Samsung monitor won't turn on. (Syncmaster 941BW)

[TheCockroach]

Hello.

So, I woke up this morning, and my monitor won't turn on. This caused a horrible start to my day, as I had a lot of work to do. I use a three monitor set-up, but for what all I do, it's barely enough as it is, so having even one go out can be disastrous for me. I'll try and lay out exactly what happened, in hopes that someone is able to help me figure out if it's salvageable.

My PC is always running, but I turn my monitors off via the power button each night before bed (still plugged in though, of course), and then switch them back off when I get up. Well, I woke up this morning, and it seems my PC was actually OFF, so not sure what happened there.. But I'm fairly sure it wasn't a power outage, since my microwave time didn't get reset. So it was either shut-down by a windows update, or by a power surge in my room. (The monitor in question, WAS hooked up to a surge protector, though.)

So I went to turn my PC back on, and then hit the power button on each of my monitors. My far left monitor though, did NOT come back on. There is no indication of it receiving power whatsoever, and I've even tried different outlets. The power button doesn't flash at all, even when unplugging/plugging it back in.

Monitor: Samsung 941BW

I've taken it apart as well, just to check and see if a cap had went bad, but it "looks" as if they are all fine. I know kind of what to look for to know if one was bad, but I know sometimes you can't tell by just looks. Either way though, I thought I'd ask here since I'm not an expert on the topic. Just trying to figure out what went wrong, so I can order the part(s) needed to get it fixed. I've also included some pics of the board as well.

Please let me know if there is any other information that you need to know, in order to offer advice about this. Or if there is anything in particular that I can try. Thank you ahead of time, I will be checking back very frequently.

[CapLeaker]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

First you have to check if the PSU puts out STBY voltage 5V, which is always there. That goes to the main board. When you press the power button, the main board returns a PSon signal to the PSU and the PSU turns fully on.
Unfortunately I am not able to read the pin out on the PSU. Anyhow, this will get the ball rolling to determent what board is at fault.

[Joe Black]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Your monitor probably wont have V+ on the connector, and most likely main power board is down.

Take care all the time, that big cap can hurt. I am not sure if it has a bleeding resistor...
Maybe someone else will see it.

Check:
-Fuse (If blown should be replaced with 20-30W bulb in series for now, that's good practice for us beginners)
-NTC (Varistor)
-Both sides of graetz bridge for short (outer legs are output inner are input)
-Test graetz out of circuit
-Test switching transistor (U101 it seems)
-Good luck

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

First you have to check if the PSU puts out STBY voltage 5V, which is always there. That goes to the main board. When you press the power button, the main board returns a PSon signal to the PSU and the PSU turns fully on.
Unfortunately I am not able to read the pin out on the PSU. Anyhow, this will get the ball rolling to determent what board is at fault.

I have someone bringing a multi-meter later, so hopefully I can find out then, thanks! Is there any good guides here for testing different components with one, though? I've only had to use a multi-meter a few times, and never on anything this complex. Just want to make sure I get it done right, obviously. I always like to be as knowledgeable as possible about things I'm trying to fix though, although I guess that's a good thing haha!

And also, there is a darker spot on the board as well, but I'm not sure if it's a burn mark, or just a stain of some sort. Any ideas?

Your monitor probably wont have V+ on the connector, and most likely main power board is down.

Take care all the time, that big cap can hurt. I am not sure if it has a bleeding resistor...
Maybe someone else will see it.

Check:
-Fuse (If blown should be replaced with 20-30W bulb in series for now, that's good practice for us beginners)
-NTC (Varistor)
-Both sides of graetz bridge for short (outer legs are output inner are input)
-Test graetz out of circuit
-Test switching transistor (U101 it seems)
-Good luck

Not exactly sure what ALL of that is, but I'll definitely find out, and test it all as well, thank you for the guidance! And also, where is the fuse at for the board? Is it what I'm pointing to in the below picture?

