BENQ2420 LCD turns off after 2 seconds

[catsunami]

So the monitor I have had for about 3 years started flickering yesterday, and today just randomly turned off video.

The following is happening:
-- The monitor is still recognized by my computer
-- The monitor is not sleeping, the green light remains on
-- The screen is completely black, I can not see anything on it even if I use a flashlight
-- I tried turning down the brightness to see if it stays on, but I haven't been able to get into the brightness menu for long enough to do this
-- If I cycle power, the desktop image comes on again for 2s then goes black. I can seemingly do this forever. Anyway it doesn't really help.

So! I took it apart, and measured the voltages across most (I tried all, but I missed some) capacitors. I don't see any bulging, I don't really see anything out of norm physically. I'm attaching the images. Some just raw ones, and then ones with the voltage values on them. I'm particularly interested in knowing why those blue ones measure 0V...? I guess?

I would appreciate some ideas! I can buy a new one, but if I could fix this one, that would be really nice. I know how to solder and stuff.

[budm]

Re: BENQ2420 LCD turns off after 2 seconds

Those blue caps are high voltage ballast caps, do not try to measure the voltage on those point, it around 2000V, it will ruin your meter.
Can we get good clear closed up pictures of the bottom side of the board where the two inverter transformers are (they are next to the blue caps), we need to measure the resistance of the high voltage windings of those two inverter transformers.

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Oh woops The meter seems ok anyway. I would have thought 2000V would have read OL on the meter?

Here are some pictures, I hope they're what you're looking for (those gray boxes on the left side, right?).

Sorry I don't know why I'm having so much trouble getting the top one better. Let me know if it needs to be clearer, and I'll redo it. I tried a few times, but this is the best I could do.

Edit: Oh btw, I checked the fuse and it is still a short (I just did this, no power on).

[Jonny Accelerant]

Re: BENQ2420 LCD turns off after 2 seconds

I just read about this last night. Called "two seconds to black" what the circuit does is look to see if the backlight bulbs are turned on during the "boot" process, and if they are not it shuts down (to black). Any of the bulbs being out will do this, and also if there is no high-voltage power to the bulbs, the system will shut down. From what I've read, usually there is a capacitor that is burnt open after the step-up transformer on the circuit board, so you should measure those and inspect them closely. I'm still learning, and may have some of the words wrong but that's the basic idea. The guys that do know everything don't always break down the whole story for every single person. You might read my thread on my LP2626 as it's current and there's a bunch of good stuff in there, including some pdf files of circuits that are similar to my, and your's looks similar to mine also. The idea that there are two transformers stepping up high voltage output and a set of caps as (I guess) "ballast". I have no idea what ballast is or does, but the output of those transformers needs caps, and those caps are under-specced for the high voltage and they go bad and the good news is it's an easy fix.

Also the bulbs (called "CCFL"s?) can burn out and there's at least one good thread on how to test them and then a nice link on where to by them for about $13.00 each.

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Hi Jonny,

I was reading the "two seconds to black" thing and the basic description throws me off - it says that I should be able to still see stuff if I shine a flashlight on the screen. I can't though, it's just black.

As I said, I already measured and inspected the caps, but I don't see any burst ones or obviously misfunctioning ones.

Anyway, thanks for the info! I will check out your thread.

[diif]

Re: BENQ2420 LCD turns off after 2 seconds

If the desktop image is coming on for 2 seconds we know that there is at some point voltage coming out of the transformers.
The 2 seconds to black is a safety feature. If the monitor detects a problem it shuts off to prevent any further damage, that is why you won't measure any voltage. But as budm said above, DO NOT try and measure the voltage of these or the output from the transformers.
The last Benq i repaired, had poor wiring joining at the ccfl tube.
I bought 5 CCFL for $12 delivered.

[Rtech]

Re: BENQ2420 LCD turns off after 2 seconds

diif..Love to know where you got the CCFL's from ??.I am wiating for some from HK at the moment at $1.80 each,but add the Delivery costs and it certainly increases that.
FOR Catsunami, the flashlight trick is not effective in all cases,it depends on quite a few things, so do not worry about it.

