Samsung 305T Plus black screen

**budm** · 03-30-2013, 10:56 PM

Re: Samsung 305T Plus black screen

Did you open up the unit to inspect components yet? Pictures?

**andrew1** · 03-30-2013, 11:36 PM

Re: Samsung 305T Plus black screen

Hi budm and thank you for your quick answer. At first glance caps look ok. I will post pictures tomorrow. Could be the tcon board itself? or this is not the usual symptom. Someone did a reflow earlier for altera chip...that's the only thing I saw until now.

**Lumberjack777** · 03-31-2013, 03:37 AM

Re: Samsung 305T Plus black screen

Andrew. What you can do is get the monitor into a dark room. Turn on the monitor, and shine a flashlight on the screen from different angles. See if you can see a faint video image. If so, that means the video is ok; and we can focus on the inverter section and the backlights. That would help narrow it down a lot. And Budm and the others could get you fixed up a little faster. And pictures too like he asked. Thx

**andrew1** · 03-31-2013, 05:26 AM

Re: Samsung 305T Plus black screen

Originally posted by Lumberjack777

Andrew. What you can do is get the monitor into a dark room. Turn on the monitor, and shine a flashlight on the screen from different angles. See if you can see a faint video image. If so, that means the video is ok; and we can focus on the inverter section and the backlights. That would help narrow it down a lot. And Budm and the others could get you fixed up a little faster. And pictures too like he asked. Thx

Hi Lumberjack777. Something does happen I'm not sure what...the flashlight spot seams to increases a little when I hear the hi pitch noise and then at the low pitch noise is lowered somehow. I don't see any colors though. I don't have a DVI cable to test it on a PC...but I ordered one this week (tomorrow will arrive).

**Lumberjack777** · 03-31-2013, 08:41 AM

Re: Samsung 305T Plus black screen

Ok Andrew. I don't think I explained it well the first time. My apologies. The way an lcd monitor works is that you have the video going on inside the thin lcd screen. There are thin flourescent lights behind it (ccfl's). They light the video up from the back side. So when you say you have a power led on. And a black screen. I'm thinking maybe the video is there. But the lights behind it are not shining through. By shining the flashlight on the screen you are looking for faint images of the video that is there; but not illuminated by the backlights. Sometimes you can barely see it. That's why you need to look for that video image with a flashlight. Try shining it from different angles. Then you'll need to open it up like Budm mentioned and get some pics.

**andrew1** · 03-31-2013, 10:39 AM

Re: Samsung 305T Plus black screen

Originally posted by Lumberjack777

Ok Andrew. I don't think I explained it well the first time. My apologies. The way an lcd monitor works is that you have the video going on inside the thin lcd screen. There are thin flourescent lights behind it (ccfl's). They light the video up from the back side. So when you say you have a power led on. And a black screen. I'm thinking maybe the video is there. But the lights behind it are not shining through. By shining the flashlight on the screen you are looking for faint images of the video that is there; but not illuminated by the backlights. Sometimes you can barely see it. That's why you need to look for that video image with a flashlight. Try shining it from different angles. Then you'll need to open it up like Budm mentioned and get some pics.

I understand what you said. I tried again with a brighter light source. The entire screen seams to have two stages. At least that's what I can see. Gray and black again. Without a light source I can't see anything...so obvious if the backlit does nothing...screen pixels are responsible for this. I will post the pictures tomorrow. Thanks Lumberjack777, you really helped me alot today

**selldoor** · 03-31-2013, 10:56 AM

Re: Samsung 305T Plus black screen

Without a source connected ,as I believe you said you dont have a connector then it may be there is nothing to see - maybe a no signal message might flash on but hard to spot. Give it up until you get a cable

**andrew1** · 04-01-2013, 06:53 AM

Re: Samsung 305T Plus black screen

Originally posted by selldoor

Without a source connected ,as I believe you said you dont have a connector then it may be there is nothing to see - maybe a no signal message might flash on but hard to spot. Give it up until you get a cable

Ok. I got the DVI cable and did another test with a PC attached. No backlight confimed. I can see the desktop now. I have a soldering gun and a cheap multimeter so let's do it.

Attached Files

**selldoor** · 04-01-2013, 07:12 AM

Re: Samsung 305T Plus black screen

ok start easy - on the inverter there are at least 2 fuses small white
Power OFF Meter on ohms 200 one probe to each side of fuse.

In Pic 1 - Power supply in the middle near the legs for the BIG cap there is a green fuse.
Same test as above.

Main board pictures are not good enough too dark and out of focus

**andrew1** · 04-01-2013, 08:20 AM

Re: Samsung 305T Plus black screen

Originally posted by selldoor

ok start easy - on the inverter there are at least 2 fuses small white
Power OFF Meter on ohms 200 one probe to each side of fuse.

In Pic 1 - Power supply in the middle near the legs for the BIG cap there is a green fuse.
Same test as above.

Main board pictures are not good enough too dark and out of focus

Hi selldoor.
powersupply - The green fuse is good.
inverter - I found two white rectangular devices with silver endings (there is a text on the motherboard pointing to them [5A/125V]. I don't get any readings. (tried both diode and 200ohms methods).

New motherboard pictures:

Attached Files

**selldoor** · 04-01-2013, 09:25 AM

Re: Samsung 305T Plus black screen

Yes those are the fuses.
There is no such thing as "no readings" unless your meter has died and you have a blank
screen on it.
We would expect your meter to read 0.00 or close (is that what the green fuse read)
OR it will display a 1 or OL on the left hand side of the meter?

