Samsung 2493HM power board NO 24v to inverter

[FoxD]

Hi, this is my first post requesting for some help fixing a Samsung 2493HM power supply.

This is the third monitor I try to fix by replacing the electrolyte caps with decent Panasonic one. All of them failed with a black screen while the blue power led in on. The first two were fixed well and are still in business after about six months now.

The third one is currently on my bench and after I thought to have it fixed by replacing all elco's it proved not to be by showing a black screen again after about three hours operation time.

Troubleshooting learned I had made a mistake. I put in a 2.2 uF 50v cap where I should have put a 22uF cap. This cap is parallel to a zener diode if I look at the diagram found in another thread. On the PCB it is numbered CB804. In the diagram it is CB802. The diagram is slightly different which makes sense as the PCB is numbered different BN44-00195A.

https://cdn.badcaps-static.com/pdfs/...f8a620fd8c.pdf

My knowledge on the power circuitry is not good enough to understand what this mistake may have caused further in the line. But replacing it with the correct value cap did not bring back the backlight. Measuring the 24V output indicates it does not provide it.


I have tested the panel with the power board of one of the working monitor and all works well. The problem is isolated to the power PCB.

Anyone knows what the first and next steps would be to troubleshoot and fix this?

[budm]

Re: Samsung 2493HM power board NO 24v to inverter

That 22uf/50V cap is the startup-running cap for the 5.2V Standby power supply to operate, when you put in 2.2uF the power supply will not run so you will not have 5.2VSTBY.
Nothing will be damaged.
So do you have 5.2VSTBY now with the correct value cap in place?
Can I see the pictures of YOUR board? For you to have 12V/24V circuit working, the power supply board has to receive the PS-ON command signal from the LOGIC/Control board to turn on the PFC Voltage booster circuit to provide close to 400VDC to run the 12V/24V SMPS section. The PFC Controller IC and the 12V/24V SMPS IC get the VCC to run form the AUX winding of the STBY Power transformer. So if STBY power supply section is dead then other power supply sections will not function.
Bottom line, you need to have 5.2VSTBY first.

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Thanks budm for the quick reply.

I have not yet measured the 5.2V standby. Will test that tomorrow when I have some time... Can I test that with the board isolated or do the Controller PCB and inverter PCB to be connected? And where to best test this voltage?

The blue led works BTW. And the buttons seem to work since my laptop responds to the source selection function. But since there is no display visible, also nothing with a bright flashlight I'm not able to confirm this.

I attached two pictures. Please let me know if you need some close-ups.

Thanks again! highly appreciate your help!

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

One additional comment. With the wrong elco installed the display did work for about three hours, including the front controls. After that it went to this state.

[momaka]

Re: Samsung 2493HM power board NO 24v to inverter

I have not yet measured the 5.2V standby. Will test that tomorrow when I have some time... Can I test that with the board isolated or do the Controller PCB and inverter PCB to be connected? And where to best test this voltage?

For the standby voltage - YES, you can test the power board without anything else connected to it.

As for where: find connector CN851. You can see its pins on the lower left side of this picture you posted:
https://www.badcaps.net/forum/attach...7&d=1455401164
I believe that connector is for a wire harness that goes to the logic/video board. On connector CN851, there is a pin labeled "S_E", right next to pin "PS_ON" in the right corner of the connector (I'm using the same picture above as reference).

Let us know what voltage you see on that pin with respect to ground (pins labeled "GND").

Because of this:

The blue led works BTW. And the buttons seem to work since my laptop responds to the source selection function.

... most likely, there is a proper standby voltage on that "S_E" pin. Nonetheless, you should still test it and report back what you get.

My guess would be that there is something wrong with the main power supply - which supplies power on pin "+5.3V" on connector CN851 and pin "+24V" on the connector at the top of that picture above that I was using as a reference.

To test the main supply, however, you will need to have the power board connected to rest of the monitor, have the monitor turned ON, and have a signal going to the monitor to keep it ON. Then check the voltage on those pins, again with respect to ground. Let us know what you get.

[budm]

Re: Samsung 2493HM power board NO 24v to inverter

If you do have PS-ON signal (>2VDC) when power switch is activated, then you need to find out if the PFC Voltage Booster is working or not, you should have close to 400VDC between the two legs of the main filter cap.
BTW, I can still see some of the parts are still covered with glue, especially the little blue disc snubber cap next to QM803, also CY803, etc.
If the PFC is OK but still no 12V/24V then you need to check the resistor (0.22 Ohm) that is standing up next MOSFET QM802 and the capacitance of the blue rectangular cap with '12nJ 630V' printed on top.

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Did some measuring:

Standby voltage is 5.23V so is ok.
Voltage on main filter cap is 310V. Not close to 400V. Is this too low?

The standing resistor is not easy to measure on my (cheap) DMM. It measures a little higher than shorting the leads, about 0.3 Ohms. Is that close enough? You mention 0.22 Ohm as is correct with the code on the resistor.

The blue rectangular cap does have capacitance using my Ohm meter, although it seems polarity sensitive while in the circuit. I have no capacitance meter so no idea how much this is.

