Dell 2407wfp backlight problem

[leapius]

Hi guys, I have a Dell 2407wfp with a backlight problem. I disassembled it and took a couple of photographs with the backlights powered on.

As you can see the u-bend CCFLs are not full lit to the end. They also flicker a lot too. It's as though they don't have enough power to be lit at full brightness to the ends.

I don't have an ESR meter or Ring tester - only a multimeter. I did the hairdryer test and heated up the capacitors on the inverter board which did have a slight effect on the bulb brightness at the ends. There was no effect when doing the same to the power board.

I switched around the bulbs onto different inverters but there was no change. When I unplugged 3 of the bulbs the other 3 lit up perfectly brightly (but obviously only for 2 seconds due to the feedback loop). This happened with any combination of 3 bulbs and inverters. Adding more bulbs back made the problem progressively worse. There were no obvious signs of damaged components on any of the boards.

What do you think the problem is? Should I try replacing the capacitors on the inverter board? Could it be a power board issue?

Many thanks for any help,
Leo

[jetadm123]

Re: Dell 2407wfp backlight problem

Can you please post a good focused photo of the inverter board.

[leapius]

Re: Dell 2407wfp backlight problem

Sure, here's the front and back:

[budm]

Re: Dell 2407wfp backlight problem

Please check all the transformers secondary side windings. Those 3 green TAPO caps on the inverter board should also be replaced. Check the 19V power.
Please see this post: https://www.badcaps.net/forum/showth...highlight=dell

[jetadm123]

Re: Dell 2407wfp backlight problem

Not sure which power supply you have, but other members have reported leaking/bad main filter caps. See post #10 in link below:

https://www.badcaps.net/forum/showth...hlight=2407wfp


Also, what's the voltage being fed to the inverter board? You should be able to read it at the wiring harness, or at one of the inverter board fuses.

And if those 3 caps on the inverter board are made by NKCON, they should be replaced as they known bad caps on boards made by TDK.

[leapius]

Re: Dell 2407wfp backlight problem

Thanks for the info - is there any way to check the secondary side windings without a ring tester? Can I do this in place or do I have to desolder. Any way you can see which pins I need to check from the photograph? I'm a bit of an amateur at this.

[leapius]

Re: Dell 2407wfp backlight problem

Not sure which power supply you have, but other members have reported leaking/bad main filter caps. See post #10 in link below:

https://www.badcaps.net/forum/showth...hlight=2407wfp


Also, what's the voltage being fed to the inverter board? You should be able to read it at the wiring harness, or at one of the inverter board fuses.

And if those 3 caps on the inverter board are made by NKCON, they should be replaced as they known bad caps on boards made by TDK.

Yes they are 180uf 30v NKCON capacitors. I have found some 180uf 35v Nichicon capacitors - will they be a good replacement? The wire loom coming from the power board has 7 wires in this order:

blue
green
yellow
orange
red
brown
black

I don't want to blow anything up randomly checking these wires for voltages. Any clues what I should test here? Thanks.

[jetadm123]

Re: Dell 2407wfp backlight problem

Probably, your best bet would be to place your black probe on one of the mounting screws and your red probe at either end of fuse F2, located next the middle cap, C3. It could be different on yours, but the TDK inverters typically run off of 24Vdc.

[leapius]

Re: Dell 2407wfp backlight problem

Probably, your best bet would be to place your black probe on one of the mounting screws and your red probe at either end of fuse F2, located next the middle cap, C3. It could be different on yours, but the TDK inverters typically run off of 24Vdc.

There's a fuse F1 just to the right of the far right capacitor in that picture (obscured by it) should I try that one?

[leapius]

Re: Dell 2407wfp backlight problem

Ok so I just checked the standby voltage from F1 to GND and it is a solid 19v as per this photo: https://www.badcaps.net/forum/attach...2&d=1339721233

I (think) I checked the resistance of the secondary windings of each inverter and they are all around the 960-990ohm range.

[budm]

Re: Dell 2407wfp backlight problem

Do the lamps stay on like that or they shutdown after a few seconds?

[leapius]

Re: Dell 2407wfp backlight problem

Do the lamps stay on like that or they shutdown after a few seconds?

The lights all stay on like that permanently (just dim and flickering on the far side). If I unplug 3 of the 6 bulbs the remaining 3 will come on fully bright (but only for 2 seconds because of the feedback loop obviously). The more bulbs I add back in the progressively worse the problem gets. It doesn't matter which bulbs I unplug just the number. They will all stay on if I plug at least 4 bulbs in.

[tom66]

Re: Dell 2407wfp backlight problem

So, otherwise, the picture is good, but obviously going to be dim on one side? Did it get worse progressively or suddenly?

[leapius]

Re: Dell 2407wfp backlight problem

The picture is fine - obviously just dimmer on one side as can be seen in the photographs. I'm not sure whether it suddenly went like that or if it was a gradual deteoration - it was just presented to me like that.

What could possibly cause the CCFLs not to light up fully in this way?

[budm]

Re: Dell 2407wfp backlight problem

They are not driven fully, I would try those three caps on the inverter board first. I have seen this symptom on inverter board with bad transformer and bad caps but since ALL the lamps are acting the same then not likely to be the transformers. Those 3 caps are connected in parallel.

[jetadm123]

Re: Dell 2407wfp backlight problem

Agree with budm that you should replace the 3 caps on the inverter board, as TDK had a bad habit of using poor quality caps in their inverter boards.


Also, if you have the same power supply I mentioned a few posts above, then you might also consider replacing the large 450V cap as it has given other members problems.

[leapius]

Re: Dell 2407wfp backlight problem

Right, I'm going to order some new capacitors and replace them. Thanks for the advice guys. I will report back my findings when I've changed them!

[leapius]

Re: Dell 2407wfp backlight problem

Well I have now installed all new nichicon capacitors on the inverter and sadly there is no change in the backlight. It exhibits the exact same symptoms as before. Any ideas anyone? Would it be worth trying to source a whole new inverter board?

Thanks

[budm]

Re: Dell 2407wfp backlight problem

Can you monitor the DC voltage on that 450V cap when the monitor is off and when the monitor is on?

[leapius]

Re: Dell 2407wfp backlight problem

Can you monitor the DC voltage on that 450V cap when the monitor is off and when the monitor is on?

Can you tell me how to do that? red wire onto positive side and black wire to negative side and turn it on with multimeter set to volts?