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Old 06-05-2013, 01:30 PM   #1
HAHOMETP
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Default Dell 2408WFPb / 2408WFP - 7 seconds to black

Folks,

I am trying to figure out why this Dell shuts off the backlight in a few seconds (sometimes as long as 7 seconds if from a cold start).

Nothing seems to be blown or burnt. No swollen capacitors. No blown fuses. All voltages are present. All 7 CCFLs light up uniformly, no purple hue, no hissing noise from their terminals.

I have attached a file with pictures and a table of voltages on the boards.

Several things seem to be out of the ordinary.

1. The big electrolytic capacitor "C605" on the PSU shows 168v DC when the power LED is OFF. When I turn the monitor ON (power LED is ON) its voltage spikes to about 417v DC and immediately falls to around 292v DC.

I also measured the AC voltage across this cap. It shows 60mv AC when the power LED is OFF. When I turn the monitor ON its voltage rises to about 2.5v AC and in a few seconds (after the CCFLs go out) the voltage falls to around 164mv AC.

2. The "ON/OFF" voltage on the Inverter PCB is 0v DC when the power LED is OFF. When I turn the monitor ON its voltage rises to about 3.25v DC and in a few seconds (after the CCFLs go out) the voltage falls to around 0v DC.

3. The "DIM" voltage on the Inverter PCB is 0v DC when the power LED is OFF. When I turn the monitor ON its voltage rises to about 3.25v DC and in a few seconds (after the CCFLs go out) the voltage falls to around 2.87v DC.

4. The inverter controller OZ9966SN has a mystery chip resistor R015 next to it that shows open circuit. It is slightly larger than the other resistors and does not have any markings on the body except for two parallel thin lines looking like this: ||. Could it be a blown SMD fuse?

5. There is a mystery 2-pin connector (CN002) on the Inverter PCB that goes to the T-Con PCB (LTM240CS05FFCC4LV0.4). I don't know what it does.

Please share any thoughts that you might have. Thanks much!
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Old 06-07-2013, 04:46 PM   #2
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

On a known-good, working 2408WFPb Inverter board I have, I'm measuring about 55KOhms across R015, in circuit, both polarities. I also have wondered about the purpose of the mystery 2-pin connector (CN002) on the Inverter PCB that goes to the T-Con PCB. I have no other ideas about your issue, other than this: I had a 6-CFL lamp 2407WFPb that would similarly cycle off the inverter in a few seconds, and even though all 6 lamps DID illuminate before cycling off, there was one of the 6 that when disconnected allowed the inverter to run OK. I replaced that one lamp and voila- issue resolved.
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Old 06-08-2013, 01:03 PM   #3
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

The 292V across the main cap seems low. Because your power supply has PFC, I would expect to see around 390Vdc.
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Old 06-08-2013, 02:10 PM   #4
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Thank you for the information, bicycle6228! Re-tested my R015: it is 54k in both directions.

Unfortunately I don't have a spare U-shaped CCFL to test this monitor.

I also could not find a datasheet for the inverter protection IC OZ9966SN but I found a service manual for the Funai LC5-D32BB TV that shows how this protection IC is connected and what voltages to expect in all 24 pins. In my case after the CCFLs go out no voltage is present on any of its pins except for +23.35v (Vcc) on Pin 20.

Also in yet another forum I found the way to temporarily turn off the protection IC by grounding Pin.15 via a 10k resistor in order to see if any of the 7 CCFLs are acting differently from the rest:
http://tel-spb.ru/inv_za.html#OZ9966SN
The caption in Russian states: "The way to disable protection in a OZ9966SN". I did this but the monitor still exhibited several seconds to black.

I am beginning to suspect that the inverter circuit might not be the culprit after all. Also, after I disconnect the Inverter PCB from the PSU PCB at CN001 the Pink wire at Pin 12 ("ON/OFF") of CN702 on the PSU still goes from +3.25v down to 0v in a few seconds. I am wondering if the culprit is upstream of the Inverter PCB turning off the ON/OFF voltage to the Inverter PCB thus shutting down the CCFLs?

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Old 06-08-2013, 02:14 PM   #5
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Quote:
Originally Posted by jetadm123 View Post
The 292V across the main cap seems low. Because your power supply has PFC, I would expect to see around 390Vdc.
That's what I thought too jetadm123 and the later findings (please see my reply to bicycle6228 above) reinforced the suspicion that the Inverter PCB was not faulty.

