I've even done a 2200+470 in an emergency case three months ago). Removing the fuse to isolate the inverter section for further tests (as per Wizard suggestion) is a good idea.
Again, what (brand,) V Rating/Series/capacitance are the remaining caps you have in there? I have a 940T board that has two 1000uf@10v and another 330uf@10v - those might be the ones on the 5v power rail. If the reference voltage (5v) is wrong, the other one would be wrong also. As per Wizard suggestion, measure the 5v output. Here's a pic of my 940T board (from a Dead screen LCD unit). It should be the same as yours (pictured before recap).

[EGuevarae]

Re: Problems with Samsung 740N monitors

What about the small capacitor in the hot side of SMPS?
This filters the hot side DC supply for the SMPS IC.

Oh yeah, I forgot about this little guy.... it makes the 5v line bounce ....

[ethernet]

Re: Problems with Samsung 740N monitors

Sorry for the late answer, i was sick these couple of days.

Still no improvements, I replaced ALL capacitors from a working 740n monitor and i still cannot find the problem. 5V line is ALWAYS ok. As soon as i turn it on, 5V line reads 5.1V and stays that way. All resistor have been checked and they have the same value as the working one. I am starting to think that the big green transformer is the problem, because every other part has identical readings as the working monitor.
Can i attach an external power supply to supply the 13V line and where should i attach it?

[EGuevarae]

Re: Problems with Samsung 740N monitors

Sorry for the late answer, i was sick these couple of days.

Still no improvements, I replaced ALL capacitors from a working 740n monitor and i still cannot find the problem. 5V line is ALWAYS ok. As soon as i turn it on, 5V line reads 5.1V and stays that way. All resistor have been checked and they have the same value as the working one. I am starting to think that the big green transformer is the problem, because every other part has identical readings as the working monitor.
Can i attach an external power supply to supply the 13V line and where should i attach it?

I need a senior help here : correct me if I'm wrong, please.
Honestly I don't know (at this moment) what to suggest to you, but I doubt the Transformer is bad. It serves to change the AC (120/240) into DC (it "downsteps" the voltage for it to be rectified). I guess it is changed into 13v and the downstreamed once again to 5v.
Once I did what you are trying to do (inject voltage) on a Samsung monitor (a 173s) and the issue didn't finish as I expected. Wait for suggestions from PlainBill, Wizard, PCBONEZ, Toasty or one of our other seniors.

But something echoes in my head : what replacements did you use? You took them from a working unit, but what did you take? CapXons? Post info.

[ethernet]

Re: Problems with Samsung 740N monitors

Thanx for all the help so far. Yes, they were all Capxons. Caps definately arent the problem. Thats the third set of caps in this board .

[PlainBill]

Re: Problems with Samsung 740N monitors

Thanx for all the help so far. Yes, they were all Capxons. Caps definately arent the problem. Thats the third set of caps in this board .

Ethernet, I've just reviewed your posts, and it seems like we're missing something. The symptoms you describe are very unusual (yeah, I'm sure you've figured that out). I'm trying to rationalize them against what I know about electronics, and I can't. That means either I don't know as much as I pretend to, or some of the things you have reported are incorrect.
Perhaps you can answer a few questions I have.

1. I noticed you reported 750 volts across the big capacitor. That is VERY unusual. What is the line voltage in your area? In the USA we use 120 / 240 VAC, monitors use 120 VAC. For 120 VAC you should measure 170 volts, for 240 VAC you should measure about 335 volts - DC in each case.

2. Wizard had suggested you disconnect the inverter section from the power supply section. Did you do that?

3. If you did, does the green transformer still make noise?

4. What test equipment do you have - VOM or DMM? Make and model number?

PlainBill

[Wizard]

Re: Problems with Samsung 740N monitors

And incorrectly done on measuring. NO way that you are getting 750 volts. We're talking about POWER supply. Not the lamp inverter we must disconnect it first, and we need to fix that 13V issue is high first then reconnect inverter's because lamp inverter transformers/transistors are very sensitive to power issues. Remember always measure with DC scale on your meter for DC voltages. 750V is not possible even on 230VAC mains, which is around 300VDC-ish. Even the 115VAC mains it is around 160VDC on that big cap.

