Broken Samsung 2493HM - blinking LED

[LAS]

Re: Broken Samsung 2493HM - blinking LED

First of all, newbie1's long shot: I removed transformer TM801 and indeed there was some glue on the legs. I scraped it clean. Readings on each side had two legs with continuity. Both sides matched.

Euromatlox's suggestions: I reviewed the link and tested the componenents listed. I found nothing out of the ordinary.

And now ICP801S. Before the morning coffee and armed with nerves of steel, I powered up the board and measured the voltage on all eight pins with respect to the ground on the negative side of the big cap. Pins 1 and 8 had 1v, and the rest read 0v. Big cap still reading 162v.

Feeling like it's coming to the end of the line with this board. Prior efforts by other members have met with definitely mixed results, and the bn44-00173a board seems to be a better bet all around.

My concern though is that the main board may have an issue too. Can they be purchased? No easy answers for this monitor unfortunately.

[euromatlox]

Re: Broken Samsung 2493HM - blinking LED

The BZX84C27 zener diode can be broken without any signs, impossible to detect it's condition using multimeter. It's 27v zener, possibly you can use conventional 27v zener diode instead too with careful soldering. That was the last part I replaced (had to wait it for a while), and after that 24v line showed no more zero volts.

[LAS]

Re: Broken Samsung 2493HM - blinking LED

The diode cannot be checked using a diode function on a multimeter? Maybe I can find a replacement on another board. Or I can get one next time I place an order.

[euromatlox]

Re: Broken Samsung 2493HM - blinking LED

Username Toasty has written about this matter earlier. I got my help by reading his suggestions. The broken zener shows like a conventional diode on multimeter. I have bought from eBay a kit of assorted zener diodes of different voltages, to help my future repairs/tests. That seems to be very handy component to have available at all times.

https://www.badcaps.net/forum/showth...t=18966&page=2

http://en.wikipedia.org/wiki/Zener_diode

[retiredcaps]

Re: Broken Samsung 2493HM - blinking LED

Pins 1 and 8 had 1v, and the rest read 0v. Big cap still reading 162v.

According to the datasheet on page 14, the SMPS IC needs at least 12V DC to start. Since you are getting 1V DC, something else feeding VCC (pin 8) is probably bad.

[retiredcaps]

Re: Broken Samsung 2493HM - blinking LED

The BZX84C27 zener diode can be broken without any signs, impossible to detect it's condition using multimeter.

Most diodes that fail that I have seen fail shorted. You can test a zener diode using the diode test function on a multimeter. If it is shorted, it will shows 0.0V DC both ways.

[LAS]

Re: Broken Samsung 2493HM - blinking LED

I'm thinking that my best plan of action at this point it to put it all back together and wait until a Samsung 245bw comes my way with a bn44-00173a board in it. Then I will be able to get a better picture of what's going on with this one. If it's just the power board, I have no issues buying a new board to replace the broken one - I am reluctant though to do this with the possibility of two bad boards.

[euromatlox]

Re: Broken Samsung 2493HM - blinking LED

Check also video board (under metal plate) 240CT01C2LV0.2, I did change that part too. I think it's not very common part to fail. There is a small burn mark in the tiny IC situated under 10uH coil in the broken board.

In the replacement part I did change the (sometimes noisy) coil, it's now 8.2uH better quality coil, slightly bigger and heavier one.

[budm]

Re: Broken Samsung 2493HM - blinking LED

I looked at the SCH I posted, the connections are really close to what you have on your board. Are you still getting the 5.2V standby power supply?
Pin8 of the PFC IC gets the power through linear regulator transistor (TO-220, QB801) sitting next to the 5.2V SMPS IC. You should see about 25VDC between the metal tab (Collector of QB801) and the Negative leg of the main large DC filter cap. The voltage between the Base pin of the transistor and the negative leg of the main filter cap should have about 16VDC.
By the way, what resistance reading between the VCC PIN 8 and the Ground pin6 of that PFC IC?

[LAS]

Re: Broken Samsung 2493HM - blinking LED

Thanks Budm. I have been out of town but hope to get back into this on Saturday or Sunday.

[LAS]

Re: Broken Samsung 2493HM - blinking LED

Hi Budm,

Just a brief update that I have not given up on this monitor yet but in the meantime have been swamped with tax deadlines, trade shows and entertaining my 6 and 8 year old boys who have no plans to give me the peace and quiet required to troubleshoot this. But today I prevailed.

I was able to read 6k ohms between pins 6 and 8 on the ic on the back side of the board. And when powered up with no other boards attached and with the jumper installed on the standby pin and PSON, I get 4.5v on the standby side and 2.8v on the pson side.

0v between the QB801 collector and neg side of the cap;
0v between the base of QB801 and the neg side of the cap.

In the meantime, I was extremely close to acquiring a Samsung 245BW which I believe would have held the key to a quick assessment of this monitor. Someone beat me to it, and in the meantime my search continues. One way or another this monitor isn't going away any time soon.

[budm]

Re: Broken Samsung 2493HM - blinking LED

"0v between the QB801 collector and neg side of the cap;"
OK, that means it is not getting the DC voltage from the Diode and the fusible reisistor.
Please check that resistor 1R0 and the Diode.

[LAS]

Re: Broken Samsung 2493HM - blinking LED

Part of the challenge for me is finding the corresponding parts on the board. Usually it takes me ten minutes or so, but this time I found it quickly. A minor victory and means I'm getting better at this game.

Fusible resistor R01 has 1.2 ohms - as retiredcaps pointed out in a prior post, essentially the reisistance of the probes.

And the corresponding diode DB804 also checks out ok.

Thanks budm for your continued interest in what has become a particularly troublesome power board.

[budm]

Re: Broken Samsung 2493HM - blinking LED

Something is not making sense right now since that transformer is producing the standby 5.2Vdc OK. The AUX wind is for producing the voltage (through the resistor and the Diode I mention before) for feeding that Collector of QB801.
So it is either the Diode is bad or the AUX winding is open, you may want to find out if it has open AUX winding or not, otherwise I am running out of idea even though we now know that you are not getting the DC voltage at that point for the board to work.

[euromatlox]

Re: Broken Samsung 2493HM - blinking LED

About checking zener diodes, here's the correct way to do it:

http://www.youtube.com/watch?v=CtQaGT1qA2M

But it's so cheap component that if 'repair tip' says that it might be broken, much easier just change it. I have replaced a zener diode to this power board, to enable 24v line..see earlier postings. Multimeter diode testing for broken zener showed that the 'normal diode function' was okay, but it did not test zener function at all.

In this case zener is smd component, and it might also easily broke when removing for testing, replacement must be at hand.

Probably the zener is there to prevent overvoltage (at startup), and if it's broken, voltage limit is exceeded and some kind of IC circuit decides to cut the 24v line.

Same time as replacing zener I would check also other components for failures (like Sanken 8008hfe regulator and some other smd components), replace if needed, and change some electrolytic capacitors there (if still original ones).

[budm]

Re: Broken Samsung 2493HM - blinking LED

Can you tell me the designator number of this 27v Zener you are talking about? Where is it at on the board.

[euromatlox]

Re: Broken Samsung 2493HM - blinking LED

I learned this from Toasty's comments. It's perhaps the most helpful writing about this power board.

https://www.badcaps.net/forum/showth...t=18966&page=2

The zener is somewhere under the transformer (other side board)..see the url for details.