Samsung 2493HM - black screen, blue LED steady on; intermittent

[SolderTrouble]

Four days ago my 2493HM didn't wake up when I woke up my computer in the morning. Its screen remained black, and its blue LED was lit (not blinking). Turning it off & on several times didn't help (so I've temporarily swapped in an old 19" monitor). Before it failed I saw no signs of trouble. (Not counting 5 years ago, when it kept failing unless I kept it from sleeping, and while failing made a sound like electrical arcing which sounded like it was somewhere behind the LED. Samsung fixed it for free since it was still under warranty, but they didn't tell me what the problem was; perhaps a bad solder joint would explain the sound of arcing.)

I partially disassembled it to visually inspect the capacitors on the power board and inverter board. The caps look okay: none swollen and no signs of leaked electrolyte. The two fuses on the power supply board seem okay: near zero resistance (measured out of circuit). I reassembled it (except for the back cover and stand) in order to verify it still isn't working, and noticed the backlights sometimes lit up. So it's an intermittent problem. A few times when switching it on, I heard a momentary 'tick' sound, which sounded like it came from the vicinity of the inverter board or power supply or perhaps from one of the backlights. Due to the intermittency and the sound, I'm inclined to suspect there's a bad solder joint somewhere. In another thread about a 2493HM with the same "black screen, blue LED solidly lit" symptoms, someone opined that the problem is probably on the inverter board... so maybe a bad solder joint on the inverter board?

While the backlights weren't working, I measured the voltage at the inverter board pins labeled 24V at the connector from the power board (with the black probe on pins labeled GND). It measured about 0.35V, assuming I didn't make a mistake.

I'm attaching 3 closeup photos of the inverter board (one that shows the upper 2/3, one that shows the lower 2/3, and one that shows the two yellow electrolytic caps). Also attached is a photo of the power supply board; its caps look fine, but it has a lot of the notorious yellow glue. Also attached is a photo of my multimeter for a bonus point. (I have another multimeter; can I earn a second bonus point?)

I'm unsure how to visually identify a bad solder joint; Youtube tells me to look for a circle around the base of the lead, if I'm interpreting Youtube correctly. Meanwhile, if anyone can identify bad solder joints in the photos, I'll be grateful.

Covering about a quarter of the bottom of the power supply board is a black rectangular insulator, perhaps made of cardboard. It's held in place by two small white plastic "pins." I'd like to be able to remove the two pins so I can inspect underneath the insulator, but I haven't been able to manage it; I used a needlenose pliers to squeeze the two tabs on the sides of the pin while trying to pull out the pin, but no joy. Is there a trick to it, or should I just keep trying?

I found Samsung's Service Manual for the 2493HM, in case anyone is interested. It can be freely downloaded at:
http://elektrotanya.com/samsung_2493.../download.html
It contains a troubleshooting flowchart, wiring diagrams, schematics, and other potentially useful information.

Thanks in advance to anyone who tries to help!

[budm]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

First, you need to remove ALL those brown glues form the power supply board and inspect all those leads of the component that made contact with those glues to make sure they are corroded by the bad glue.
You are not getting the 24V to the inverter board (and the power supply board is getting the PSON to turn on the PFC Voltage booster (this PFC has to work for the 12V/24V power supply to work) and the switched 12v/24v) because the switched 12v/24V power supply is not working.
The blue rectangular cap is also a common failure part.
This is what bad glue can do to the board:
https://www.badcaps.net/forum/showthread.php?t=34692
Power supply board is the one you should concentrate on at this point first.
BTW, that service manual does not have the diagram for the power supply board BN44-00195, only the diagram of the main board.

[SolderTrouble]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

Thanks, budm, for the speedy reply!

By "brown" glue do you mean the yellow glue? Does the glue change color from yellow to brown as it ages, or are there different colors that both age poorly?

EDIT: The thread about bad glue that budm linked says the glue darkens and becomes conductive with age. But the glue on mine is still yellow. Does this mean it's not yet conductive?

[budm]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

They turn from yellow to brown and dark brown and become corrosive (it will eat away the component leads and traces) and become conductive.

[SolderTrouble]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

Since it's still yellow, is it unlikely to be the cause of problem?

[budm]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

Bottom line, remove the glue!
You see what the color of the glue on the two blue caps on the left look like, the rest of the glues are already turning brown, and if you scrape off the glue to look at the inner of the glue, you will see darker inside, or look at around the edge of the glues, you can see darker brown already.

[SolderTrouble]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

I've attached 9 photos of the power supply board, which I describe at the end of this message.

I measured the resistance of the yellow glue in several places, including the spot that seemed darkest (tan), by placing the probe tips about 1/10" apart. It was "infinite" each time. It was also infinite in the three spots where I scraped off the glue's surface. Of course, it might conduct at a higher voltage than the multimeter puts out, but hopefully the fact that it has very high resistance at low voltage can help narrow the "bad glue" suspects to the small number of components that see high voltage. The intermittent nature of the problem may help narrow the suspects further.

I'm reluctant to simply remove all the yellow glue because there appears to be a real risk that the process could damage the board or components. After all, I've never done it before. Also, it's likely to be very time-consuming.

