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Repair guides and support for computer monitors produced by Samsung.

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Samsung 305T, backlights work, black screen 90% of time

Hey folks, I have this neat older monitor that I'd like to preserve. It still has a very functional resolution and size (2560 x 1600 @ 30"). These are notorious for having overheating scaling chips on the T-con, yet I don't think that is my problem as the screen is simply black (or it occasionally works) and T-con failures are colorful, right? At least, there are telltale artifacts and such before they fail completely, correct? I never had any hint of it going bad. Just lots of instances where I would have to shut it off, fiddle with the DVI, and plug it back in to get a signal. I'm honestly not sure the plugging had any effect more than the power cycling. Although, if there is a bad connection in a solder somewhere, the plugging may have moved it slightly enough to make contact.

Taking it apart, I found some obviously bad caps on the PS board, so I recapped all the capxons except the large main cap and two Rubycons. This changed nothing. I since pulled the two Rubycons and the large main to test. The Rubycons (35V1000uf) test at Vloss 0.5~0.8, C is at ~930uf, and ESR is 0.00-0.01ohms. The main (450V150uf) tests at Vloss 1, C is ~135uf, and ESR is 0.46ohms. Should I replace these?

I'm starting to think the problem lies in the main, or the ribbon connectors.

Any ideas?

See pics here

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Hi,

When the screen is black, shine a torch at an angle close to the screen and check for images.

They will be very faint if they are there, so trying this in a darkened room may help to see them.

If there are images to be seen on the screen, there's a backlight problem

Here's the block diagram from the service manual which may help although it's not a schematic.

Check if there is +24V on CN 402 (I don't know which pin so you may have to check all of them).

If not then the problem is in the inverter section of the power board.

If there is check for voltage on the BL_EN(backlight enable) lead on CN 402. Again I don't know which pin and but maybe look for a voltage between 3V-5V as this is mostly the case with display backlight enable.

Usually there is always voltage supplied to the backlights but it is turned on and off in the screen by a signal voltage sent/removed on the BL_EN lead.

The way to find the lead maybe is to check the voltages on CN402 (all pins - write them down) when there is a display for the 10% of the time and then check again when there is no display.

If there are no images then it's looking like a mainboard problem, especially if there is +24V for the backlights.

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Hi Jayeff, thanks for the reply. I appreciate the time you have taken to help me. Unfortunately, you missed in the title of my question where I say that the backlights do work. I should have reiterated that in the body of my post though, I apologize for that. I also am thinking it's likely a main board problem, though I'm also going to replace the big cap on the power supply as it seems be down 10% of it's capacity.

How do you suggest addressing the fault on this main board? They seem to be fairly irreplaceable at this point. Should I try baking to reflow it?

Cheers

von

Hi @Alex

No worries but I didn't miss the fact that the backlights worked. I went on the fact that they don't work at the times when they should be i.e. initially I suspected a backlight "problem", not no backlights at all. That's why I asked whether there were images on the screen to further isolate whether it involved only the backlights or the video as well and then what to do in either case

Did you try the torch test when the monitor is on and the screen is black, you didn't say? (torch = flashlight - different name for the same thing depends where you are)

In this instance to isolate it further is there still video being sent to the screen but it can't be seen due to no backlights or no video at all as well as no backlights? The torch test will let you know

If there is still video, then concentrate on the backlights to find out why they're not on when they should be e.g. inverter circuit or mainboard backlight control circuit or even the backlights themselves somewhere

If there isn't any video then it is something that affects the video as well as the backlights e.g. mainboard control only.

Board level fault finding without the schematics is difficult though. The service manual may give pointers but that's all.

e.g. if there is video then if if you can find the backlight power lead or the backlight enable lead as per my suggestions above you may be able to trace them back from the connector pins to where their circuitry is on the board and check components there. As I said, difficult

In the end in may come down to finding a replacement mainboard. Try searching for the mainboard "board number" for a replacement.

Here's the section in the service manual (i think) that has the part numbers that may help to find one. Verify with what is printed on the board itself.

baking can be dangerous if you don't get it right , you could lose the whole board .Baking is only for suspected faulty solder connections as it reflows the solder to make a good connection but it won't fix faulty components e.g. if a component is getting too hot for some reason and this affects something else etc.

von

Ah, ok. For clarification, the backlights do appear to be working as they are supposed to. On power on, they all light up, and stay lit up until the power button is pressed again. There seemingly is no signal to the LCD and no video display of any kind. When I say the screen is black, I mean only that the LCD is displaying nothing, but is clearly illuminated by the backlights.

Unfortunately, this mainboard is unavailable from any used parts dealers as well as from ebay. So I'm stuck with component level repairs or junking the whole thing, which seems wasteful. The only capacitors on the main board are poly caps, which are probably ok. There are also some mosfets as possible culprits, or maybe some of the ICs. I've actually never baked a board before, but willing to give it a go if the next step is taking it to the recycling center.

von

@Alex

One more try before throwing into an oven.

Check posts# 19 & #24 in this thread. It may work and you won't run the risk of damaging the board.

If still no good then try baking it. Here's a link for a similar model that may help with this.

von

Both of the posts you list are in reference to the notorious T-con failure that plagues this machine. I'm pretty sure that's not my issue. Either way, they're also both referring to baking a board, just not the one I am.

von

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could be a connection issue, try unplug and replug all the connectors a few times, then test it.

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I did this. I also tried using some isopropyl on the pins and removing and reinserting the plugs a few times to clean out both sides of the socket. No luck, unfortunately. Thanks though.

von

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Alex wird auf ewig dankbar sein.
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