Where would you look for the possible cause of the low voltage on the capacitor? It is not swollen and is not leaking electrolyte.
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Old 06-08-2013, 02:22 PM   #6
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

It does sound like the BL_ON signal is the problem. You do have the monitor connected to the running PC? Picture of the main board and the power supply board? the 292VDC on the large cap is not right either
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Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

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http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809

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Old 06-08-2013, 02:37 PM   #7
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Lightbulb Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Quote:
Originally Posted by HAHOMETP View Post
I am trying to figure out why this Dell shuts off the backlight in a few seconds (sometimes as long as 7 seconds if from a cold start).

Nothing seems to be blown or burnt. No swollen capacitors. No blown fuses. All voltages are present. All 7 CCFLs light up uniformly, no purple hue, no hissing noise from their terminals.

I have attached a file with pictures and a table of voltages on the boards.

Several things seem to be out of the ordinary.

1. The big electrolytic capacitor "C605" on the PSU shows 168v DC when the power LED is OFF. When I turn the monitor ON (power LED is ON) its voltage spikes to about 417v DC and immediately falls to around 292v DC.
Is that the big smoothing/filter capacitor right after the bridge rectifier? Usually the voltage across that big capacitor should be quite constant, no matter whether its on or off. The voltage should be roughly 1.4*Vrms where Vrms is the mains voltage. Possible dry capacitor there?

Quote:
Originally Posted by HAHOMETP View Post
I also measured the AC voltage across this cap. It shows 60mv AC when the power LED is OFF. When I turn the monitor ON its voltage rises to about 2.5v AC and in a few seconds (after the CCFLs go out) the voltage falls to around 164mv AC.
You're measuring the ripples there? Should stay almost the same, unless the capacitor could not charge up enough to maintain the current flow...

Quote:
Originally Posted by HAHOMETP View Post
2. The "ON/OFF" voltage on the Inverter PCB is 0v DC when the power LED is OFF. When I turn the monitor ON its voltage rises to about 3.25v DC and in a few seconds (after the CCFLs go out) the voltage falls to around 0v DC.

3. The "DIM" voltage on the Inverter PCB is 0v DC when the power LED is OFF. When I turn the monitor ON its voltage rises to about 3.25v DC and in a few seconds (after the CCFLs go out) the voltage falls to around 2.87v DC.
You could hardwire those Lamp enable control pins. Usually I would use a 1K resistor between the +5V line and the control line to force the backlights on. If it stays on then the problem is not the inverter section.

Quote:
Originally Posted by HAHOMETP View Post
Unfortunately I don't have a spare U-shaped CCFL to test this monitor.

I also could not find a datasheet for the inverter protection IC OZ9966SN but I found a service manual for the Funai LC5-D32BB TV that shows how this protection IC is connected and what voltages to expect in all 24 pins. In my case after the CCFLs go out no voltage is present on any of the pins except for +23.35v (Vcc) on Pin 20.

Also in yet another forum I found the way to temporarily turn off the protection IC by grounding Pin.15 via a 10k resistor in order to see if any of the 7 CCFLs are acting differently from the rest:
http://tel-spb.ru/inv_za.html#OZ9966SN
I did this but the monitor still exhibited several seconds to black.

I am beginning to suspect that the inverter circuit might not be the culprit after all. Also, after I disconnect the Inverter PCB from the PSU PCB at CN001 the Pink wire at Pin 12 ("ON/OFF") of CN702 on the PSU still goes from +3.25v down to 0v in a few seconds. I am wondering if the culprit is upstream of the Inverter PCB turning off the ON/OFF voltage to the Inverter PCB thus shutting down the CCFLs?
If you have a bench power supply, try powering the inverter section directly, plus the hardwired lamp enable to keep the lamps on. Make sure to disconnect the mains power during this test.
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Old 06-08-2013, 04:09 PM   #8
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Okay, here's my play on it:

A similar power supply (made by the same company) is used on the Dell 2407 series monitor. Some users have reported a bad main cap as the source of their problems. Your main cap appears to be made by the same company. While the main cap rarely fails, it's not impossible. Eventhough you're measuring 292V (versus 385V) across the main cap, you're still getting the 23V output at the wiring harness, which is really odd. I believe the main cap is something like 120uf 450V? If you've got a spare, even if it's only 100uf, try substituting it in.

Last edited by jetadm123; 06-08-2013 at 04:11 PM..
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Old 06-08-2013, 05:39 PM   #9
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

"Is that the big smoothing/filter capacitor right after the bridge rectifier? Usually the voltage across that big capacitor should be quite constant, no matter whether its on or off. The voltage should be roughly 1.4*Vrms where Vrms is the mains voltage."

