Samsung Syncmaster 740BF repair

[soopacrucial]

Hi folks,

I'm new here and was wondering whether you might be kind enough to give me some advice on troubleshooting the following problem:

Quite a few years ago now, a couple of years from new, my Samsung Syncmaster 740BF LCD monitor starting playing up by making a buzzing noise and then the display would go blank. I seem to remember the blue power led would also go off when this happened but that I'm not 100% sure of. By pressing the power button I do remember I could at first get it working again but eventually the problem would come back. At the time I had read about caps going bad and had planned to try and have a lookd at them but I never got around to it...that is until now!

When I press the power button today, however, the problem has become worse: the blue led on the button does come on and stays on, but there is nothing at all displayed on the screen, i.e. no samsung logo, and the menu button also display nothing. When I power it off, I do detect some sort of backlight going off, but barely. On the computer side, the monitor *is* recognised by windows, i.e. it's reported as a samsung syncmaster with the correct resolution!

I'm not trained in electronics...I'm a computer guy with a soldering iron and a multimeter and I like to learn something new, so I'm now having a go at fixing this.

After opening it up to have a look, none of the larger caps on what I'm calling the power board in the attached photos looked bulged nor leaky but I read they might still be problematic so I ordered some replacements and swapped them out but it hasn't changed the behaviour. I have found what I think are two fuses, red and circular on the power board, and I've tested their continuity with the multimeter on Ohms and they seem fine, i.e. not open/blown.

Any idea what I could do next please? I presume I should test the 5V and/or 13V power at various places on the power board to maybe check what is arriving to the flat panel but not sure how to proceed, thanks.

PS: there is also a long slim board on the flat panel itself, with all surface mount components but no larger caps so I haven't included a photo of it.

[momaka]

Hi soopacrucial and welcome to BCN forums!

Looks like you already did some key troubleshooting already, particularly with the recap on the power supply board, so that's a good start. Only thing I would advise for next time is to use better quality capacitors, seeing that you have use ChongX and probably other no-name brands. That said, I doubt this would be the cause of the issue you were seeing originally. So let's continue with more troubleshooting.

For starters, let's determine if the backlight is working. You mentioned you could kind of see it turn On but aren't 100% sure.
Probably one of the easier ways to confirm if the backlight is working is to disconnect the ribbon cable between the video/logic board (the board with the VGA and DVI connectors) and theT-con board (the long board attached to the LCD TFT matrix.) Then power up the monitor and see if you get a white or white-ish (or light-gray) screen. Also see if the monitor responds properly to the power button commands in this state - it should. If it does, then I can think of two possibilities in regards to the original problems you were having:

1) The issue might be that one of the boosted voltage rails on the T-con board is acting up or has become shorted, causing issues to other power rails on the monitor... hence the erratic behavior observed initially. When a boosted rail on the T-con has issues, this is typically due to either a ceramic cap shorting out or the TFT matrix developing internal issues. The former is usually the case, though... and when a ceramic cap on the T-con board does fail, it will sometimes take out the fuse on the T-con board. On most T-con boards, the fuse is typically a small SMD component located close to the ribbon cable connector, labeled with an "F" pre-fix. Check if this fuse is still good on your T-con board.

Another possibility is that perhaps some of the (SMD) electrolytic caps on the video/logic board have gone bad. Since these are SMD type (and can be difficult to remove with a plain ol' soldering iron), you can just try to solder some electrolytic capacitors in parallel to these, directly on their leads. Something like 16 or 25 or 50V 100 uF caps should do for a test. Just observe the proper polarity, as with the other electrolytic caps. Then re-connect everything and see if any of the symptoms change.

A third possibility (though very unlikely) is for something in the firmware to have become corrupt... but I doubt that would be the case here.

[soopacrucial]

Hi, momaka,

Thanks, that is awesome...exactly the sort of patient help I was hoping for! Noted about the cheap caps...well-spotted.

Ok, I'm afraid the power up test with the logic board to T-con board ribbon cable disconnected gives no white-ish or light-gray screen at all so it looks like there's no backlight. The blue power button comes on and off when pressed and when it's on I can hear a faint whining/whistling which I believe is coming from the power board.

We're probably on another track now but while I was at it I found and tested the fuse on the T-con board and it's ok.

Could I do some further testing on the power board...maybe the transformer(s)? And/or maybe trying the soldering of 100 uF capacitors in parallel to the SMD caps still applies here? (and yes, the existing ones are indeed 100 uF)

[momaka]

Ok, I'm afraid the power up test with the logic board to T-con board ribbon cable disconnected gives no white-ish or light-gray screen at all so it looks like there's no backlight. The blue power button comes on and off when pressed and when it's on I can hear a faint whining/whistling which I believe is coming from the power board.

OK, sounds like the backlight indeed isn't working then.

In that case, check the small brown cylindrical fuse on the power supply board near one of the inverter transformers. It has the text "T3,15A250V" written on it. If it's blown, then the inverter for the backlights won't work. If it's not blown, then it's possible the inverter is detecting an issue with one of the backlight tubes or the associated transformer driving them and shutting down. The transformers can be checked fairly easily - at least their secondary sides (the pins connected to the blue caps that then connect to the backlights / bulbs. Use your multimeter set to 2 KOhm or 20 KOhm scale (no need to do anything if it's auto-range) and measure the resistance of the inverter transformers. Post what results you get here. Both transformers should show fairly equal resistances, matching to a few %.
See these guides in regards to troubleshooting inverter / backlight issues for further things to test if the above tests don't reveal anything:
https://www.badcaps.net/forum/troubl...conds-to-black
https://www.badcaps.net/forum/troubl...-using-old-cfl

We're probably on another track now but while I was at it I found and tested the fuse on the T-con board and it's ok.

Still good to know.

Could I do some further testing on the power board...maybe the transformer(s)? And/or maybe trying the soldering of 100 uF capacitors in parallel to the SMD caps still applies here? (and yes, the existing ones are indeed 100 uF)

Surely. Won't hurt anything.

Just and FYI, though... you won't be able to test the main transformer with a plain multimeter. Most of its windings will appear as a very low resistance (under 3-5 Ohms) or completely shorted, because it's a (relatively) high-frequency transformer with very few turns. But then again, there's no need to test the main transformer, because the power supply appears to be working fine - otherwise you would not have gotten the power LED to work on the monitor at all.

[harp]

Momaka... "Then power up the monitor and see if you get a white or white-ish (or light-gray) screen."................... can you be more specific about testing, what missing signals cause white screen?

[momaka]

Momaka... "Then power up the monitor and see if you get a white or white-ish (or light-gray) screen."................... can you be more specific about testing, what missing signals cause white screen?

There are 2 very important power rails on the T-con board used for driving the transistors in the TFT screen to change the opacity: V_GH and V_GL. When you disconnect the T-con board, these two rails become "missing". As such, most LCD screens assume a state where the TFT matrix is left mostly transparent... hence getting a white-gray-ish screen when the backlight turns on. This allows more easily to see if the backlights are turning On or not or whether a CCFL has any issues (pink or orange-tinted light coming from the affected CCFL.)

[harp]

Thank you momaka, actualy I try to learn more about white screen issue, and have white screen mobile to experiment... https://www.badcaps.net/forum/troubl...screen-repairs ?.. I know that isnt the same like in tv, but maybe some good will outcome...