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Old 03-18-2013, 08:49 AM   #1
Denz
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Smile Dell 2405fpw - Backlight problem

Hi,

I have casually been reading posts on this site for about a year hoping that “one day” I'll have enough knowledge to fix my Dell 2405fpw. The monitor turns on (power button lights) but nothing shows up until I use a flash-light and see that the “please connect input“ default error screen is bouncing around. So the back-light needs more power.

I was hoping it was the usual capacitor problem but after changing all of them the monitor would only have a brief flash. I would also hear a click/buzz sound which sounded like someone was flicking a switch on and then off from the TDK inverter circuit board. The sound may have also originated from the CCFLs.

On the power board, I replaced the big capacitor because it was leaking and the smaller ones were bulging. I also replaced mosfet and resistor near the fuse as it was causing other posters problems due to their capacitors leaking.

The power-board is outputting 21.3 V when it is switched on and 21.7 V on standby.

TDK board

While tracing the current, I found that it stops just before the transformers at the surface resistor/fuse (F1, F2 & F101). I'm not too sure what those little white components are but one has a much different reading than the other two;

F1: 0.3 Ohms
F2: 78.8 Ohms
F101: 0.4 Ohms

I also tested the output while the monitor was switched on and got 21.3V for all three of the components.

For some reason I'm unable to get a reading from more than 1 leg of the transformer.

The output voltage to the TDK inverter board (while monitor is switched on) is:

Black: 0
Brown: 0
Red: 21.7V
Orange: 21.7V
Yellow: 21.7V
Green: 1.4V
Blue: 3.2V

The output voltage to the Benq logic board:

White: 3.2V
Black: 3.2 V
Brown: 1.4V
Red: 0
Orange: 0
Yellow: 3.2V
Green: 6.6V
Blue: 0
Purple: 0
White: 0

Am I right in thinking the power board is good and I should be diverting my attention to the other boards ?

Does anyone have any suggestions/tips how I could test the inverter board more vigorously and what components are more likely to fail ?

If the monitor flashes for a split second, would this mean the CCFLs could be still alive and intact ?

Sorry if this post is a little disjointed, I wrote this over months so I wouldn't forget.

Thank you in advance for all your help, the threads on here have already been very helpful but sadly I have come to a grinding halt so any suggestions are welcome.
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File Type: jpg P1000102.JPG (1.45 MB, 52 views)
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File Type: jpg P1000113.JPG (1.51 MB, 38 views)
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Old 03-18-2013, 09:16 AM   #2
selldoor
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Default Re: Dell 2405fpw - Backlight problem

F2 is a fuse and it has blown

If it has a K on it - flash has blanked it it may be 1.5a
There is no standard marking but I have seen a few from different sources with k=1.5. They tend to be a bit expensive and having to buy 10 makes it moreso.
Some mebers use small pico fuses and solder them in first as from some sources
they can be a bit cheaper.
Cpc seem to have a FREE postage promotion on at the moment
http://cpc.farnell.com/cooper-bussma...-bb45-00001003
might do - they have others as well you will need to get the right physical size
- it is often easier to solder the new one piggyback on top of the old one - less chance of damage to the fuse and the board.

Last edited by selldoor; 03-18-2013 at 09:49 AM..
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Old 03-18-2013, 01:34 PM   #3
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Default Re: Dell 2405fpw - Backlight problem

Thank you Selldoor, I've removed the fuse and yes, it's broken. I've googled SMDs and and yes the "k" stands for 1.5 amp.

A even bigger thank you goes for the CPC free delivery link. That will save me a fair amount of money since I usually order components from RS.

Fingers crossed, this solves the issue unless the power supply has blown something else.

Thanks once again for your guidance

Last edited by Denz; 03-18-2013 at 01:44 PM..
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Old 03-18-2013, 01:52 PM   #4
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Default Re: Dell 2405fpw - Backlight problem

Good - I hope it works for you. Dont worry about voltage on the main transformer - you cant measure it with a standard multimeter. Dont try and measure voltage on any inverter transformers they can be 2000v and will blow your meter and you!!.
When you get the fuses we can look at the rest. Just one query where you have ~21v
that is ok if its meant to be 20 but not good if its meant to be 24v
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Old 03-18-2013, 09:47 PM   #5
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Default Re: Dell 2405fpw - Backlight problem

Quote:
Originally Posted by Denz View Post
The output voltage to the TDK inverter board (while monitor is switched on) is:

Black: 0
Brown: 0
Red: 21.7V
Orange: 21.7V
Yellow: 21.7V
Green: 1.4V
Blue: 3.2V

The output voltage to the Benq logic board:

White: 3.2V
Black: 3.2 V
Brown: 1.4V
Red: 0
Orange: 0
Yellow: 3.2V
Green: 6.6V
Blue: 0
Purple: 0
White: 0
Thanks Denz. I can't help you with your problem. But you've helped me with mine.

Seldor also helped by making me realize that F701 and F702 on the power board are fuses.

Thanks guys.
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Old 03-19-2013, 12:16 PM   #6
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Default Re: Dell 2405fpw - Backlight problem

Glad the data helped you out. If you need more readings for confirmation, I'll be happy to post it once I get my fuses delivered.
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Old 03-21-2013, 08:09 AM   #7
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Smile Re: Dell 2405fpw - Backlight problem

Hi,

I've just replaced the fuse and after I switched the screen on it blew again.

