Samsung 2493HM power board NO 24v to inverter

[momaka]

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

See the bottom part of post #5 above again.

Basically, you need to test the main power supply. See if there are voltages on the pins "+5.3V" on connector CN851 and "+24V" on the other connector above it.

To test the power board, you will either need to have it connected to the rest of the monitor OR pull pin "PS_ON" high on connector CN751 (i.e. connect PS_ON to 5V standby). Sometimes, it is beneficial to do both, to see if you get different results.

If we know these voltages above come up, then we will know where to go from there. The PFC booster may refuse to run if the power supply detects a short-circuit or some other abnormal condition on the output. Therefore, I always start troubleshooting from the output side and then move towards the input (high voltage) side. It's also safer this way.

[FoxD]

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

Thanks momaka, that looks like a good approach.

Unfortunately I found TM801 to be open on the side where the glue is. I desoldered to look at it from the bottom. Bad news: the wire from the middle pin is covered in black burned glue and no more sign of the wire. This might have happened after removing some glue to remove the cap it was glued to. Or this transformer has been overloaded due to the wrong value cap. Can some explain what the role of this transformer is and if it is related to the standby voltage of the 22uF cap? If not, it's probably a coincidence this transformer toasted after recapping and touching the glue.

Bottom line, I don't think this transformer is repairable. I will try to clean it with alcohol to see what is left of the wire.

And I would think this PSU cannot work without this transformer.

And a replacement transformer is hard to get I read somewhere else since it most likely is custom made for this board.

Any suggestions? I really appreciate all the help here!

[CapLeaker]

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

Well, the wire could have come off for different reasons. The black may be just from arcing, as the wire didn't make proper contact. Depending on what winding it is and how much slack of wire there still is, you may be able to solder it back on, or undo one turn of the winding, then solder it back on to the leg.

[FoxD]

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

Thanks capleaker... You just gave me a handful of precision-work!

What about the chance repairing the transformer is the fix? I could try install the transformer from the working PSU, but if the fault is somewhere else I could end up with two dead transformers..

[FoxD]

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

Just cleaned the transformer. The wire is gone, like evaporated. Trying to open the transformer to unwind one round seems like a destructive action. It's glued together and I will break the magnet when opening. I guess I run out of luck and need to find a replacement board or monitor.

Unless there is a way to replace this transformer with a different one that has similar specs. I can determine the winding count "relations" per leads in the working PSU.

Any last ideas how to overcome this problem are very welcome.

[stj]

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

did you search the part-number on the transformer?

[FoxD]

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

I searched for the part-number but they are not sold anywhere. I just find troubleshooting forums on this monitor and pictures.

I thought I could take the risk and I installed the good transformer (which I bottom-cleaned thoroughly while it was out!) in the faulty PSU and everything works as it should now.

Conclusion: Glue killed the TM801 transformer and without the TM801 no 5.3VDC and 24VDC.

Once more it is clear that cleaning the glue off is worth it, preventing damage which is very difficult to repair like this transformer.

I find it hard to understand why the glue is there anyways? Especially since it is spilled generously over metal parts carrying high voltage and high frequency signals.

My question to the people in this forum with a dead 2493HM (or 245B) but operational TM801 transformer, I'm am very glad taking it over from you! Just the TM801... I will reward you generously!

Or anyone with a bit of understanding on these kind of transformers to help me figure out what kind of make and type could be a replacement.

[FoxD]

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

Before I put the working monitor back together I measured the VAC on the transformer with my DMM. I suspect the frequency of the signals to be higher than normal AC since my readings are drifting.

If I'm correct the primary winding is connected to the further separated legs. This measures 6VAC (readings from 5.7 to 6.0). The two secondary bindings are then connected to the closer separated legs. These measure 13.6-14.0 VAC and 6.8-7.0VAC.

Readings when monitor and display is on.

Any help on finding a transformer that can replicate this in this circuit are very welcome.

[stj]

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

what is printed on the actual transformer?

[FoxD]

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

QGAH00064 / DW07515

I will post a picture later.

[FoxD]

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

See the attached picture for the working TM801 transformer (taken from a different fully operational 2493HM PSU board), next to the heatsink, installed in the "faulty" PSU board being subject in this thread.

The board is now working properly. Hope to find another transformer or alike to get the known working other board back to life so I have both monitors running again.

So please! If you have one in a dead PSU board or just broke/damaged 2493HM or 245B you can make me a very happy man by offering this small part to me

[momaka]

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

Just cleaned the transformer. The wire is gone, like evaporated. Trying to open the transformer to unwind one round seems like a destructive action. It's glued together and I will break the magnet when opening.

Well, look at it this way: it's already broken. So might as well try and fix it, no?

