Dell ST2410b Backlight Shuts Off

[PohTayToez]

Hey I've got a 24" Dell LCD. When it first gets video input the backlight will work for about one second before turning off. None of the caps were visibly bad but I went ahead and replaced all of them except the big one. Unfortunately it's doing the exact same thing still. Any ideas?

Higher Res Image

[1supertech]

Re: Dell ST2410b Backlight Shuts Off

If you have one bad CCFL bulb they all shut down, spare bulbs come in handy for testing

[jetadm123]

Re: Dell ST2410b Backlight Shuts Off

Please post a photo of the back side of the board.

[PohTayToez]

Re: Dell ST2410b Backlight Shuts Off

If you have one bad CCFL bulb they all shut down, spare bulbs come in handy for testing

If a bulb is bad would it be visibly noticeable? For the split second it turns on it looks like both bulbs are working fine (i.e. not dim on one side)

Pictures of the back side:
Higher res image

[jetadm123]

Re: Dell ST2410b Backlight Shuts Off

Suggest you check mosfets Q801 and Q802 for shorts. I can't read the part numbers.

[PohTayToez]

Re: Dell ST2410b Backlight Shuts Off

I've never tested MOSFETs before but I found this guide saying how:
https://www.badcaps.net/forum/showpo...4&postcount=19

So I tested the resistance between the pins and didn't find any shorts, everything was at least a few kiloohms.

Oh at the part number on them is A2790 and "930" on the second line.

[jetadm123]

Re: Dell ST2410b Backlight Shuts Off

Try checking the resistance of the secondaries of inverter transformer T801. They should be approx. within 3% of each other. If they check good, then you'll have to take look at the ccfl wiring and the ccfl's themselves. If you have a spare ccfl around, then that would be even better.

[PohTayToez]

Re: Dell ST2410b Backlight Shuts Off

Currently looking for a datasheet on it, it's part number 1J.26049.161

There are six pins on one side and four on the other, but two of the pins on the side with four are not connected. Anyway to tell which are the secondaries just be looking?

[budm]

Re: Dell ST2410b Backlight Shuts Off

The transformer only has one primary (Half bridge drive circuit to drive the primary side) and one secondary winding, the secondary winding is connected to the C816, you can measure right at the C816 legs but you will not be able to tell if it is good or bad since you do not have another transformer to compare it to. You may have to have test lamp to find out where the problem is.

[jetadm123]

Re: Dell ST2410b Backlight Shuts Off

Guess I wasn't paying attention, but this 24" monitor has only 2 ccfl's? If yes, as budm suggested, using his link build a test ccfl that you can use for troubleshooting.

[budm]

Re: Dell ST2410b Backlight Shuts Off

These new DELL were cheaply made with just 2 lamps now instead of 4. Pretty lousy for 24" monitor, seen so many of them like that now.

[PohTayToez]

Re: Dell ST2410b Backlight Shuts Off

Alright, I rigged up a couple of 14W test bulbs and they both stayed lit so I guess I'm looking at at least one bad bulb in the monitor. Thanks a lot guys!

EDIT: So one things has me scratching my head. I hooked up one of the screens CCFLs at a time along with a test lamp and determined which bulb wasn't working. But in both configurations my test bulb stayed lit, where as I would have expected to shut off after a second or two when hooked up along with the bad bulb.

[PohTayToez]

Re: Dell ST2410b Backlight Shuts Off

Any recommendations on where to get a replacement? The closest I can find are these off of eBay, which are supposed to be for a 24" screen but are only 500mm (19.7in) long. The ones in this screen are right about 21" long.

[hemanjo]

Re: Dell ST2410b Backlight Shuts Off

Hi,u can buy it at CCFLwarehouse.They don't have the 24 inches ccfl but I ordered 2 ccfls which is 2.4 x 545mm full spectrum costing $9.95 each and I use USPS International
2-5 weeks no tracking but it took only 8 days to reach me in Kuala Lumpur,Malaysia in tip top condition.The ccfl fits nicely in place too and u might want to see the tutorial on soldering the ccfl beforehand and to be very careful with the ccfl and the dismantaling of the whole lcd screen.

[hemanjo]

Re: Dell ST2410b Backlight Shuts Off

I'm using Dell ST2410b oso.Thanks.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Hi Guys,
Loved reading the thread, thanks to everybody for contributing.

I have the exact same monitor, with a bit of a story.

Firstly it used to stay on for about 30-50 seconds before turning off. I thought, definitely the caps. So hopped on to RS components, ordered my self a set of full capacitors. However, the two 330uF capacitors on the CCFL transformer side were rated at 35V and somehow I bought them as 16V. I replaced them any how, still the same behaviour.

I bought but hadn't replaced the mains side rectification capacitor, even though skeptical it would make any difference, changed it anyway :- no difference.