I clean the board by spraying some isopropyl alcohol and found a dirty looking component C31 which I assume is some kind of surface capacitor ? It looked like it had been soldiered by myself (messy) and had some yellow residue beside it. I'm not fully sure if it has exploded or it's just dirt.

I've only got a cheap £30 multimeter so it's unable to test capacitors. Is it possible to test capacitors with a basic multimeter ? I'm going to have to remove another capacitor from the board to compare the results. I usually just replace the standard capacitors with spare capacitors lying around but in this case I'm not going to have a surface mount one.

Does anyone else have any other ideas of what could be disturbing the F2 fuse ?

Sorry about the photos, it's really hard for my camera to focus on the miniature components.

Thanks for your help
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Old 03-21-2013, 08:59 AM   #8
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Default Re: Dell 2405fpw - Backlight problem

Please do the resistance testing of the MOSFET, start with Q106. We are looking for low resistance reading between pin 1 and 5 (or 6, 7, 8 since they are tied together), do the same for the MOSFETs and report the readings.
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Old 03-21-2013, 03:15 PM   #9
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Smile Re: Dell 2405fpw - Backlight problem

Quote:
Originally Posted by budm View Post
Please do the resistance testing of the MOSFET, start with Q106. We are looking for low resistance reading between pin 1 and 5 (or 6, 7, 8 since they are tied together), do the same for the MOSFETs and report the readings.

Hi, I'm unsure if I've numbered the pins correctly. I assumed the pin number starts with the dot/indent. The 5,6,7,8 pins are all linked according to my continuity test. I've attached my version of the pin numbering system just in case it is wrong.

From pin 1 to pin 5 (diode/continuity test mode)

Mosfet model - 4512C AA5MV

Q106 - 531 ohms
Q105 - 532 ohms
Q6 - 532 ohms
Q7 - 533 ohms
Q4 - 532 ohms
Q5 - 530 ohms

Thanks for the help
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Old 03-23-2013, 08:43 AM   #10
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Default Re: Dell 2405fpw - Backlight problem

I decided to reattempt measuring the resistance using the Ohms function on the multimeter since the diode function try was wrong.

I could only get a reading if I set the multimeter to 200M Ohms which was probably why I ignored it in my first try. I'm not entirely sure I've used the correct setting ?

From pin 1 to pin 5 (200 M Ohms setting)

Q106 - 1.2 M ohms
Q105 - 1.2 M ohms
Q6 - 1.2 M ohms
Q7 - 1.2 M ohms
Q4 - 1.2 M ohms
Q5 - 1.2 M ohms

It seems like all of those Mosfets are working. There is still that suspicious looking ceramic capacitor near the broken fuse and between Q6 & Q7 which does not look good so I am going to remove and test it.

Is it possible for a ceramic capacitor to take out a fuse or am I barking up the wrong tree ?

There are no markings on these capacitors so I'm probably have to sieve through the internet and this forum for some clues.

Thanks
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Old 03-25-2013, 04:09 PM   #11
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Default Re: Dell 2405fpw - Backlight problem

There will be few clues - I think you can only test them for short with the meter you have.
You could also test on ohms from each side of the fuse to ground.
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Old 03-25-2013, 09:49 PM   #12
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Default Re: Dell 2405fpw - Backlight problem

OK, I just look at the spec sheet of the AF4512, it is dual MOSFET's (P and N channels), so you need to take the reading between pin 3 and 5 also.
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Old 03-26-2013, 04:35 PM   #13
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Smile Re: Dell 2405fpw - Backlight problem

Quote:
Originally Posted by budm View Post
OK, I just look at the spec sheet of the AF4512, it is dual MOSFET's (P and N channels), so you need to take the reading between pin 3 and 5 also.
From pin 3 to pin 5 (200 M Ohms setting)

Q106 – 1.1 M ohms
Q105 – 1.1 M ohms
Q6 – 1.0 M ohms (tested many times)
Q7 – 1.1 M ohms
Q4 – 1.1 M ohms
Q5 – 1.1 M ohms

Quote:
Originally Posted by selldoor View Post
There will be few clues - I think you can only test them for short with the meter you have.
You could also test on ohms from each side of the fuse to ground.
I tested the SMD ceramic capacitors. I can do simple low capacitance tests on multi-meter .I can test using 100u, 2u, 200n, 20n.

All of the following capacitors are located next to the mosfets which I measured in the previous post.

Capacitance test – On the board ( 100u)

C106 – 14.5 uF
C105 – 14.5 uF
C30 – 2.2 uF
C31 – 0.8 uF
C2 – 14.5 uF
C4 – 14.5 uF

I originally thought C30 & C31 were broken because the values were considerably less than the other capacitors on the board. Then I decided to test it out of the board and the results came back all the same.

Capacitance test – Independently ( 100u)

C106 – 2.2 uF
C105 – 2.2 uF
C30 – 2.2 uF
C31 – 2.2 uF
C2 – 2.2 uF
C4 – 2.2 uF

This led me to test the bigger capacitors which were Nkcom 180uF 30V. All three capacitors were testing out of the circuit and came back with completely different results with the same capacitance settings. I know my meter can't read 180uf but I still expect to see some consistency with the results.

I've bought some Panasonic 180uf 35V caps to use as replacements because something must have be wrong with them. Hopefully these caps have enough surges and irregularity to break fuses.


Thanks for your help guys, I can tell I'm/we're making progress.

I know it would be a lot easier if I had a lot more knowledge and the correct equipment, lol.
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