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

And ohh... speaking of actually.. one of my other monitors, is the same kind. I could potentially just take the power supply from the other (working) monitor, and put it in the bad one and test it out that way. Although, I'm also a bit hesitant to do it that way also, knowing my luck.

Would it be a safe option, as long as I'm grounded properly, and (of course) very careful during the exchange? I'm always extremely careful anyway, but I'm also unaware of how sensitive these components can be. And I guess another concern being, how long should I have the power off before removing the (good) power supply? Better to know, then to take a shock because the capacitor is still holding too much charge.

[CapLeaker]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

When the power supply is plugged in, be very careful on the primary "hot" side of the PSU. There are lethal voltages. For now I would avoid it, just do all the measurements in dc volt.

You can swap power supplies if the other one is the same or a sufficient alternate.

[momaka]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

I could potentially just take the power supply from the other (working) monitor, and put it in the bad one and test it out that way.

If you don't mind opening the other monitor(s), then that would be the best thing to do, since then you will know weather it is the power supply board or the main/video board in the monitor that is not working

Would it be a safe option, as long as I'm grounded properly, and (of course) very careful during the exchange?

Yes, it is safe both for you and the monitor. Just take these precautions:
1: Never disassemble or work on the monitor while it is plugged in (except for measuring voltages - in those cases, just avoid touching any metal parts on the monitor or the circuit boards).
2: Discharge yourself before touching the boards on the monitors with your hands (especially if you live in a dry climate area). This can be done by touching the metal case of the monitor.

how long should I have the power off before removing the (good) power supply?

That depends on the power supply design. The best practice is to unplug the monitor while it is either in the Standby mode (i.e. monitor LED lit with no signal going to it, like when the PC is off) or turned ON. In this case, the cap should discharge instantly.

On the bad power board, the high voltage electro cap could sometimes hold a charge indefinitely.

Either way, just avoid touching the "HOT" side on the board, and you should be fine. On the board markings, there is a thick white line, near the yellow transformer. It separates the power board into a "HOT" and "COLD" sides.

Now, as far as troubleshooting the possibly bad power board (should you find that the dead monitor is not working due to a bad power board)...
First watch this video to get more familiar with your multimeter (if you already aren't):
https://www.youtube.com/watch?v=bF3OyQ3HwfU

The next thing you will want to do is check the HOT side fuse and thermistor, as Joe Black suggested. No need to test the bridge rectifier yet (if it is blown, this will almost always cause the fuse to blow as well... so unless the fuse is blown, don't bother checking it). You can check these components while they are on the board (i.e. you don't have to remove them). The fuse is that round red cylinder, right behind where the power cable jack is. The thermistor is to the left of that (using your second attached picture as a reference) - it is green in color, and sits right next to that white-box capacitors (an X safety cap, to be exact). The thermistor is marked on the board as "TH01".

What you have to do to measure the fuse and the thermistor is set your multimeter to continuity (beep) mode and make sure that you get a audible beeping sound when you put the multimeter probes on the metal leads of those components (first check the fuse, then the thermistor). Alternatively, instead of using the continuity mode, you could also use the lowest resistance scale on your multimeter (usually 200 Ohms on manual-range meters) and see that both the fuse and the thermistor read less than 5 Ohms. Regardless of which way you do this, make sure the monitor is unplugged! The only time you will ever need the monitor plugged in while troubleshooting is if we ask you to measure DC voltages. For everything else, keep it unplugged.

Once you get these troubleshooting steps done, we will continue from there.

[Joe Black]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

And also, there is a darker spot on the board as well, but I'm not sure if it's a burn mark, or just a stain of some sort. Any ideas?

Many components get hot during normal operation, and heat will darken the board as in your case. Sometimes it indicates overheated component... My guess is those diodes are OK in your case. Of course, when in doubt, you have to test component to be sure. In this case, leave them be for now.

And, if you find that fuse is open circuit, resist the temptation to replace it and power on the monitor. Just report back here.

