Samsung 24" power supply BN44-00195A

[Trofazni]

So, we meet again badcaps

I got monitor with psu mentioned, that was in some repair shop, but they tried something, with no sucess, and it came finnaly to my hands.

Problem was (or is still) on primary side, QM802/803 were short, as well as driver MC33067p, along with fuses, resistors, etc.
Changed all that I found faulty, but still it does not run. What you can see in a video, is psu+logic+pwr on/off button, without backlight or monitor connected.
As I switch it on, by pressing power button, pfc is working and steps up to ~390V, 5VSTB is steady, but 24V comes on very briefly. I detached 24-5V regultor, to eliminate problem in 5V rail, but is it the same situation.
When i switch it off, pfc drops, and that is it. I measured almost everything for shorts and values, can't find the problem...
Multimeter shows voltage of pfc, green is 5,2stby, upper is 24V, down is 5V meter..

Can anyone point me in some direction?

https://www.youtube.com/watch?v=iMELsqLluYE

Update: I checked voltages on MC33067, and i don't think there should be 14V on output pin 12?

[R_J]

Re: Samsung 24" power supply BN44-00195A

Check if CM808 is still good. It might be CM812 12n/630v

[budm]

Re: Samsung 24" power supply BN44-00195A

CM812 (Blue rectangular cap) is good? That schematic does not really match -00195A.

[Trofazni]

Re: Samsung 24" power supply BN44-00195A

CM812 (Blue rectangular cap) is good? That schematic does not really match -00195A.

Yes, tested it with one of those ebay "LCR-T4 ATmega328 Digital Transistor Tester LCD Capacity ESR Meter" which works really fine btw, cappacitance is 11n tested. Also CM809 47pF/1KV beside him is ok. Measured esr of electrolytic caps in that area, also ok.
When turned on by button press, MC33067 gets 15V, 24V rail gets some Voltage, and that is it. IC is still on steady 15V, but its like it does not oscilate. Also, changed that IC along with fet change, but i have soldered another one mc33067, and new ic does not make difference.
Next?

Correct me if I am wrong, but isn't MC33067 in charge of driving two fets that produce 24V over transformer?
Pins 12 and 14 are changing states giving impulses to tm802 trafo, wich are then rectified and limited and they drive gates of fets, one at a time, providing high frequency pulses, say voltage, to trafo that produces 24v. Well, this behave like it switches one just one fet for one time only. Although, optocoupler gets and loses voltage one time only, the driver ic 33067 still gets 15V steady,and stays on, regardles of no presence of voltage on diode part of optocoupler. Why ic stops sending pulses to the coil that drives fets?
Am I getting this right?

[Unspun01]

Re: Samsung 24" power supply BN44-00195A

Good job removing that crusty yellow adhesive glue stuff. That stuff was green on a few of my "Dong Yang" power boards in Samsung monitors and could possibly be conductive...double check you got all of it.

From the photo of the backside of your power supply board, check the condition of gate resistors RM813 and RM820. They look poorly soldered, and RM813 looks discolored possibly burned?. Should be 22 ohm.

Also check soldering of gate of QM803 for good connection or damaged trace.

Check coil continuity of transformer TM801. I don't know if it would happen here, but I had a small transformer with similar winding design that had an open winding on a different model Samsung 24" monitor. The transformer had one primary that drives 2 secondary windings, and one of the secondary windings was open.

[budm]

Re: Samsung 24" power supply BN44-00195A

BTW, if the inverter board is not connected to the 24V output of the power supply, does the 24V stay at 24V?

[Trofazni]

Re: Samsung 24" power supply BN44-00195A

BTW, if the inverter board is not connected to the 24V output of the power supply, does the 24V stay at 24V?

It does not matter if it's connected or not... PSU should stay on even if inverter board is disconnected...

From the photo of the backside of your power supply board, check the condition of gate resistors RM813 and RM820. They look poorly soldered, and RM813 looks discolored possibly burned?. Should be 22 ohm.

They are ok. Problem is not that obvious, I THINK I checked all of usual suspects..

Also check soldering of gate of QM803 for good connection or damaged trace.

It's ok.

Check coil continuity of transformer TM801.

It's in the picture

[Trofazni]

Re: Samsung 24" power supply BN44-00195A

Ok, so the problem was in one zener diode on error amplifier, it should be 27V, but it had leakage... 9V. Shouldn't pass voltage under 27. On multimeter testing diode has voltage drop within range, but voltage check did not go that well, and it reports fault in psu and shuts down.. Removed it, and put 2 zeners in series, 13V and 12V, and its working, and fault circuit is once again working You have details on pictures.. How i found it, when psu shuts down? I applied external 24V directrly to 24V psu rail, and measured... This is the first time I had problem like this, and I'm proud to say that i solved it

[budm]

Re: Samsung 24" power supply BN44-00195A

Real good job finding the cause of the problem.

[Unspun01]

Re: Samsung 24" power supply BN44-00195A

Well done Trofazni! Thanks for posting your success so others can follow your steps.

[markusb]

Re: Samsung 24" power supply BN44-00195A

Hi,

First, thank you very much for posting the schematic. It helped alot!!


I just repaired one of these boards in my 24" samsung screen.
After I moved to a new house a few months ago the screen just blew up as soon as I reconnected it to mains again.


It also blew a 5AT ceramic fuse,
the 2.2nF cap after the PFC was toast (CM810).
RM801, the 0.22R was open.
Leaking zener, the 15V ZDB805

ICP801 (PFC ic) was dead,
so was the MC33067 (5.1V ref was not there) so I changed both.

The main problem I think was the 47p/1kV cap (CM809) connected to the fault input on the MC33067 ic. After changing everything else the 24V line just got up to about 5V when switching the board on. The MC33067 latched instantly to off state.
Tried bridging the LED side of PC802S, but no difference, only 5V on the 24V line and then latching off, so thats when I suspected something else on the fault input pin.
Changing CM809 got it running again.

Hope this can be of assistance to someone else having problem with this.

Also, my board is the BN44_00195a but component numbering does not really match the schematic, and some values are different, like the cap CM808 is 12nF/630V on my board.

[markusb]

Re: Samsung 24" power supply BN44-00195A

Update: The MC33067 blew again. After reconnecting to mains without series load it just died.

Any idea of why? With a series load of about 200W incandescent light and it was working fine and without it it died almost instantly.
PFC is still ok if I remove the dead MC33067 thats clamping the M_Vcc supply to ground (only 13 ohms between vcc and gnd pin).

To bad.. just got the IC from ebay France all the way to Sweden..