Dell 2408WFPb / 2408WFP - No Power

[eaarnoudse]

Re: Dell 2408WFPb / 2408WFP - No Power

I am also trying to repair a Dell 2408WFPb. I have the same problem as mentioned in one of the previous posts. A led light that blinks one time and the monitor won't turn on. The power supply is OK, because CN701 has about 24V on the output. The reason that the inverter board doesn't power on is simple. The first black wire on CN701 is the ON/OFF signal and it's 0V.

I started to suspect the mainboard and I found a way to power up this board. It seems like there are two main sections on it. Each section has a big chip, a crystal and a voltage regulator. First, I measured the section at the side where the power supply cable comes in. The crystal has a perfect sine wave and the voltage regulator shows a perfect 1.8V. But there is no voltage on the second crystal and voltage regulator.

I heated the heatsink and the onboard chip in the first section very carefully with a hot air rework station. Then I tried to power up the monitor and it was working great again. I waited some time until the heatsink was cooled down. The monitor did not start up. I heated the heatsink and chip again and the monitor was working fine.

I think there is a cold solder joint in one of the pins of this chip. Maybe a reflow precedure will solve the problem, although this method is not very reliable. I hope this post will help other people to troubleshoot this specific problem.

Greetz,
Erwin

[RadekZ]

Re: Dell 2408WFPb / 2408WFP - No Power

Hi All,

I know that this post is really old but I found some activity on last days here. I came to this forum with exactly same issue as was described by member nickname Brainwash. On my desk is DELL Ultrasharp 2408WFP. After connection of supply cable green led blinks (not every time !!) and it is all. it is not possible to switch it on. Power source works fine, on output it has 24.5V exactly as reference (TL431) sets. So I can say that root cause is not in switching power supply but very probably in mainboard. I completely checked controll board with buttons and LEDs, flat cable and all is OK. I also checked some supply sources on mainboard and they looks also OK and switching. Also LDOs works fine.

I would like to know more about CN701 - is there any feedback from backlight inverter ? Is there power OK signal or something like this ?

Do you have any information about mainboard in this DELL monitor ? I would like to find problem.

Thanks a lot
Radek

[pvb_astro]

Re: Dell 2408WFPb / 2408WFP - No Power

my 2408 failed, I've applied information shown here to debug it. Seemed like the voltages were OK, symptom was-- plug power cord in and the power LED might flash for 1/2 second, but then stayed off and would not power on. Probing today in the power supply, all seemed OK, voltages normal, then I look down and the green LED is on-- monitor is working. I'm thinking there's a cold solder joint! I'm going to let it run for awhile and see what happens. I guess if there's a repetition, I'll have to disassemble the supply and just reheat all the solder joints.

[brainwash]

Re: Dell 2408WFPb / 2408WFP - No Power

Hi all,
My monitor started not turning on: either the LED was on but the power button did nothing or the LED would blink for a 1/10s when the monitor was plugged in.
Initially, I had something around 23VDC on the power board output but also 50VAC superimposed on the output (referenced to chassis). The USB ports were receiving 5V.
I've replaced the old TL431 (which tested fine with 1k resistor on a power supply) with another one (also tested) but now I get 65VDC output and a burning smell from the area around R611.

All the diodes and transistors were tested, nothing was shorted, nothing seems open.
The new TL431 shows 2.65V on the reference terminal when powered on, I was expecting ~2.5V.

I don't see any bulging caps and no obvious burn damage.

What is the expected output of the voltage supply?
What voltages waveforms should I look for?

EDIT:
Ok, I did some thinking, the TL431 reference si given by Vout going into the R716/R715 divider which means the output will be 2.5V*(1+28/3.32)=23.58V. I've put back the old TL431 and now I'm getting 23.4V on the output with no AC voltage. Still not sure what's happening.

[gipiv]

Re: Dell 2408WFPb / 2408WFP - No Power

Hi to All,
I am in much the same predicament as Sparrow.
I have a very very very basic knowledge of the matter, but thanks to these forums I have successfully repaired several monitors, except for this one...
The readings (very basic VOM) seem to quite regular throughout the board (comparing them to previous/other threads except for the diode D701 & D703 which seem to be giving a value regardless of how I place the tester leads on the pins.
The only thing I haven't tried is measuring the Vcc value of IC601 as I am not sure how too.
I've uploaded some images which I hope are of some use.
At this point short of changing all the caps, IC's, shunts, I am not sure of what to do.
Thanks in advance for any ideas !