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

The next thing you will want to do is check the HOT side fuse and thermistor, as Joe Black suggested. No need to test the bridge rectifier yet (if it is blown, this will almost always cause the fuse to blow as well... so unless the fuse is blown, don't bother checking it). You can check these components while they are on the board (i.e. you don't have to remove them). The fuse is that round red cylinder, right behind where the power cable jack is. The thermistor is to the left of that (using your second attached picture as a reference) - it is green in color, and sits right next to that white-box capacitors (an X safety cap, to be exact). The thermistor is marked on the board as "TH01".

What you have to do to measure the fuse and the thermistor is set your multimeter to continuity (beep) mode and make sure that you get a audible beeping sound when you put the multimeter probes on the metal leads of those components (first check the fuse, then the thermistor). Alternatively, instead of using the continuity mode, you could also use the lowest resistance scale on your multimeter (usually 200 Ohms on manual-range meters) and see that both the fuse and the thermistor read less than 5 Ohms. Regardless of which way you do this, make sure the monitor is unplugged! The only time you will ever need the monitor plugged in while troubleshooting is if we ask you to measure DC voltages. For everything else, keep it unplugged.

Once you get these troubleshooting steps done, we will continue from there.

Ok so, I just now finished checking them. The fuse was showing up at 1.0 Ohm, and the thermistor way up at 7.5 Ohms.

I'll assume that that means it's definitely the power supply, so I guess now it's just up to what to do about it, and if it's something I can fix myself or not. We'll see though, I guess!

[budm]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

The nect thing you need to do is to find out if you do have 160~170VDC reading between the two legs of the main filter cap (the big 450VDC Cap laying on its side), this is where you MUST be real careful since you will be making the measurement in the PRIMAY DEADLY SIDE of the circuit, you need to have at least the Voltage on the cap before going further.
BTW, the small white connector of the power supply/inverter board that goes to the main board has pin names and numbers to tell you what they are, did you verify that none of the pins have Voltages?

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

The next thing you need to do is to find out if you do have 160~170VDC reading between the two legs of the main filter cap (the big 450VDC Cap laying on its side), this is where you MUST be real careful since you will be making the measurement in the PRIMARY DEADLY SIDE of the circuit, you need to have at least the Voltage on the cap before going further.

Should I assume that the best way to do that, is to lay the board back into it's "slot" in the case, and then test it by placing the leads onto the associated pins on the bottom side of the board? And if so, what should I set my multimeter at, DCV (20)? And last, but not least, I'm assuming that in this case, it doesn't matter which lead is on which pin, right?

BTW, the small white connector of the power supply/inverter board that goes to the main board has pin names and numbers to tell you what they are, did you verify that none of the pins have Voltages?

If you mean, the small white connector that goes from the power supply board, to the other board, then yes. Assuming that's the one you mean though, since the other 2 white connectors appear to go to the monitor. I set my multimeter to DCV (20) and placed the COM lead onto one of the NEG pins, and then tested each of the other pins. If I didn't test the correct one, OR if I did it incorrectly, please do let me know though.

[budm]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

OK, let test the DCV on that white connector of the power supply board that goes to the main board, you can test the power supply board by itself without having other board or the lamps connected, just the power cord.
There are the GND pins (3) that the black probe of the meter will be connected to (any one of the 3 pins), then the red probe will be connected to the +5V pin. Meter set to DCV 20 will be fine.

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

OK, let test the DCV on that white connector of the power supply board that goes to the main board, you can test the power supply board by itself without having other board or the lamps connected, just the power cord.
There are the GND pins (3) that the black probe of the meter will be connected to (any one of the 3 pins), then the red probe will be connected to the +5V pin. Meter set to DCV 20 will be fine.

Yup, it's showing around 5.13V.

[momaka]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Ok so, I just now finished checking them. The fuse was showing up at 1.0 Ohm, and the thermistor way up at 7.5 Ohms.