The way to take these apart would be to first unwind and remove the yellow tape on top (the one with the part number). Next, try to separate the two ferrite core halfs of the transformer. They are usually glued together so you will have to pry them apart. If you break them, don't worry - in that case, just keep all of the pieces from the core (if they are all glued back together later on when the transformer is fixed, its core properties will not change too much). Finally, take out the bobbin/plastic separator with the windings. Make sure to label how each wire is connected (either on a piece of paper or on the transformer) and show us some pictures too. Depending on how badly the wire is burned, you may have to rewind just a section of the transformer or the entire thing. But it is not that big of a deal. You will just need to find similarly-sized wire. A little thicker than the original is okay if you can make it fit. Thinner is okay too, but you may have to wind it in double-strands.

Can some explain what the role of this transformer is and if it is related to the standby voltage of the 22uF cap?

The transformer may or may not have been related to the 22 uF cap (although unlikely, I'm thinking the lower capacitance cap perhaps made the PWM controller oscillate, which somehow burned the transformer out).

Transformer TM801 is basically used to drive the gates of those two MOSFETs attached to the heatsink right below it. And those MOSFETs drive the main transformer, which is what provides the 5.3V and 24V rails. The standby supply has its own transformer and drive circuitry, so it is not affected.

Before I put the working monitor back together I measured the VAC on the transformer with my DMM. I suspect the frequency of the signals to be higher than normal AC since my readings are drifting.

Yes, most likely in the 10's or 100's of KHz. Not use trying to measure that with a multimeter.

It's a better bet to take the transformer apart and get the following info from that:
** number of windings
** which pin is connected to which winding
** number of turns of wire in each winding
** DIRECTION of the wire turns in each winding

With all of that, you could technically make your own transformer if you find a similarly-sized core. Cheap PC ATX PSUs usually have one.

Any last ideas how to overcome this problem are very welcome.

That's all I have above. Unfortunately, you really do need that transformer if you want to get the power supply fixed.

That said, you could get the monitor fixed with a different power supply if you really wanted to save it. It doesn't even have to be the same exact replacement - just needs to have 5V and 24V output with appropriate current ratings.

[budm]

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

TM801 (same as TM802 in the diagram) is the GATE drive transformer for the two power MOSFETs QM802, 803 whcih drive the power transformer TM801S to produce 24V, 13V, 5.3, 12V.
Nothing to do with the wrong cap value in the STBY power supply. You are not the first one with transformer legs being eaten away by the glue.

[FoxD]

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

Thank you very much great people of this forum! You have given me enough confidence to try and repair the transformer. Later this week I should have some time to start this small project. I will document it with pictures and share it here. I hopefully can help others do this type of repair successfully (in case I succeed )

I would end this message with my reasoning behind the "coincidental" short by the glue in this monitor which could prevent some other failures for others when this is taken into account.

This monitor, the transformer, failed after about three hours operation. But the last few minutes of operation were after it has been installed in our office location. During transport from my home to the office it was stored half a day in my car, standing outside in just above freezing conditions (4-5 degrees celsius). After taking it inside the office I let it stand for about 15 minutes to warm up and allow the condensation to evaporate. My guess is that the old glue absorbs condensated water and becomes VERY conductive and it takes a long time to dry. 15 minutes was just (way) to short to allow the glue to dry.

So my recommendation with especially this type of monitor, recapped or not, but with the glue still "spilled" in all these places: leave it for a couple of hours after moving it from cold to warm and don't use it in too moist environments. Only if you have removed all the glue you can safely keep a shorter time to accommodate and safely use it in a bit moist (but not condensating) environments. Nevertheless I don't recommend 15 minutes, just too short . I was not thinking well and was a bit in a rush having to do so much else that day...

[FoxD]

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

Amazing! The transformer repair was a success!

I opened the transformer breaking the magnet in 5 pieces, found the burned wire, extended it with a small thin wire and connected it to the pin, firmly taped it back in place and glued the magnet into one piece using super glue. Finished it with re-taping the magnet. See pictures. Result: a working 2493HM again!

It can be done . I needed about an hour for this repair.

Thanks all for the tips and support.

[CapLeaker]

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

given: It's not an easy task, but it is doable. Good job FoxD!

[budm]

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

Amazing! The transformer repair was a success!

I opened the transformer breaking the magnet in 5 pieces, found the burned wire, extended it with a small thin wire and connected it to the pin, firmly taped it back in place and glued the magnet into one piece using super glue. Finished it with re-taping the magnet. See pictures. Result: a working 2493HM again!

It can be done . I needed about an hour for this repair.

Thanks all for the tips and support.

Glad to hear you are able to get it running.

[momaka]

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

Amazing! The transformer repair was a success!
...
Result: a working 2493HM again!

Nice work! Looks as good as new.

That's why you never give up until you've tried.

[FoxD]

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

Thanks! Agreed, just do it

[doomi]

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

I had almost the same issue just week ago.

The QGAH00064 short-circuited every time I tried to power on the monitor. A small visual light of arc was emitted for some milliseconds from below the component.

I desoldered the component, removed all the glue attached to it and to the board below it with a cutting tool, then soldered the component back to its place, and voila. The monitor seems to work again.
It seems the component or any other component was not too severily damaged which amazes me.