To make it a little more convenient to measure some of the operating voltages, I disconnected some of the cables and left the Power Supply board, Main board and Inverter board cabled together. (In other words, I unplugged the LVDS cable from the Main board to the top of the LCD panel, I unplugged the 6 cables from the Inverter to the backlight lamps, and I unplugged the two cables from the Main board to the chassis (speakers etc).) It still showed intermittent behavior--sometimes the Inverter board received 24V. It still sometimes made a 'tick' sound that may have been the pop of an electrical arc. It sounded like that sound originated on or near the power supply, more specifically from or near the lower-right quadrant of photo 5 (the upper-right of photo 1), but sounds can be deceptive. I can also smell a stronger "burn" odor in that area of the Power Supply than elsewhere on the Power Supply or Inverter, but I don't see any visual signs of overheating, and I don't know if it's abnormal to have a stronger odor there. The sound scared me from measuring additional voltages.

I'd really appreciate it if someone would consider the intermittent nature of the problem and the glue's still-high resistance, use that info to narrow the list of "bad glue" suspects, and comment on whether a bad solder joint is a more likely culprit in this case since the problem is intermittent. (I've read that bad solder joints are often the problem in monitors.)

Regarding the Samsung Service Manual that I linked in a previous post... yes, it's a shame that its Wiring Diagrams chapter lacks wiring diagrams; it only shows connectors' pin-outs. The manual focuses on troubleshooting the Main board. I also noticed it labels the Power Supply board's connectors slightly differently than what I've actually got. For example, pin 15 of the 16-pin connector to the Main board is labeled S_B on my board and is labeled +5_MICOM on page 52 of the manual; I assume it's the +5V Standby Power output. Also, the connectors themselves have different numbers on the board and in the manual; for example connector CN852 on my power board appears to be CNM801 in the manual.

Descriptions of the photos: The first 4 are views of the top side; the last 5 show the bottom (where bad solder joints might be seen). The first 2 photos are from slightly different angles, both nearly directly overhead, taken without camera flash to avoid reflections. The 3rd used flash and was shot from a much shallower angle, to give a better view of some of the yellow glue. The 4th is a close-up that shows where I made a couple of scrapings of the glue surface to see whether it's lighter or darker beneath the surface; it's a paler yellow color (contrary to budm's prediction; perhaps he meant deeper where the glue contacts a hot component; I didn't scrape that deep). The 5th is an overall view of the bottom. Photos 6-9 are closeups that each show a little more than a quarter of the bottom; for example photo 6, which is labeled upper-right, shows the quadrant that's in the upper-right corner of photo 5. (I managed to remove the black rectangular insulator shown in photo 5, before shooting 6-9.)

[SolderTrouble]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

I think I've made progress. I video-recorded the bottom of the power supply under dim lighting and recorded the arcing, which showed me exactly where it was arcing. A small transformer is on the top side there. Some of the yellow glue touches the transformer, a neighboring capacitor, and something I'll call an uninsulated jumper wire. Attached are two frames of the video that show the moment the arc begins and the moment of brightest arc. Also attached are three photos: two of the bottom of the power supply with the arc location circled in red (the second of the two is a closeup) and one of the top side after I used an exacto knife to scrape a few "fire breaks" in the yellow glue: along the base of the transformer, between the cap and the jumper wire, and some of the glue under the cap. (You can also see a lot of scratches that the exacto knife left in the board.)

After I scraped off that glue, I applied AC power and toggled the on/off switch about a dozen times. No arc. Maybe it's fixed.

The video of the arc was shot while the Power Supply and Main boards weren't connected to anything else except the AC power cord. The test after scraping off the glue around the transformer was made under the same conditions.

It's tempting to reassemble the monitor, at least partially, to see whether it works now. Before I do that, would anyone care to comment on whether the scraped "fire breaks" in the 5th photo appear sufficient to solve the problem? Thanks in advance.

I didn't see any burn marks anywhere. Is that surprising?

[budm]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

"I measured the resistance of the yellow glue in several places, including the spot that seemed darkest (tan), by placing the probe tips about 1/10" apart. It was "infinite" each time." Your Ohm meter does not put out high enough Voltage to cause the Voltage breakdown on the glue, you saw the arcing already on that Gate drive transformer legs, those glues on the board are in the high Voltage section, around 400V and higher. Any carbon deposit developed around the arcing area also needs to be removed.
That is why I indicated ALL the glues must be removed or you can wait for more damages to the board.
Use Alcohol to soften the glues.

[SolderTrouble]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

@budm: "Use Alcohol to soften the glues."
I tried putting 91% isopropyl alcohol on a patch of the glue and didn't notice any softening, neither quickly nor after a few minutes of soaking. Can you provide more detailed information about how to remove the glue? If it's going to be very time-consuming or risks damage to the board, leaving most of the glue in place might be the wisest course of action, especially if it's likely to be years before the glue degradation causes another problem. Do you have any information about how rapidly the remaining glue will break down? Also, could the alcohol cause the glue to become conductive sooner than it otherwise would?

[budm]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

Well, as you already see that the arc already occurred on the so called glue that did not look bad.
I just soak and soak the glues with Alcohol and use Exacto knife, Diagonal cutter, Dental picks, it is time consuming.
At this point I already told you to remove all the glues but if you do not want to it is up to you to do the research yourself and take your own chance on the matter.
I have no more suggestion for you.

[SolderTrouble]

Re: Samsung 2493HM - black screen, blue LED steady on; intermittent

@budm, Thanks for all the time you contributed. Now that my monitor's immediate problem appears to be fixed, I have a dilemma about whether to work on more of the glue or hope it will last years before the next glue-related problem arises.

I did a little more research and found the following forum thread:
http://www.electronicspoint.com/thre...-boards.61364/
The last message there says about the glue: "You can't get it off until it starts discoloring. I've tried."

Maybe a moto-tool (like a small drill with a small grinding bit) will work. Amazon sells a Dremel for about $24. If I try something like that, I'll try to remember to report the result.