This power supply has PFC Voltage booster, at standby the voltage on the cap is 120 x 1.414 = 169Vdc, when power switch is activated, the PFC will kick in to boost the voltage to around 380~390VDC.

"You could hardwire those Lamp enable control pins. Usually I would use a 1K resistor between the +5V line and the control line to force the backlights on. If it stays on then the problem is not the inverter section." You will have to first disconnect this control line that goes back to the main board since this signal line is held LO (or stuck LO) by the switched transistor on the main board.

The power supply only has one output (rectified by 3 diode in parallel), the output is then step down (done by the main board) to multiple voltages to run the main board.
http://s807.photobucket.com/user/bud...?sort=3&page=1

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Old 06-08-2013, 09:01 PM   #10
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

OK, I got it working now. An outstanding monitor, BTW. Was well worth the effort. Below is the recap for the archives.

I bought it with the classic "two-seconds-to-black" symptom. Took it apart and began to diagnose. First of all I went into the inverter board. In this particular monitor the backlight is comprised of 7 U-shaped CCFLs. Instead of the standard wires with a 2-pin jack each end has a metal pin protruding outside the glass envelope. The lamps are arranged in a grid in the backlight housing. There is a black plastic piece on the right edge of this housing which has spring loaded "forks" that click onto these pins providing electrical contact. The Inverter PCB has 7 fiberglass tabs connected with the secondaries of the 14 step up transformers, 2 transformers per each lamp. In order to remove the Inverter PCB it has to be lifted by its left edge so that the board pivots on its right edge pretty much like a grand piano lid on its hinge. When this assembly pivots (roughly about 30) its tabs with "forks" pivot on their respective CCFL pins. The whole design is rather flimsy and fragile looking. Imagine applying a bending stress to a U-shaped glass tube approximately 2mm in diameter x 530mm long and then applying yet more force in order to slide the 7 PCB tabs out of this whole contraption all while trying not to allow the 14 lamp pins jump out of their respective "forks" much less break the tubes.

So, when I carefully removed the Inverter PCB I did notice that a few lamps were not nested in their respective receptacles. Why they jumped out on the previous owner will forever remain a mystery. I scrutinized the inverter board under the microscope for cold solder joints, tested all p-n junctions with the multimeter, tested all the capacitors yet didn't see anything wrong. I reconnected the 7 lamps back to their respective inverters making sure they all clicked home and from that point forward have been conducting all the tests on the monitor lying with its face down on the rubberized grounded antistatic mat covering the work bench. The PC was on the desk across the room.

I probably would've kept on chasing my own tail trying to figure out why the monitor refused to work despite all the voltages present were it not for the hint by budm above basically telling me to plug the damned thing in. Doh! I am such a moron. All of a sudden it has dawned on me that the monitor might have been simply going to sleep in the absence of the signal.

I reconnected the monitor to the PC's VGA port as a secondary display and lo and behold the Dell logo popped up followed by the "check the input" prompt. I switched to VGA and voila - the monitor came to life.

So looking back I'd venture a guess that at first it was indeed the "2 seconds to black" because several CCFLs jumped out of their nests. When I fixed that the monitor switched to the "7 seconds to black" which was nothing else but the sleep mode. Of course I didn't know that because the monitor was not connected to the PC and I didn't see the prompts on the display because the monitor was lying face down.

I took some measurments on the working monitor.

1. The big electrolytic capacitor "C605" on the PSU is 150f x 450v (124f actual, no leak, no drift). It shows stable 168v DC when the monitor is OFF and stable 292v DC when ON.

I even took it out and substituted with a known good capacitor 150f x 450v (140f actual), no leak, no drift that I last used to troubleshoot the very dead 24" Acer (into whose screen I am staring right now while typing this text). The voltages across this cap remained the same as above.

2. The small start up electrolytic capacitor next to it was also good: 47f x 50v (41f actual) no leak, no drift.

3. The "ON/OFF" voltage on the Inverter PCB (PNK wire at Pin 12 of the CN702 on the PSU) is 0v DC when the power LED is OFF. When the monitor is ON its voltage rises to about 3.25v DC and stays there. It falls down to zero when the monitor goes into the sleep mode.

4. The "DIM" voltage on the Inverter PCB (BLK wire at Pin 13 of the CN702 on the PSU) is 0v DC when the power LED is OFF. When the monitor is ON its voltage rises to about 1.91v DC and stays there.