I have done some comparing measuring on a working (recapped) PSU from a different 2493HM. All readings are the same:

5.2 volt standby
310V on main filter cap
Similar reading on standing resistor
Similar reading on the blue cap

About the glue, I have been trying to remove most, especially the glue that touches the legs. But some may still touch the legs since I cannot clean further without taking the components out. Measuring across the leads of the glued legs does not indicate a short or something strange however.

The working PSU has still all glue in its place, except the glue that was removed for the recap.

What is next? I am figuring out a good setup for testing the monitor in full operation. Hope to have that ready later today.

Thanks.

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Just tested the power board connected to the other parts and there is only 0.5VDC on the 5.3VDC supply to the video/control board.

[CapLeaker]

Re: Samsung 2493HM power board NO 24v to inverter

310V on the main filter cap means that your PFC isn't working. 220 x 1.414 = 311.08V.
No boosted voltage = no 12V or 24V or both.

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Thanks capleaker. Makes sense. Just compared the faulty board with the working PSU.

The working PSU has 388VDC on the main filter cap when the screen is on. When I switch the screen off using the front panel button the voltage drops to 311VDC. Switching the screen (and blue led) on and off with the faulty PSU installed does not have any effect on the voltage: it stays 311VDC all the time.

Does this mean the PFC is broke or can it also be something else causing the PFC not kick in?

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

After reading back and trying to do some logic reasoning I have a question.

1. With the wrong value capacitor installed (2.2uF instead of 22uF) the monitor had worked for about three hours. So the standby voltage should have been working. Despite it shouldn't work according to what budm said.
2. Could this wrong value cause extra stress on other components like the other new capacitors installed? Or any other component. Although I cannot see any component has died/been damaged, we now can tell for sure the PFC does not work while it worked before, even with a wrong value cap in that area.
3. Is the PFC the heatsinked transistor and diode on the bottom left (see picture)? I try to understand the layout but this seems to be too high of knowledge for me .

Cheers and thanks for the help!

[budm]

Re: Samsung 2493HM power board NO 24v to inverter

'
1. With the wrong value capacitor installed (2.2uF instead of 22uF) the monitor had worked for about three hours. So the standby voltage should have been working. Despite it shouldn't work according to what budm said.' The problem is that since the diagram does not match the board, so are you sure that cap is the filter cap for the SMPS IC.
So the PFC is not working then you need to find out if you do have VCC to run the PFC IC or not.

[CapLeaker]

Re: Samsung 2493HM power board NO 24v to inverter

That TDA4863g is the PFC IC you are looking for.

[budm]

Re: Samsung 2493HM power board NO 24v to inverter

OK, I verified that C804 is connected to the VCC of the SMPS STBY IC and it is 22uf. I have no expalnation as why the STNY 5V was working with capacitance that low, it should have cause the STBY SMPS to be in startup/shutdown loop (we have seen that many times here when the startup/running lost its capacitance) but will not cause any damaged to other circuit.

[budm]

Re: Samsung 2493HM power board NO 24v to inverter

That TDA4863g is the PFC IC you are looking for.

FYI: The diagram is for 00192A which doe snot exactly match which makes it difficult to troubleshoot the 00195A.
The 00195A uses FAN7530 PFC Chip.

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Thanks budm. But this leaves me questioned where to go from now. Could it be a coincidence that the screen blacked while a wrong cap was installed? It's kind of strange the monitor worked based on your experience. But I also understand this does not stress any other component in the rare occasion the

I can't find the PFC ic looking for the FAN7530 , could it be the MC33067P? This ic has sixteen pins and I read this is a crucial part of the supply of 24V.

https://www.badcaps.net/forum/showpo...3&postcount=37

After reading this post I started cleaning the glue as good as possible, especially the small film cap CM809. I'll report back when I cleaned off all glue on legs of components.

If you have any suggestions on how to troubleshoot this board further, I would appreciate hearing them! I refuse to give up on this monitor just yet.

[budm]

Re: Samsung 2493HM power board NO 24v to inverter

https://www.badcaps.net/forum/attach...7&d=1455401164
Bottom right 8-pin SMD is the PFC IC.

[CapLeaker]

Re: Samsung 2493HM power board NO 24v to inverter

FYI: The diagram is for 00192A which doe snot exactly match which makes it difficult to troubleshoot the 00195A.
The 00195A uses FAN7530 PFC Chip.

Well... o.k. different schematic it is!

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Thanks, found the ic! And three steps further:

1. More glue is removed, though not all, but visually all legs are now glueless.
2. Measured the two standing 100KOhm resistors next to the main filter cap which are covered in glue and cannot be cleaned without breaking them. One measures correct at 100KOhm. The other (next to the transformer) is around 71KOhm. Compared this with the working PSU and the values are both 100 KOhm. How big of a problem is this? I could replace these although I need to order them.
3. measured VCC (pin 8) on de PFC chip against pin 6 (ground): 14.8VDC. The working PSU measures the same.

Where does this bring us? I hope you can help me look further...

[FoxD]

Re: Samsung 2493HM power board NO 24v to inverter

Small update: Managed to get the glue off the standing resistor without breaking it. It now measures 100KOhm (instead of 71). So they are now both matching their intended value. No power unfortunately. The glue here was definitely conductive.