Samsung B1940 blue LED but blank screen

[robcar]

Hello everyone,
I got this Samsung B1940 which got the following symptoms.

When I connect to a pc and turn the video on the blue LED turns on firmly, but screen is blank, actually I should say the screen is white. Same problem with VGA and DVI port.
If I wait let's say 5 minutes the video starts blinking, I guess because of standby.
I opened it up and couldn't see any visible problems; I measured all the caps with an ESR meter and the majority are in the 0.0x ohms range.
I measured the voltages on the secondary and they seem fine too.

With power button of the video turned ON:

B-Dim: 3.257V
+13V: 12.71V
GND: 0V
GND: 0V
GND: 0V
+5.1V: 5.12V
+5.1V: 5.11V
N.C.: 0V
O/F: 3.083V

With power button of the video turned OFF:

B-Dim: 3.261V
+13V: 13.27V
GND: 0V
GND: 0V
GND: 0V
+5.1V: 5.12V
+5.1V: 5.11V
N.C.: 0V
O/F: 0V

I looked at the logic board and couldn't visually detect problems there, all SMDs which seem fine.

Could someone please tell me what to check next?
Thank you very much
bye

rob

[budm]

Re: Samsung B1940 blue LED but blank screen

The Voltages and the signals are fine.
Fuse F901 OK? It supplies B+ to run the Inverter circuit so it should have 13VDC at both ends with ref. to GND.

[robcar]

Re: Samsung B1940 blue LED but blank screen

Hi budm,
I managed to measure both ends of F901: 13.31V
So are you telling me that power board seems fine? Maybe I should look elsewhere? in the T-Con board maybe (never troubleshot such a board before)? The main symptom for this lcd is a white screen.

[robcar]

Re: Samsung B1940 blue LED but blank screen

One thing that I didn't mention is that this video has got integrated audio. So one thing that I can check is if sound at least comes out of it, so maybe I can rule out power supply issues. Let me try and I'll post the results.

[robcar]

Re: Samsung B1940 blue LED but blank screen

So, I wasn't able to hear audio, don't know if it was because I was missing something macroscopic...
Anyway I attach other pics, one of the digital board and two of the T-Con.
I was able to do measurements on the T-con contacts, just above the flat cable (I guess LVSD something ).
Here they are:

GND1: 0V
RX00N:1.361V
RX00P:1.130V
RX01N:1.365V
RX01P:1.132V
RX02N:1.337V
RX02P:1.150V
RX0CN:1.259V
RX0CP:1.289V
RX03N:1.361V
RX03P:1.127V
RXE0N:1.363V
RXE0P:1.139V
RXEIN:1.364V
RXEIN:1.136V
RXE2N:1.344V
RXE2P:1.162V
RXECN:1.256V
RXECP:1.289V
RXE3N:1.358V
RXE3P:1.132V
GND2: 0V
SDA_D:3.76V (cool: I2C bus )
SCL_D:3.84V (i2c clock)
WPN_D: 0.44V
5V: 5.10V

Now I have to go BUT it seems like F1 on the T-Con is broken! It is located just above the T-Con connector, on the right. So I'll have to check it further when I come back.

[robcar]

Re: Samsung B1940 blue LED but blank screen

So F1 on the t-con was blown. I managed to order a bunch of 3A SMD 0603 fuses, to desolder the blown one with ChipQuik and to solder one of the new teenie-tiny bastards.
Then I made some measures; continuity was fine across it and also between the lead facing the flat cable of it and one of the legs of IC3 shown in the third pic.
So I connected back everything again: nope, still white background, even if I got brief flashes of black. Disconnected cables, made measures: new fuse blown...

[boyebaran]

Re: Samsung B1940 blue LED but blank screen

ic3 is a 3.3v regulator please check all pin in IC3 to GND.mybe IC3 damaged or IC4 and IC1 vcc shorted to gnd.

[robcar]

Re: Samsung B1940 blue LED but blank screen

Thanks for the tip; IC3 is fine, but CA16 is shorted. That one should be a cap.
Now the question is: how can I tell its value? I guess I cannot...

[boyebaran]

Re: Samsung B1940 blue LED but blank screen

Thanks for the tip; IC3 is fine, but CA16 is shorted. That one should be a cap.
Now the question is: how can I tell its value? I guess I cannot...

first remove CA16 from board and testing .if you sure the cap is shorted
so you can remove the cap without replacing by new one.just remove cap and turn on monitor.be careful about removing CA16 .just remove by Soldering System and dont use hot air.

[Joe Black]

Re: Samsung B1940 blue LED but blank screen

first remove CA16 from board and test.

Indeed, you should try this. If you happen to have a schematics, you could confirm if CA16 is on the same rail as F1, so it could possibly short that rail to ground, blowing F1 as all that current rushes through.

Also, you could confirm if CA16 is actually a cap, (which most likely is), because it could be some other component which normally reads low on DMM.

Can you lower the brightness, or is the BL on max all the time?