I hope that we have documented this particular monitor reasonably well in this thread for the archives. Thanks much to all participants!
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Old 06-08-2013, 09:33 PM   #11
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Glad you found the problem and thanks for the hint on the lamp connections, will have to keep that inspection in my memory when I work on another one in the future, learning something new everyday.
I do sometime forget about the simple stuff like forgetting that I set the PC on my work bench to go to sleep after 4 hours. Sometime simple thing like that can trip you, the bad part is that some people get upset at me when I ask if the PC is running or not and take it as an insult, the fact is that since they have been working on the monitor for hours and the PC has gone into sleep mode, so when they try it out, the monitor will do the 2-second to black (some monitor will do that in 2~3 seconds if it does not see the video signal).

Example: http://www.badcaps.net/forum/showthr...585#post345585

By the way, the DC voltage on mine has about 385VDC.

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Old 06-08-2013, 11:14 PM   #12
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Wink Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Quote:
Originally Posted by budm View Post
This power supply has PFC Voltage booster, at standby the voltage on the cap is 120 x 1.414 = 169Vdc, when power switch is activated, the PFC will kick in to boost the voltage to around 380~390VDC.
The mains voltage in my place is 230V~250V thus my readings are always around 330V~380V+...

Quote:
Originally Posted by budm View Post
You will have to first disconnect this control line that goes back to the main board since this signal line is held LO (or stuck LO) by the switched transistor on the main board.

The power supply only has one output (rectified by 3 diode in parallel), the output is then step down (done by the main board) to multiple voltages to run the main board.
http://s807.photobucket.com/user/bud...?sort=3&page=1
That's the usual routine, simply disconnect the Dscaler board first. See attachments. That's one of my "franken" testing to check the LCD panel, kinda messy but you should be able to make out the setup there. The diodes are blocking the DC to the transformer, and I just power up the inverter section only. With this method, I could test the inverter section, Tcon board and Dscaler board easily..
Attached Images
File Type: jpg 1k.jpg (121.6 KB, 138 views)
File Type: jpg 1m.jpg (170.0 KB, 93 views)

Last edited by lexwalker; 06-08-2013 at 11:24 PM..
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Old 06-08-2013, 11:42 PM   #13
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

"The mains voltage in my place is 230V~250V thus my readings are always around 330V~380V+..." That will be true if the power supply does not have PFC Voltage booster (@250vac the DC will be about 354VDC), as you can see the picture of the Dell, it has PFC IC for the PFC Voltage booster circuit. Most newer TV/Monitor required to have PF as close to 1 as possible.
If you remove the cover off your power supply board, then I can tell if it has PFC Voltage Booster or not.
See PDF of example of the PFC V booster.
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File Type: jpg PFC V BOOSTER.JPG (255.8 KB, 83 views)
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File Type: pdf TDA4863 PFC.pdf (293.2 KB, 67 views)

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Old 06-09-2013, 12:58 AM   #14
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Quote:
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the bad part is that some people get upset at me when I ask if the PC is running or not and take it as an insult,
Yep, we get people who test and "repair" monitors without ever hooking up a source on a weekly basis now.

Many many years ago I worked on a laser printer for someone and fixed it after hours.

The next day, the user phoned me promptly at 8am and screamed and accused me of ruining her printer and how I had to get my a** over there NOW to fix it.

I, calmly, asked her if it was turned on. She screamed at me for a few more seconds and then said who the f*** turned it off?
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Old 06-09-2013, 01:04 AM   #15
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

LOL! I just do not understand why when we ask simple questions and they get upset, we are not in front of the unit to be able to tell what was done or was not done, or get the answer like the voltages checked OK on the LDOs but found out later after asking them to post the results then we see that some of them are wrong.

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Old 06-09-2013, 06:52 AM   #16
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Quote:
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Sometime simple thing like that can trip you, the bad part is that some people get upset at me when I ask if the PC is running or not and take it as an insult, the fact is that since they have been working on the monitor for hours and the PC has gone into sleep mode, so when they try it out, the monitor will do the 2-second to black (some monitor will do that in 2~3 seconds if it does not see the video signal).
Naah, I am in fact very grateful. Many years ago, almost 1/4 century to be exact, I, relatively fresh from the college kid and therefore full of myself, was hired to work for a high tech company and was lucky to report directly to their tech lead, a brilliant engineer then in his late 50s. Once we returned from lunch and the PWM circuit I was working on suddenly would refuse to trace the inclination of the tilt sensor. My boss walked into the lab, saw me busting my brain and asked (no kidding!) if the power were on. I said something like of course it is on, it is always on, feeling rather insulted. Then I actually looked into the mess of cords in the corner of the bench and discovered that the lab power supply was in fact disconnected from the wall outlet. My boss chucked and walked away and I was forever humbled having discovered first hand that assumption was indeed the mother of all screw ups.

