Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

[OxygenX]

Hello there,

I have a problem with the BN44-00195A power supply.

History
The screen turns of when I was working during the day. The blue LED on the power button was still on I guess (I'm not quite sure, but I guess). Suddenly, maybe an hour later, the scree turns on again, which I can see because there was this message "no signal" on the screen, because I plugged out the HDMI cable from the notebook. Before there was no more picture on the screen. I plugged the HDMI cable back in an worked for one day just normal on it, since i heard something that sounds like a electronic part was blowing up. Since then the blue LED on the Power button didn't shine anymore.

I opened the screen and saw that there is a bad cap on the power supply CB853 (2200uF, 10V) which i replaced whit a new one that i bought. I taught I fixed the problem, reassembled the screen, but the blue LED was still not shining and the screen doesn't work. Well maybe I fixed only a part of the problem. And I learned something new about caps.

I took my multimeter because I would like to measure the voltages on the board. Suddenly I heard little explosion. Now both fuses are blown and the resistor RM801, which was full with glue was black like the bottom of the board around this resistor, which I have disoldered afterwards. Maybe it was the connectivity of the old glue? I don't know, but it was not possible to measure a resistance of that resistor after I solder it out.

Now i soldered out a few other components to test them. There was two other resistors which have no value to measure (also full with glue).

Now I'm not sure what the correct values of the resistors are and the other components and I hope to find somebody who can help me to fix this problem.

Equipment
-Multimeter
-LCR-T4 ESR Meter

I'm just a noob and hope somebody can help me.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

check this thread https://www.badcaps.net/forum/showthread.php?t=49288
RM801 is likely 0.22Ω If that resistor is open, the ICB801S is damaged RB801 shoould be 100kΩ 2watt
There are many resistors on this board, you need to provide the identifier, not "There was two other resistors which have no value "

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Hey R_J,

Thank you for your reply. And sorry for the bad English. You're right with the two values of the resistors (I posted a picture of those two ones, because I came to the same result in my learning process, by reading the color-code on the resistors.)

Unfortunately I was not able to provide the identifier, because I can't read the number on the board (it was behind the glue, and after removing it, the font was no longer readable because the upper side of the board is scratched now). On the back of the board it's also not readable on one resistor. But there's also a wrong information on my first post...

...let my try to correct that...

...the two resistors i wrote about has the same color-code. It means that they are identical. One of them is RB801 (100kΩ). The other, identical one, is on the left side of it (the one with the identifier that is not readable). I measured this two ones with my multimeter now. One Value is 95kΩ which is okay. The other one is 91kΩ which is a little to low, but I'm not sure if that's rely a problem?

I posted also a picture of the two measured values for better understanding.

The RM801 (0.22Ω) is defective, because I cant measure that value with my multimeter. I post a picture of that multimeter. I hope it's good enough to measure resistors with small values like that? But yeah, that was also the black resistor, which blows up before I want to measure the voltages. All the glue was still on the board at this time...

Maybe the problem can be solved by removing the RM801 (0.22kΩ) resistor and then try it again with new fuses and no glue.

Are you sure if RM801 (0.22Ω) is open the ICB801S is damaged? I ask because this resistor is not close to the ICB801S. But RB801 (91kΩ measured instead of 100kΩ is, and has a direct connection to it). Maybe you mixed up these two ones?

Thank you very much for your patient.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

The meter will be ok for testing, You might want to check RB808 (2.0Ω), It is from Pin1 to hot ground, it is the current sense resistor from the internal fet source to ground, sometimes if the ic's fet shorts it will open RB808 as well.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

I made a mistake, I thought RM801 was the supply resistor for the standby ic. Sometimes these power supply circuits start to all look alike. So the standby ic, ICB801S should be good.
RM801 is the fuse resistor for the two mosfets, QM801 and QM802, check that they are not shorted, RM801 could have opened up if you shorted any of the 24 volt or 12 volt lines.

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Hey R_J,

Thank you for helping me. That's fun!

First of all i checked the resistor RB808 (the measured value is 2.1Ω). The resistor also looks brand new.

Next I desoldered the two mosfets QM802 and QM803 (picture added). Both of them are shorted. That's bad because they are shorted, but good on the other hand because I found the error (only with your help!). And I learned how to check mosfets with the multimeter. Unfortunately the schematic didn't match exactly (PDF added) which is confused for a beginner like me, because the two mosfets I desoldered are FQPF9N50C, not FQPF5N50C like in the schematic. It's written in another thread as well (that the schematic does not match exactly).