So I ask what spots did you measure these? Remember cold ground is for cold side voltage only while hot side ground is for hot side voltage,

You see, the power supply and the optisolator is the isolation between hot and cold side for SAFETY and takes knowledge where do you pick a ground reference to measure against.

Cheers, Wizard

[ethernet]

Re: Problems with Samsung 740N monitors

750V was due to a broken DMM i forgot to mention that later.
1. Line voltage here is 230V AC
2. & 3. I disconected the fuse to the inverter and the green transformer is making less noise and does not stop working (when the fuse is in place, transformer is working for 2 sec then stops working, then starts for another 2 seconds...)
4. DMM Velleman DVM 760, ESR meter

13V and 5V line was measured on CN2

[Wizard]

Re: Problems with Samsung 740N monitors

Broken DMM is not to trust and cost of a new good DMM is too inexpensive against repair one so we buy another. Unless we have fluke which I'd certainly send it in for repair.

That noise is cycling on overload or overvoltage tripping. We call it buzzing, tweeting, chirping etc. That's what a good SMPS design should do.

If it still making tweeting with this disconnected, then SMPS still have a problem.

Cheers, Wizard

[PlainBill]

Re: Problems with Samsung 740N monitors

As Wizard said. Check for parts generating excessive heat, in particular the diodes on the output of the transformer.

One relevant thing to remember: Transformers RARELY fail. But that is not the same as NEVER. It is usually a royal pain to change them, tho.

PlainBill

[ethernet]

Re: Problems with Samsung 740N monitors

Thanx for all the help guys!! Even if i fail to fix this thing, im really happy that i found this forum. So much to learn here. If i find a solution to my problem i will post it, but i will not post anymore questions.

Big thanx again to all

[Wizard]

Re: Problems with Samsung 740N monitors

With this fast reaction of SMPS IC, we hardly could detect overheating. Solution is use meter with PSU disconnected from mainboard and not plugged in for power, unsolder the main power transformer completely off the board. This will give you easier time measuring in ohms (diode check is not always 100%) on everything not just outputs.

Very RARELY we find a bandit PSU that had bad SMPS IC! :o

Keep asking. It will get fixed.

Cheers, Wizard

[EGuevarae]

Re: Problems with Samsung 740N monitors

Keep asking. It will get fixed.

Agree. It will, and then this info can be used by other in the future. It would be a very didactic experience.

[LIUSPEED]

Re: Problems with Samsung 740N monitors

hi im new here does anyone do this already ? and if i ship a few to you can you repair them for me?

thanks

[EGuevarae]

Re: Problems with Samsung 740N monitors

hi im new here does anyone do this already ? and if i ship a few to you can you repair them for me?

thanks

Better yet: if you post info and pictures, and answer the question that either I or other members do, you'll save money, do it yourself and save some landfill space.
And BTW, Welcome to Badcaps.net

[LIUSPEED]

Re: Problems with Samsung 740N monitors

ill post pics of the systemboards i have.

i got about 12 of these monitors at my office that all have the same symptoms

[EGuevarae]

Re: Problems with Samsung 740N monitors

ill post pics of the systemboards i have.

i got about 12 of these monitors at my office that all have the same symptoms

I can take a look if we are not so far away from each other, but sending (to me and from me, or to/from anyone else) can be expensive man. If we are close, I can take a look, see what parts are needed, order them, fix one or two if possible and let you fix the remaining units based on what I did to the fixed ones (no $$ involved, of course). But the odds that we are close are really small......

92254

[LIUSPEED]

Re: Problems with Samsung 740N monitors

i live in dallas tx so ya not that close

thanks though

[undisclosed]

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

[undisclosed]

Re: Problems with Samsung 740N monitors

Here is the picture of the board with the resistors lifted. Oh and i forgot i ask what watts would they be.