Whenever I happen to work on a TV set I usually lie it face down on a large glass dinner table in the living room. I also have a large trapezoidal mirror that I scavenged off a dead projection TV that I disassembled for parts a while ago. I typically place this mirror on the floor face up right underneath the table. This way I can see both the circuitry in the back of the TV and the picture on its screen all at the same time.

This time around since the PC monitor was so relatively small and the problem seemed so easy to troubleshoot I didn't bother to carry the equipment to the living room. You know the rest of the story.

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Old 06-09-2013, 08:00 AM   #17
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Thanks for sharing your fix with us. Also, I am reminded of the time I was at a party at a co-worker's home. We decided to make some coffee and added the water and coffee to the coffee maker and pressed the on/off switch. No light/activity. A number of engineers started looking at it and made suggestions as what could be wrong. Finally, I asked the host if there was any "special" way of turning it on. She replied yes: "try plugging it in"

Your power supply does has PFC and I am still puzzled over the low voltage across the main cap, which is why I suggested trying a different main cap.
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Old 06-09-2013, 02:37 PM   #18
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Hilarious. "How many engineers would it take to brew a cup of coffee?"

Oh, I forgot to mention yet another interim silly move by the yours truly. In the very beginning I tested the inverter board step-up transformers one by one. Five out of seven tested fine but the last two, which are slightly offset in the budm's picture above, showed infinity in their secondaries. Excited by the fact that I have solved the problem so quickly I went on ebay, found those transformers and ordered two from China for less than 5 bux each. In three weeks the transformers showed up in the mail. Naturally I first tested them prior to soldering them into the circuit. To my surprise they also show infinity in their secondaries. I almost went back to ebay screaming bloody hell to file a dispute but then I asked myself, what are the odds of two transformers go bad at the same time, and two new ones being bad exactly the same way from the factory? Well, after looking carefully at the transformer through my Magnavisor I suddenly realised that the two pins on the end of the ferrite core that were facing the CCFL did not belong to the secondary. One of them did but the other one didn't. It was just an extra reinforcement leg. The remaining pin for the secondary was located on... that same end of the ferrite core where the two primary pins were located. Why was it designed in such an unconventional way? Beats me. But I did buy two transformers that I didn't need and wasted three weeks time while waiting for them to arrive only because I *assumed* that the two pins on the same end belonged to the same winding like was the case with like a gazillion assorted transformers that have passed through my hands over the past 4 decades.

Assumption is indeed the mother of all screw-ups.

No clue why the voltage across the big cap in my power supply is almost 25% less than it should be. I am not too familiar with boosters. Replacing the cap did not change the voltage even by 1v. The monitor works fine though.

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Old 01-29-2014, 02:10 PM   #19
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Hello again, folks.

Well, this monitor is acting up again albeit in a slightly different fashion.

Recently when woken up from the sleep approximately 1/3 of the left side of the screen would remain dark followed by the classic two seconds to black syndrome. Regretfully this happens way too quickly to snap a picture.

What puzzles me is the bizarre pattern of the dark spot. The CCFL backlight in this monitor is comprised of seven U-shaped lamps. A failure of one of those lamps (or of its inverter) should result in a horizontal dark band, not vertical. I can't figure out what might cause all seven bulb be to be significantly darker in their "U" parts.

A couple of days ago the backlight has finally quit completely. Now upon the start up the backlight goes from barely perceivable grey to black in about two seconds.

Also when the monitor was still working I had noticed that immediately upon turning it on the display would be quite a bit dimmer and yellower than after a few hours of working. Could this be an indication of worn out lamps?

I have attached a .pdf file with pictures and my comments.
Attached Files
File Type: pdf Dell.2408WFPb.CCFL.pdf (2.18 MB, 244 views)
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Old 01-30-2014, 07:02 PM   #20
Davi.p
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

"Could this be an indication of worn out lamps?"
Yes, it seems the major explanation..
check also the resistance on the outputs of the 14 transformers and compare them..
you can also try to disable the protection to see if the lamps stays on, if yes you can trim the protection level by changing one/two resistors like i said in one of my past messages, and continue using the same lamps until them are more exhausted.
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