Well, now I know that I need one resistor RM801 (0.22Ω, 1W) and two mosfets NQPF9N50C. I will order them from aliexpress, because it's not a urgent project.

Is there something else I can test before ordering? For example I don't now for what reason these two mosfets are shorted? Maybe because they are old?

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

If those fets are shorted, check if the gate is shorted to the drain or source, because if it is it might have damaged the gate resistors and diode
The FQPF9N50C is the 9 amp version. the FQPF5N50C is the 5 amp version, both are 500 volt rating They are also TO-220F which is full plastic case. So be sure to get FQPF not FQP. They also make higher current versions of this mosfet.

There is also a ferrite beed on the drain, be carefull removing it the are fragile, and install it on the drain of the replacement.

I see you have removed CM812, Did it look bulged and not nice and square? It could be bad, sometimes they go open.
If CB853 was bad, that would have put some extra strain on those mosfets.
I would also replace CM806 (10µf/50v) and CM807 (47µf/50v) They might have different location numbers on this board. Likely CM814 and the one next to TM801

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Everything is shorted on those two mosfets (picture added). Which brings me to the question, which is/are the gate resistor/s and the gate diode/s (to me it's not clear on the schematic right now, but that's my fault I guess)?

Thank's for all your additional information about the mosfets. To me as a noob I first taught that the ferrite beed is shrinking tube.

CM812 don't look bulged. The measured value of it is 11.48nF (see added picture).

I also soldered out both of the caps you recommended to replace. CM814 (10uF/50V, measured value: 10.75uF) and the one next to TM801, which I can't read the number anymore (47uF/50V, measured value: 48,89uF). The values of those two caps are good enough, right? Or not? Sorry for asking as many, maybe for you, stupid questions. For me it's just highly interesting and I really appreciate all of your information.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

CM812 is likely ok, it is usually the white SIC ones that fail. The gate diode and resistor are the ones connected to the mosfet..... Gate. for QM801, they are DZM803,DZM804,DM803, RM811. DM803 is hard to check because of RM811 (10Ω) in parallel.
You can reuse the 10 & 47µf caps if you like.

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Uff, you mean the small smd-parts on the back.

I tested the four z-diodes with the multimeter just like a normal diode, which means that the current flow from anode (+) to cathode (-) is okay. I painted that in red (picture added).

The two mosfets are painted yellow.

Painted in blue are the two resistors, which are in parallel to the other, normal smd-diode (not z-diode). In my case these two ones have 22Ω. I measured both of them and they are okay.

Painted in Orange are the two normal diodes (not z-diodes), which are hard to measure, as you wrote, because they are in parallel to the resistor. Nothing done there, because I'm a little afraid to solder them out... :/

What do you think shall I do at this point? Do we have enough information? Or shall I give it a try and solder the two smd-diodes out for testing them (never done this before)?

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

The gate drive components seem to be ok. It must be just the mosfets and the resistor that are bad. If they were bad the diodes would check shorted and the 22Ω would liklely check open and be burnt.

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

one thing confuses me.

The z-diodes are okay like I wrote (painted in red). I also can measure the resistance of the two 22Ω resistors (painted in blue).

But...

...when I measure the two resistors (22Ω) in the diode-mode of the multimeter, not in the ohm-mode, the resistor is shorted. The same with the two diodes (painted in blue) parallel to it? The diode-mode is also the one where I can hear a sound like "peeeeep" if something is shorted. It's not the case for the four z-diodes.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

You will need to lift one end of the diode to test it, that is the only way to be sure.

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Hmm, I have to learn how to solder smd-parts on a trash board first. For that PSU it means I risk it without testing them now. Maybe later...

Thank you very much for sticking with me until this point. It's just nice to see how people like you help beginners like me.

I order the parts today (delivery will take a few weeks). If they are here I'll give you a feedback.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

You can't go by the meter BeeeeP, Most meters will beep between 0Ω and 120Ω so a 22Ω resistor will make it beep. thats why you need to rely on the meter resistance numbers when checking resistance, or voltage drop in diode mode.

[OxygenX]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Today I received the last electronic parts for the power supply and soldered everything in.

The good news are: The Screen works without any problems.

Thanks again R_J for helping me working thru this. I'm happy.

[R_J]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Good to here it is fixed.

[mtbsrac]

Re: Samsung Syncmaster 245B Plus (BN44-00195A Power Supply))

Hello!

Mine is bad too, but there was only an amperes smell 0 burn trail.
What to check?

Thanks.