[diif]

Re: BENQ2420 LCD turns off after 2 seconds

http://www.aliexpress.com/item/Unive...789456586.html
and
http://www.aliexpress.com/item/5pcs-...668342533.html

[budm]

Re: BENQ2420 LCD turns off after 2 seconds

Check the resistance reading between those two points in the pictures, set the meter to 2K scale.
You should have two test lamps that will make it easier to narrow down the problem.
https://www.badcaps.net/forum/showth...er+testing+cfl
You also need to check those 4 power MOSFETs (8-pin SMD), but lets verify if the transformers are OK or not first.

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Thanks everyone for the info and the links!

budm, I will measure the resistance when I get home from work today and report back then. Would the flickering I saw the day before make it more likely to be the CCFLs? Just had a thought that sometimes when a light is about to burn out (like a normal light at home), it flickers a bit.

[budm]

Re: BENQ2420 LCD turns off after 2 seconds

You may have bad lamps or bad solder joints at the end of the lamp, for examples:
http://s807.photobucket.com/user/bud...?sort=3&page=1

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Ohh another thought occurred. I have a second monitor that is exactly the same (brothers). If I take that one apart too, and swap the power boards, then I can make sure it is/isn't the CCFLs, right? Without having to maybe rip apart the metal frame (this monitor is such a pain to take apart).

For example, if I swap boards, and the good one stops working, then I know it's something on the power board. If it keeps working, then it's probably something else, like the CCFLs. Any risk in doing this? Can I possibly wreck my good one doing this?

PS. Some of the connections in that album you sent me are horrifying. Is that a fairly popular problem?

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Ok, measured those points!

If I reference your images:
For transformer #1 the value is ~2000 ohms.
For transformer #2 the value is OL on that particular setting (20kohm).

Edit: While I had everything out, I also got to the CCFLs and checked those out. All 4 seem to be working.

[budm]

Re: BENQ2420 LCD turns off after 2 seconds

# 2 is bad. I would remove it and look at the pins to see if you can see broken wire.

[retiredcaps]

Re: BENQ2420 LCD turns off after 2 seconds

Oh woops The meter seems ok anyway. I would have thought 2000V would have read OL on the meter?

The voltage at those inverter transformers is ACV. So yes, if you had your multimeter on AC, it would should 0L. If you had your meter on DCV, then it would show 0 (probably).

As budm mentioned, some meters, especially the cheaper ones without input protection or poor build quality, will not survive when measuring around 1500V+.

[retiredcaps]

Re: BENQ2420 LCD turns off after 2 seconds

# 2 is bad. I would remove it and look at the pins to see if you can see broken wire.

The usual places like lcdparts.net and ebay show no replacements for T2504 transformer?

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Thanks, budm and retiredcaps!

I found a thread on a Russian forum that talked about the same part, and they seemed to find a replacement ok using the other code on the sticker (1J.26061.021). The guy reported back that he bought 1J.26064.021 and it worked anyways.

I found this one on ebay ($15): http://www.ebay.com/itm/1J-26061-021...item233f0af6d0

Also in another thread I saw this one being mentioned as a possible replacement: http://store.lcdparts.net/1J26051001-MS300T_p_2724.html

I did find a whole new power board on Aliexpress though ($32), so that's also an option I'm considering. Mostly because I don't know how to desolder that massive thing in the first place. Heatgun?

[retiredcaps]

Re: BENQ2420 LCD turns off after 2 seconds

The guy reported back that he bought 1J.26064.021 and it worked anyways.

At 1AM in the morning, I wasn't going to surf Russian websites. Good job on searching/finding for the 1J part number. Usually, I go by the T part number and had good luck in the past.

Mostly because I don't know how to desolder that massive thing in the first place. Heatgun?

Well, in your first post, you say you know "how to solder and stuff" so desoldering that transformer is easy.

I have done at least one and they desolder easily. You just have to add some 60/40 solder first and then use a solder sucker. After doing that to all the pins, the transformer should fall out on its own.

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Well, there we go, I said I know how to solder, but not necessarily desolder. I don't usually have to desolder components from my PCBs, and when I do - I find it to be very frustrating because they're all tiny SMD devices with ~100 pins.

Anyway. I don't have a solder sucker, so I'll look around and see if I can find one.

[catsunami]

Re: BENQ2420 LCD turns off after 2 seconds

Hi everyone! Again!

The new transformer came in today, I soldered the other one out (solder suckers are a godsend) and put the new one in. The monitor now turns on and stays on! But it makes a high pitched buzzing sound. Any thoughts what that could be? The new transformer? I'm thinking of reflowing all connections on it just in case.