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Old 12-27-2011, 03:21 PM   #1
Ritalin
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Arrow Dell 2408WFPb / 2408WFP - No Power

Greetings fellow badcap'ers... been awhile!

No bulged caps, nothing out of the ordinary, can't find any schematic to see if I'm getting the appropriate power. I did trace it to a transformer where there isn't any power going past it, but when I google it, nothing comes up.

On top it says... "1J.20319.001 / LTSJ

Thoughts/comments on where to start?
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Old 12-27-2011, 03:48 PM   #2
retiredcaps
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Default Re: Dell 2408WFPb / 2408WFP - No Power

1) What is the DC voltage across that small cap (right of IC652)? Is it stable or fluctuating?

2) IC601 is likely your SMPS chip.

a) What is the part number?
b) Find the datasheet.
c) Is there DC voltage on the Vcc pin?
d) Is it stable or fluctuating?

3) I assume you have mains x 1.414 DC voltage across the large filter cap? That is 120 V AC x 1.414 roughly 165 V DC?
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Old 12-22-2012, 12:30 PM   #3
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Ritalin:
I have a 2408WFP with the same no power issue, no bulged caps, shorted semi-conductors, burned components and nothing out of the ordinary.
What did you do to resolve your issue?

retiredcaps:
Answers to your questions posed to Ritalin, as pertains to the unit I have:
1) What is the DC voltage across that small cap (right of IC652)? Is it stable or fluctuating?
= 1.2 volts; very slightly fluctuating. (C716 this is)
2) IC601 is likely your SMPS chip.
a) What is the part number? = 1203P60
b) Find the datasheet. = done.
c) Is there DC voltage on the Vcc pin? = Yes, ~ 7 to 8 volts.
d) Is it stable or fluctuating? = Fluctuating between ~ 7 and 8 volts.

3) I assume you have mains x 1.414 DC voltage across the large filter cap? That is 120 V AC x 1.414 roughly 165 V DC? = Yes, ~ 165 volts.

Ideas / suggestions?
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Old 12-22-2012, 01:32 PM   #4
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Welcome to the forum- Bicycle6228
We can sometimes help a lot more if you post good clear pictures of the whole chassis, and then pictures of each board, front and back (in the same orientation) and close up of connectors, (max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Examples of what is needed

http://www.badcaps.net/forum/attachm...7&d=1280167246

http://www.badcaps.net/forum/attachm...6&d=1280167246

http://www.badcaps.net/forum/attachm...4&d=1280167246

It will be useful if you can say a bit about yourself - skills tools etc.
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Old 12-22-2012, 03:11 PM   #5
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by bicycle6228 View Post
1) What is the DC voltage across that small cap (right of IC652)? Is it stable or fluctuating?
= 1.2 volts; very slightly fluctuating. (C716 this is)
c) Is there DC voltage on the Vcc pin? = Yes, ~ 7 to 8 volts.
Hmm, I haven't worked on a thread that started 1 year ago.

Try changing the startup cap first since it is easy to do. The SMPS needs roughly 12.8V on Vcc pin to startup properly as per page 7 of

http://pdf1.alldatasheet.net/datashe...I/1203P60.html
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Old 01-27-2013, 11:30 AM   #6
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Replacing the shunt regulator, IC701 in the attached schematic, fixed my Dell 2408WFPb power supply.
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Old 01-27-2013, 11:45 AM   #7
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Well done bicycle6228 that is a good fix and the schematic will help others.
Did you check every component to get to that one or did you find it as a common failing somewhere?
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Old 01-27-2013, 03:24 PM   #8
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by bicycle6228 View Post
Replacing the shunt regulator, IC701 in the attached schematic, fixed my Dell 2408WFPb power supply.
Thanks for reporting back. Every now and then we see a failed 431, but it is very hard to diagnose.
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Old 01-27-2013, 06:04 PM   #9
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Thanks for posting the SCH. Where in the world did you find the SCH?
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Old 01-27-2013, 06:35 PM   #10
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Default 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.
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Old 01-27-2013, 06:37 PM   #11
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Yes, it was very generous of you to share that schematic. However, I noticed that some of the resistors and diodes on the schematic do not have values associated with them. It would be great if someone that still has the board out can fill in those missing part numbers/values for future reference.
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Old 02-05-2013, 06:06 PM   #12
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Default Re: Dell 2408WFPb / 2408WFP - No Power

My 2408 has just died, lasted 4 years and 10 months - shame I didn't have a 5 year warranty.....
I'm grateful to bicycle6228 for the schematic and suggestions so far, but swapping the KA431 hasn't worked for me.
Any other (polite) suggestions?
I'll get the scope out and have a poke around, but for now what voltage should be coming out of the PSU?
Thanks
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Old 02-11-2013, 07:46 AM   #13
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by faulkn_c View Post
My 2408 has just died, lasted 4 years and 10 months - shame I didn't have a 5 year warranty.....
I'm grateful to bicycle6228 for the schematic and suggestions so far, but swapping the KA431 hasn't worked for me.
Any other (polite) suggestions?
I'll get the scope out and have a poke around, but for now what voltage should be coming out of the PSU?
Thanks
What voltages do you get/ - are the pins labelled on either of the boards.

Welcome to the forum-
We can help a lot more if you please post good clear pictures of the whole chassis, and then pictures of each board, front and back (in the same orientation) and close up of connectors, (max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Examples of what is needed

http://www.badcaps.net/forum/attachm...7&d=1280167246

http://www.badcaps.net/forum/attachm...6&d=1280167246

http://www.badcaps.net/forum/attachm...4&d=1280167246
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Old 02-11-2013, 09:19 AM   #14
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Thanks for the post, which has prompted me to answer my own question.....

The PSU output is 24v, or 23.8v on my board - I found the culprit:
Referring to bicycle6228's schematic, D602 (EGP10D) was duff, measured 47R both ways.
That would explain why there was low voltage (7-8v) on C606, solely due to the chips internal start-up current source from the HV. The NCP1203 was trying but.....
I replaced it with an UG2D (all I had in my scrap box) and it fired up.

Again thanks to bicycle6228 for the schem.
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Old 09-10-2013, 12:28 PM   #15
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Default 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!
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Old 03-06-2014, 11:39 AM   #16
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Default 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.
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Old 03-07-2014, 09:14 AM   #17
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by sparrow View Post
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

Last edited by viziks; 03-07-2014 at 09:16 AM..
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Old 03-07-2014, 01:38 PM   #18
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by viziks View Post
Check diode D701,D602 (mentioned above) could be short.Also check the startup capacitor mentioned above.

Samir
Which capacitor is the startup capacitor?
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Old 03-01-2015, 05:52 AM   #19
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by bicycle6228 View Post
>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 voilă! Fixed the problem.

Very impressed also regarding the schematics!
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Old 05-03-2015, 09:49 PM   #20
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Default Re: Dell 2408WFPb / 2408WFP - No Power

Quote:
Originally Posted by sparrow View Post
Which capacitor is the startup capacitor?
Did you find the problem with your monitor _
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