On the main board near the B sticker there is a Green rectangle with X20 on it.
That is a re-settable fuse- measure the same as before meter on ohms 200.

**andrew1** · 04-01-2013, 10:04 AM

Re: Samsung 305T Plus black screen

Originally posted by selldoor

Yes those are the fuses.
There is no such thing as "no readings" unless your meter has died and you have a blank
screen on it.
We would expect your meter to read 0.00 or close (is that what the green fuse read)
OR it will display a 1 or OL on the left hand side of the meter?

On the main board near the B sticker there is a Green rectangle with X20 on it.
That is a re-settable fuse- measure the same as before meter on ohms 200.

Sorry...I mean "1" - so those fuses are bad.

X20 - good.

Thank you for helping me selldoor

.

**selldoor** · 04-01-2013, 10:43 AM

Re: Samsung 305T Plus black screen

Originally posted by andrew1

I mean "1" - so those fuses are bad.

Yes.

They are not that easy for many people to get hold of and postage makes them
quite expensive.
If you are very careful (for test purposes only)you can get away with 5A Car fuses, Well Insulated and wired up. You can solder them
to the existing fuses, saves wear on the board.
When you get the proper fuses they can also be piggybacked on top of the old fuses, again it seems easier to solder and saves making an error on the board.

**andrew1** · 04-01-2013, 11:22 AM

Re: Samsung 305T Plus black screen

Originally posted by selldoor

Yes.

They are not that easy for many people to get hold of and postage makes them
quite expensive.
If you are very careful (for test purposes only)you can get away with 5A Car fuses, Well Insulated and wired up. You can solder them
to the existing fuses, saves wear on the board.
When you get the proper fuses they can also be piggybacked on top of the old fuses, again it seems easier to solder and saves making an error on the board.

5A is the nominal fuse current? or the breaking capacity. I found something with 5A In / 8A breaking capacity.

Waiting the next step. We have to find the reason why these fuses had to stop the current. There are two of them so I think is not a coincidence.

Service Manual attached.

Attached Files

ServiceManual_305TPlus.pdf (3.83 MB, 41 views)

**selldoor** · 04-01-2013, 11:52 AM

Re: Samsung 305T Plus black screen

Originally posted by andrew1

Waiting the next step. We have to find the reason why these fuses had to stop the current. There are two of them so I think is not a coincidence.

Service Manual attached.

You are correct but I suspect it will be very difficult to trace.
What I suggest is you set your meter on ohms 2000 and test from each side of each fuse to ground. (if it goes to "1" move up the range)

These fuses are often rated quite close to the amps on the board so they do blow quite frequently. I am wondering if they have been put under extra stress
as the caps are Samyoung so may well be shot ( are only reckoned to be mediocre at best. I cant tell from the picture if they are bulged at all - inspect them .Perhaps replace them whatever.

**andrew1** · 04-01-2013, 01:04 PM

Re: Samsung 305T Plus black screen

Originally posted by selldoor

You are correct but I suspect it will be very difficult to trace.
What I suggest is you set your meter on ohms 2000 and test from each side of each fuse to ground. (if it goes to "1" move up the range)

These fuses are often rated quite close to the amps on the board so they do blow quite frequently. I am wondering if they have been put under extra stress
as the caps are Samyoung so may well be shot ( are only reckoned to be mediocre at best. I cant tell from the picture if they are bulged at all - inspect them .Perhaps replace them whatever.

ground connection test.
Both fuses have one side connected to the ground. The other end is "1" with any scale.

I can go to a professional service if you think that we can't trace the actual problem like this. Or you can let someone else continue what you started if you got tired...no problem. I really enjoy working at this project. Would be nice to do it myself if can be done. I have a diploma in electronics but I never repaired something

all I can do is try to follow directions.

**budm** · 04-01-2013, 01:52 PM

Re: Samsung 305T Plus black screen

I can see at least 10 MOSFETS that drive the inverter transformers, they should be check for Source to Drain short.
We need good close up pictures of the inverter board.

**Lumberjack777** · 04-02-2013, 01:10 AM

Re: Samsung 305T Plus black screen

So the white fuse on the left side of the inverter board is out. And the white fuse on the right side is out too? In my experience bad caps or a bad mosfet will only take out one fuse. The side that has the bad cap or mosfet. I wonder what would take out the fuses on both sides?
Andrew, can you test them both again and make sure they are both out. Thx.
And test those mosfets and get some good pictures like budm asked. As soon as we can determine what caused the fuse(s) to blow. We should be able to get you fixed up. I love fixing electronics and bringing these dead monitors back to life. Hope you do too

**andrew1** · 04-02-2013, 04:37 AM

Re: Samsung 305T Plus black screen

Lumberjack777. Both are out. I tested them a few times. I even tried to scratch the surface in case someone applied some transparent resin there. This monitor was good a few weeks ago not sure what happened. If we don't have any luck I will call a repair centre but for now I want to learn some basics about electronics and this is the best forum I found around the internet

One more thing: I don't know how to test those mosfets. If I recall was something like: pin 1-2 ,2-8, 4-5 can you explain the whole task and multimeter settings?

Pictures attached.
Correct order from left to right (first four pictures, in order)
jpg names: <--- leftside, second, third, rightside --->

You can see better the numbers on the mosfets in the other pictures. closerview1, closerview2...and so on.

Attached Files