[robcar]

Re: Samsung B1940 blue LED but blank screen

Thanks,
so it happens that CA16 wasn't itself shorted. After having removed it the pads underneath are shorted. And I figured out there are many more other caps that appear shorted to ground (sorry folks, I'm such a newbie and need to learn), namely: CA5, CB6, CB7, CB8. Then a pair on the left around the big chip. So it appears clear to me that there's a short somewhere, but where?
@boyebaran: hot air sucks, I have a station that just doesn't seem to work. I found
ChipQuik easy to use, even if bloody expensive.
@JoeBlack: I haven't looked for schematics yet and BL seems on max all the time

[boyebaran]

Re: Samsung B1940 blue LED but blank screen

seems that IC4 have problem.u need remove Ic4 to check.
one way to find short part .use power supply (1.5 V)with high current .connect (+ )to somewhere that short to gnd and( -) to gnd.
so turn on power supply(without turning on monitor) just a moment and see which part gone to hot.please be careful about this way.

[Joe Black]

Re: Samsung B1940 blue LED but blank screen

Are you positive that T-con blows new fuses?

Here's what I want you to understand: there may be short on the rail somewhere, and it blows your fuses and reads low on ohmmeter,
BUT
there may be one component along that rail which normally reads low on the ohmmeter and makes confusion.

You need to clarify how do you measure for short, using continuity on your meter, ohm setting or diode setting. That's why I'm writing this.

If you're positive on short, you can try what boyebaran suggested:

You could take desktop PSU or something similar and inject voltage to shorted rail. Voltage should be close to max voltage that rail is designed to withstand for best results.
Once you injected voltage, you feel with your hand for component which is the hottest, and that is your shorted component. However, for obvious reasons (too high V could kill many of your components), you have to know what are you doing before trying this.

Another, harder way would be to take sensitive ohmmeter and measure along the shorted rail. As you approach actual shorted component, the resistance will be slightly smaller and smaller. Patience required, ofcourse.

The actual reason why I brought BL up is that you haven't made clear if you have white screen as in white pixels or white screen as in screen to bright to see anything because of too strong backlight...

[robcar]

Re: Samsung B1940 blue LED but blank screen

Are you positive that T-con blows new fuses?

I guess so. If I substitute fuse F1with a new one it just opens as soon as I turn the video on.

Here's what I want you to understand: there may be short on the rail somewhere, and it blows your fuses and reads low on ohmmeter,
BUT
there may be one component along that rail which normally reads low on the ohmmeter and makes confusion.

Ok.

You need to clarify how do you measure for short, using continuity on your meter, ohm setting or diode setting. That's why I'm writing this.

I use continuity test.

If you're positive on short, you can try what boyebaran suggested:

He suggested that maybe IC4 is the culprit. It seems an FPGA, so no easy task.

You could take desktop PSU or something similar and inject voltage to shorted rail. Voltage should be close to max voltage that rail is designed to withstand for best results.
Once you injected voltage, you feel with your hand for component which is the hottest, and that is your shorted component. However, for obvious reasons (too high V could kill many of your components), you have to know what are you doing before trying this.

Yeah, I got what you mean. Is it possible that t-con needs only 5V to operate? That's the line coming from the logic board.

Another, harder way would be to take sensitive ohmmeter and measure along the shorted rail. As you approach actual shorted component, the resistance will be slightly smaller and smaller. Patience required, ofcourse.

Got it.

The actual reason why I brought BL up is that you haven't made clear if you have white screen as in white pixels or white screen as in screen to bright to see anything because of too strong backlight...

Backlight functioning really well, but no sign of image below.

So guys, thanks for all the help so far. I'm still uncertain if I want to further troubleshoot this sucker.
I guess that if I really want to fix it I could also look for a spare t-con on ebay.

[budm]

Re: Samsung B1940 blue LED but blank screen

It is not replaceable T-CON because you will need special equipment to do the tab bonding.

[Joe Black]

Re: Samsung B1940 blue LED but blank screen

I use continuity test.

Not good, your multimeter can beep on some low resistances that are not actually short circuits. For you, best way is to use ohm setting. However, if fuses are blowing, I'd call it a short to ground.

I'm still uncertain if I want to further troubleshoot this sucker.

Totally understandable. With the situation nowadays, and the way things are going, you have to have really strong internal drive to tackle electronics repair, meaning that many repairs require more work and effort than someone who is not an expert is willing to provide versus paying someone to do it or just buying new stuff.

At this point, it all depends on how bad do you want it fixed and what's it worth to you.

[robcar]

Re: Samsung B1940 blue LED but blank screen

Not good, your multimeter can beep on some low resistances that are not actually short circuits. For you, best way is to use ohm setting. However, if fuses are blowing, I'd call it a short to ground.

Got it; it's just that is so simple with continuity test not to have to look away from the circuit you're testing and just keep listening if the meter beeps. Maybe that's why people still use analogue meters when looking for shorts: with lateral view it's easier to see the needle going down.

At this point, it all depends on how bad do you want it fixed and what's it worth to you.

Not so badly, to be sincere. It was one my users' video, I gave him a 17" as a replacement in the meantime a batch of new 22" that we had ordered arrives
Hey, nevertheless I really appreciated all the help from people that replied to my OP! I learn a lot from knowledgeable folks that post in this forum!

I'll post updates if I manage to find the shorted component (maybe by using a thermo camera, hmmmm....)