Samsung 275T 2 seconds to black

[wetwillyskate]

I pulled my 275T out of the closet to fix it this weekend after i tossed it aside for the infamous "2 seconds to black" issue.

That being said i've been lurking around forums trying to solve this problem on my 'own' but im not getting too far. Apparently I'm the only person that bought a 27" monitor 3+ yrs ago; or at least I'm the only guy with a broken one.

these 2 forums were close, but not quite enough info for me:
https://www.badcaps.net/forum/showthread.php?t=18601
https://www.badcaps.net/forum/showthread.php?t=17325


Issue:
Monitor turns on and displays for approx 1 sec before dimming by approx 50% for another 500ms and then finally the display turns off completely. I when in the off condition i dont believe i can see a 'ghost' LCD image which some others have described seeing. Before it did this it would work for 10-15 mins when the problem 1st started. It got gradually worse until it reached the current state.

I've included pics of the following:
Disassembled monitor for live circuit measurements and troubleshooting
Power board top side
Power board bottom side
Video input board top side
Video input board bottom side
Inverter/CCFL driver board
Small mystery board (connected to the video board with a 30 pin connector an to the Inverter board with a 2 pin connector)
Suspect resistor
Suspect mystery fuse
Typically suspect Capacitors

Troubleshooting:
1. the power board seems to be working ok. All Caps appear in tact and i believe have the proper voltage across terminals (~24V for CM852, 851, 861, and ~18V for CM863). I have visually inspected just about every component on all of the boards and the only thing that looks suspicious is resistor RM802 which measures out to 320K ohm and has visual signs of excessive heat. This is a 5 band res which i believe has blk, yel, grn, blk, gold which would be 45 ohm 5%. It may also be brn, yel, grn, blk, gold which would be 145 ohm 5%. Its tough to decipher between blk and brn.
Either way if im measuring the res accurately given that it is in a circuit, im getting an extremely high impedance value.

2. with help from the thread from retiredcaps (https://www.badcaps.net/forum/showthread.php?t=10419 ) I think the CCFLs are fine. They all appear to turn on before the fault condition and any number of them plugged/unplugged does not change the condition.

3. When measuring 24VDC @ the pins @ CN803 i see 23.7VDC and when the monitor is power cycled and attempts to display an image the voltage drops to 23.55 and then returns to 23.7VDC. Im not sure if that would create an undervoltage condition though.

4. I see ~24VDC @ DM 851, 852 diodes and ~18VDC @ DM853 diode

5. I have ~385VDC at the large Cap (CP803)

6. all fuses are in tact with the exception of F601 on the Video input board top side, that measures out at 156k ohm but im not even sure if this is a fuse or not.

As you can see i've basically started trying to find where power does/does not go which will hopefully lead me to my problem. Im leaning towards a problem on the Inverter/CCFL driver board itself or one of the 4 wires in CN803 labeled "SOS, BL O/F, DIM and PDIM". Im not sure what to measure at those wires...power, signal?

Any help at all is greatly appreciated

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

...and now the images

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

...and again, just getting a feel for forum rules

[budm]

Re: Samsung 275T 2 seconds to black

The F601, is it the one sitting between the two white connectors, white body? That is a fuse. The picture is kind of grainy.
You have a lot of linear regulators and Switching regulators on that main board.
I think your main power supply is OK since the PFC is working (the voltage on the big cap did get boosted up about 400VDC when active) and you also getting the 24Vdc.

[retiredcaps]

Re: Samsung 275T 2 seconds to black

6. all fuses are in tact with the exception of F601 on the Video input board top side, that measures out at 156k ohm but im not even sure if this is a fuse or not.

I agree with budm. F601 is a SMD 3A fuse. If it is measuring 156k ohms, it is definitely bad.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

I've de-soldered the fuse. Once i re-spec it and replace it ill post the result.

[jetadm123]

Re: Samsung 275T 2 seconds to black

I've de-soldered the fuse. Once i re-spec it and replace it ill post the result.

Suggest you pick up a few spare fuses just in case you do have a short on the board.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

I've ordered a couple through Mouser, and a sample from the manufacturer themselves (littlefuse.com). Since its a time delay fuse I'm suspicious of a short.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

Back to the drawing board
Replacing the blown fuse did nothing. Im thinking that maybe the fuse was blown as part of my probing around the PCB and had little/nothing to do with the actual failure.

New suggestions? I brought an o-scope home from work so if anyone can think of any tests/checks for me to do im ready...

[budm]

Re: Samsung 275T 2 seconds to black

So now it has new fuse, next thing to do now is to check the voltages on the LDO regulators. What DC voltage are you getting at the each end of the fuse measuring with Black probe of the meter to the chassis?

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

I'm editing the post i just made, I'm assuming nobody read it. My readings were off thanks to the way i have things set, up the respective boards weren't tied to chassis. Alligator clips solved that issue.

Turns out i have ~5VDC on one side of the fuse....and a blown fuse again! Im not exactly sure when it blew.

Another thing to note that during all this i removed the resistor (RM802) i mentioned at the beginning of this thread and it measured out @ .987MOhms. The color code is black yellow green black and gold, which should be a 45Ohm resistor. I paralleled 2 100 Ohm res and ended up with a 49Ohm res that i soldered in as a quick test. I got a little whisp of black smoke from near the resistors when i powered the board back up. They are 1W resistors so maybe that is the problem? Im not sure how to tell what the power limit of the original resistor is. i removed my new resistor and put the original back in and the board is back to how is was 'working' before. So long story short maybe this blew the fuse? either way when i replaced the fuse before messing with the resistor nothing had changed so i think its still progress. I have another fuse but im leery of replacing again without a next step...

[tom66]

Re: Samsung 275T 2 seconds to black

Is the new fuse blown? Fuses either fail due to fatigue or excessive load. 90% of the time it's fatigue, but sometimes you're lucky enough to have a short circuit on the board.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

yes. I edited my last post to reflect these findings since my measurement for budm was initially wrong.

[tom66]

Re: Samsung 275T 2 seconds to black

Okay, use the 200 ohms range of your meter to test from both ends of the fuse to the chassis (metal frame.) You should see open circuit both ways - ignore a brief reading if it doesn't settle within 10 seconds.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

hmm, one side is open, the other is reading .5ohms to chassis. I guess something is shorted to gnd then?

[budm]

Re: Samsung 275T 2 seconds to black

You have really badly shorted out circuits.

[tom66]

Re: Samsung 275T 2 seconds to black

Sounds a lot like an MLCC - ceramic capacitor problem. This could be difficult to locate. Try using your meter to probe all the brown/tan coloured devices. If you find any that give readings much lower than 0.5 ohms you are getting closer to the shorted device. (Here is where using an ESR meter as a low ohms meter can be handy.)

[budm]

Re: Samsung 275T 2 seconds to black

Diagram, see page 5.
Fuse F600 is the one that blown, right? It is for T-CON power supply circuits.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

I have a milli-ohm meter at work that i can grab tomorrow if need be. tonight i'll use my meter though.

F600 is the fuse that was/is blown.

[wetwillyskate]

Re: Samsung 275T 2 seconds to black

correction, F601 is the fuse that is blown