Then I though maybe some of the capacitors I bought have the wrong tolerance (never checked them when buying) so I was suspicious whether the tolerances are throwing the circuit down. So, I don't know what I was thinking but I sequentially removed capacitors that were replaced in sets by their value at a time and powered the inverter and checked for if it would light up. If it didn't I replaced the capacitors with the ones that were on the board originally, until I removed C701 and C702 and replaced them with the originals. After this the CCFLs didn't turn on at all. I checked the LCD for an image, sure enough it was there. Then I noticed that R803 was burnt out. I struggled to read Brown, Black, Black. Happened to have one in my stash with 1% tolerance and replaced that but CCFLs still not lighting up.

Pulled apart my DELL 1707FPt and used its inverter to power up the two CCFLs on the ST2410b. It was faint and lasted for a second or two.
Then I used the ST2410b inverter to light on the 1707FPt CCFLs and there was no luck.

Came across this site and I tested the Q801 ND Q802 according to the pinout from the datasheets, resistance test showed no shorts.

I have also tested the LD1117AV33 (IC701) voltage regulator. This seems to work taking in 5V and pumping out 3V.

T801 showed 3 Ohm across primary and 0k5 Ohm across the secondary. Since this is a step-up transformer, this makes sense to me as the secondary should have much more winding than primary. This is expected right?

However, I thought I'll check the T601 and to my surprise, secondary winding is 0 Ohms on any combination of the terminals. On the primary I had existent but low resistance (<10 Ohms).

So my conclusion is, I have CCFLs that need replacement and I have stuffed up my board, possibly the T601. Would you be able to suggest further tests at all? Your assistance is much appreciated in confirming or pinpointing failure point.

Thanks in advance.

[uengin]

Re: Dell ST2410b Backlight Shuts Off

Hi Guys,
Loved reading the thread, thanks to everybody for contributing.

I have the exact same monitor, with a bit of a story.

Firstly it used to stay on for about 30-50 seconds before turning off. I thought, definitely the caps. So hopped on to RS components, ordered my self a set of full capacitors. However, the two 330uF capacitors on the CCFL transformer side were rated at 35V and somehow I bought them as 16V. I replaced them any how, still the same behaviour.

I bought but hadn't replaced the mains side rectification capacitor, even though skeptical it would make any difference, changed it anyway :- no difference.

Then I though maybe some of the capacitors I bought have the wrong tolerance (never checked them when buying) so I was suspicious whether the tolerances are throwing the circuit down. So, I don't know what I was thinking but I sequentially removed capacitors that were replaced in sets by their value at a time and powered the inverter and checked for if it would light up. If it didn't I replaced the capacitors with the ones that were on the board originally, until I removed C701 and C702 and replaced them with the originals. After this the CCFLs didn't turn on at all. I checked the LCD for an image, sure enough it was there. Then I noticed that R803 was burnt out. I struggled to read Brown, Black, Black. Happened to have one in my stash with 1% tolerance and replaced that but CCFLs still not lighting up.

Pulled apart my DELL 1707FPt and used its inverter to power up the two CCFLs on the ST2410b. It was faint and lasted for a second or two.
Then I used the ST2410b inverter to light on the 1707FPt CCFLs and there was no luck.

Came across this site and I tested the Q801 ND Q802 according to the pinout from the datasheets, resistance test showed no shorts.

I have also tested the LD1117AV33 (IC701) voltage regulator. This seems to work taking in 5V and pumping out 3V.

T801 showed 3 Ohm across primary and 0k5 Ohm across the secondary. Since this is a step-up transformer, this makes sense to me as the secondary should have much more winding than primary. This is expected right?

However, I thought I'll check the T601 and to my surprise, secondary winding is 0 Ohms on any combination of the terminals. On the primary I had existent but low resistance (<10 Ohms).

So my conclusion is, I have CCFLs that need replacement and I have stuffed up my board, possibly the T601. Would you be able to suggest further tests at all? Your assistance is much appreciated in confirming or pinpointing failure point.

Thanks in advance.

By the way, when I was suspicious about the the tolerances, I was so because it went from 30 seconds to a "2 seconds to black" operations hence arising suspicion and then thinking of tolerances.

[selldoor]

Re: Dell ST2410b Backlight Shuts Off

Its TOOOOOOOO long a story. I will try and disect it

What make and series caps have you used. If you have read any threads we tend only to recommend Panasonic FR FM & FC or Nichicon ot rubycon equivalents.
I dont understand what you say about 16v/35v caps - you can replace 16v with 35v
but not 35v with 16v. (they are likely to go bang)

[Davi.p]

Re: Dell ST2410b Backlight Shuts Off

Check if your new cap with 16v changed for the 35v is conductive..

[budm]

Re: Dell ST2410b Backlight Shuts Off

Can I see good clear pictures of YOUR board, both sides?
"However, I thought I'll check the T601 and to my surprise, secondary winding is 0 Ohms on any combination of the terminals. On the primary I had existent but low resistance (<10 Ohms)." DCR of the power transformer will be really Low, since you are getting the 5V, the transformer is working. What DC do you get on those two caps that are supposed to be 35VDC?