Those readings mean both the fuse and the thermistor are good.

Since you also checked the voltage on the white connector and you found that there is about 5V, then that means the power supply IS likely working okay.

Question:
Did you try swapping the power supplies from your working and non-working monitors? This will help determine if it is the power supply. The fact that you got about 5V from the (supposedly) bad power supply means that it actually might not be bad as it was initially assumed.

[budm]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Yep, the power supply section is working.
So for sure you do not see any indicator lights on the front bezel of the monitor at all?

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Those readings mean both the fuse and the thermistor are good.

Since you also checked the voltage on the white connector and you found that there is about 5V, then that means the power supply IS likely working okay.

Question:
Did you try swapping the power supplies from your working and non-working monitors? This will help determine if it is the power supply. The fact that you got about 5V from the (supposedly) bad power supply means that it actually might not be bad as it was initially assumed.

Nope, I haven't yet, but I guess that's the next step then. Most people would have tried that first, I guess. But I didn't for one sole reason.

Which is, with my luck, something would end up it up if I had tried that first haha.. Let's just say, my luck is so bad, that there's a better chance that I could've touched the connectors of that 450VDC Cap with a big metal object (while it was plugged in..) and only gotten a little static shock, then that my spare monitor would've still worked afterwards.

Luckily, I should be ok now, since I didn't try that first haha..

[TheCockroach]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Ok so.. I took out the power supply from the other monitor, and put it in the "bad" one. It came right on, and I'm using it as we speak. So, that means apparently the other power supply WAS bad, then..?

But according to the tests, it seemed to be good, right? Is there anything else in particular on it, that I can check, to see if I can find out where the problem is coming from?

Yep, the power supply section is working.
So for sure you do not see any indicator lights on the front bezel of the monitor at all?

And nope, I didn't before.




This is kind of the story of my life, though... Weird ass shit like this happening Lol.. If you ONLY knew..

[momaka]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Ok so.. I took out the power supply from the other monitor, and put it in the "bad" one. It came right on, and I'm using it as we speak. So, that means apparently the other power supply WAS bad, then..?

But according to the tests, it seemed to be good, right? Is there anything else in particular on it, that I can check, to see if I can find out where the problem is coming from?

I can't think of anything to check. Perhaps look at the joints on the white power connector first (the one that is used to give power to the logic/video board). If it looks good, then I guess it is time for a recap.

Sometimes, the voltages may appear correct, but the caps could still be bad and letting excessive ripple through. I had a Gateway monitor behave in a very strange manner, even though all voltages from its power supply appeared normal. After doing a recap, all problems went away suddenly.

Just because the caps aren't bulged doesn't mean they aren't bad or starting to go bad . So next stop for you will be Digikey, Mouser, or some other place to get reputable caps (avoid eBay for now - too many counterfeit caps to sort through. If you don't have the eyes for it, you will likely get burned there.) You don't need to replace the big, high voltage cap on the board, but the other electrolytic caps need to go.

If you need help picking the right replacements, please provide some info on the existing caps, such as:
brand, series (usually or or three letters), voltage rating, capacitance rating, diameter (in mm), and height (also in mm).

In general these caps brands and series will work in most monitors:
Panasonic FC, FR, FM
Nichicon PW, PS, PM, PJ, HE, HW
Chemicon LXZ, LXV, LXY, KY, KZE, KMY
Rubycon ZL, and possibly ZLJ

[momaka]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

Also forgot to mention...
the heat from those hot diodes (where the PCB is darkened) has probably shrotened the lives of the caps quite a bit (even if they are still in spec). So it is a good idea to change the caps anyways.

Now if you are feeling a bit adventurous, you should also replace those hot diodes with something better, like STPS1545D or MBR1060G. You will only need two.

[budm]

Re: Samsung monitor won't turn on. (Syncmaster 941BW)

BTW, the 15VDC is also present at the pins, right.