[cacvn]

Re: Dell 2408WFPb / 2408WFP - No Power

Hi everybody,
I have a Dell 2408 with this problem: everytime when I turn on the monitor, the power led doesn't go on. I have to unplug the power cord and then pluggin, and repeat 3 or 4 time, after that the monitor works well. But after not use about half an hour, the same thing happened.
I changed two caps 47mf/50v and the IC KA431 from another power supply board of a Dell 2047 (This Dell 2407 is working, only one thing that I cannot access the menu).
Can anyone help me ? Thanks a lot.

[cacvn]

Re: Dell 2408WFPb / 2408WFP - No Power

hi

[studioflux]

Re: Dell 2408WFPb / 2408WFP - No Power

i have the same power supply as sparrow, the chip (IC601 AND IC651) were blast due to power over loading.

[sparrow]

Re: Dell 2408WFPb / 2408WFP - No Power

Did you test the diode mentioned previously

I tested the diodes with a multimeter and they seemed to check out, unless I'm missing something.

[selldoor]

Re: Dell 2408WFPb / 2408WFP - No Power

Did you test the diode mentioned previously

[sparrow]

Re: Dell 2408WFPb / 2408WFP - No Power

The startup capacitor is C606 also C611 47uf 50V could be defective as also IC701 TL431 as per two of the posters here.

OK thanks. I tried replacing both 47uF capacitors. No luck.

[viziks]

Re: Dell 2408WFPb / 2408WFP - No Power

The startup capacitor is C606 also C611 47uf 50V could be defective as also IC701 TL431 as per two of the posters here.

[sparrow]

Re: Dell 2408WFPb / 2408WFP - No Power

Did you find the problem with your monitor _

After letting it sit for 6 months, it started working again for a few months. However, now it's back to doing the same and not turning on.

[selldoor]

Re: Dell 2408WFPb / 2408WFP - No Power

That post is 12 months old? if you need help tell us what is wrong with your monitor
what is doing- not doing and post good clear pictures of YOUR boards

[bubbs198]

Re: Dell 2408WFPb / 2408WFP - No Power

Which capacitor is the startup capacitor?

Did you find the problem with your monitor _

[pavex]

Re: Dell 2408WFPb / 2408WFP - No Power

>Did you check every component to get to that one or ...
There was nothing obviously wrong via visual or resistance checks, and I could hear the ticking in T601, so I guessed the SMPS was trying to start, and I knew it wasn't an excessive load issue. I bought another 2408WFPb with a working power supply, put the two power supply boards side-by-side, focused some resistance checks on the feedback/isolator area and found differences associated w/ IC701. Out of circuit, the KA431 shunt regulator did not behave right per simple VOM checks. Swapped in a replacement KA431, and voila!
>Where in the world did you find the SCH?
It is hand drawn, by me, from scratch. Yeah, I'm retired and have many hours to kill.
>very hard to diagnose
Agreed; normally these switcher power supplies seem to fail with the usual culprits, but not this one.

Hi all,

This is my first post to the forum and I would like to use that to thank bicycle6228 for excellent troubleshooting!

I found one 2408WFPb from bin and it didn't power-up. I also diagnosed the ticking of T601 so I was also thinking that there's something preventing SMPS starting. Then I found this post and desided to replace IC701. It was ka431 and I found from old scrap PC power-supply and ap431. First part I changed and voilã! Fixed the problem.

Very impressed also regarding the schematics!

[sparrow]

Re: Dell 2408WFPb / 2408WFP - No Power

Check diode D701,D602 (mentioned above) could be short.Also check the startup capacitor mentioned above.

Samir

Which capacitor is the startup capacitor?

[viziks]

Re: Dell 2408WFPb / 2408WFP - No Power

I just ran into this same issue, but I'm having a hard time determining what to test. The power button LED flashes temporarily when first plugging it in, but nothing after that.

Check diode D701,D602 (mentioned above) could be short.Also check the startup capacitor mentioned above.

Samir

[sparrow]

Re: Dell 2408WFPb / 2408WFP - No Power

I just ran into this same issue, but I'm having a hard time determining what to test. The power button LED flashes temporarily when first plugging it in, but nothing after that.

[sgt_az]

Re: Dell 2408WFPb / 2408WFP - No Power

I've got a dead 2408WFPb as well. F701 (5A fuse) on the output of the power supply board is open. I've got 23.8vdc on the supply side of F701. I found Q15 (J598) shorted on the board connected to the CN701 connector of the power supply board. I have replacement parts ordered. Bicycle's schematic was helpful. Thanks Bicycle!