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Old 02-19-2014, 08:29 PM   #21
HAHOMETP
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Folks, I am happy to report that my 2408WFPb is once again alive and well.

Turned out it had two problems at the same time.

1. I've built a makeshift inverter tester out of a small 5w fluorescent tube and
a couple of wires with alligator clips. All 7 inverters had fired up this tube
successfully but in all 7 cases I would still get the dreaded 2 seconds to
black. However while performing this test I noticed a faint arcing under one of
the output transformers. Per Davi.p's advice above I measured the resistances of
the secondary windings of all 14 transformers. Thirteen of them showed ~1.650k
but one transformer showed 440k. I desoldered it from the board and looked at it
under the microscope. A small portion (<1mm) of the solid Cu wire that makes up
the secondary winding had burnt off the pin it was wrapped around and soldered
to pretty much the way a glass fuse blows leaving a tiny droplet of metal on the
wire end. I can only explain the 440k (i.e. the non-infinite resistance) of this
broken circuit by the presence of the carbon deposit left by the evaporated Cu
wire which provided at least some path for the multimeter current. Luckily I had
a couple of brand new transformers on hand that I had bought from China ahead of
time so I replaced this transformer right away.

2. Back in June 2013 I had mentioned that one of the CCFL pins had dislodged
itself from the socket and that I had revived the monitor by reinserting it
back. Well, sort of. This monitor is designed in such a way that in order to
remove the inverter board one has to first remove the upper metal chassis that
holds the rest of the boards. This is not readily obvious because you can pivot
the inverter board slightly upward and wiggle it out of the 14 nests holding it
without removing the top portion of the chassis. These nests are essentially the
opposite ends of the CCFL sockets. However if you decide to take this shortcut
you will be putting enough prying pressure on the CCFL pins so that some of them
might jump out of their sockets and this is exactly what had happened with my
monitor. I have a suspicion that its previous owner did try to repair this
monitor at some point because when I opened it up for the first time one of
CCFLs was already disconnected from its socket.

Back then I thought I had managed to reinsert the pin back into its socket.
Turns out I had not. Now I know that these sockets have stiff spring loaded
contacts and that the CCFL ends require quite a bit of pressure to click their
pins home. Since the CCFL pin was just sitting on top of the clip the connection
was intermittent. This explains why for a while I was able to get the monitor to
turn on by squeezing and twisting its plastic body. Finally the arcing in the
poor connection had caused the socket to became charred to a point that the
inverter lost connection to this CCFL altogether.

I took the CCFL completely out of the housing and cleaned up the charred pin
with a blade. I also removed its socket, took it apart and gently sanded the
metal clip with some 400 grit sandpaper. Upon reassembly the monitor fired right
up and has been working fine ever since.

I have documented the above in the attached .pdf file.

One problem still remains a mystery. The big capacitor on the PSU board (C605)
used to show 168v at standby and 292v while working. Back in June I desoldered
it and measured its capacitance. It was 124f, no drift.

Now C605 shows 78v at standby and 239v when working. It's capacitance is still
124f, no drift.

I touched up each and every soldered joint with my Weller pencil thinking that
perhaps there was a cold joint somewhere. Now C605 shows 78v at standby and 241v
when working. I have never dealt with power stages before. I wonder if there is
a voltage divider with a drifting resistor or something like that.
Attached Files
File Type: pdf 2408WFPb.backlight.repair.pdf (5.43 MB, 65 views)

Last edited by HAHOMETP; 02-19-2014 at 08:41 PM..
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Old 02-19-2014, 08:58 PM   #22
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Thanks for the follow up. So what kind of 5W CFL lamp did you use? T5 type?
http://www.badcaps.net/forum/showthread.php?t=19987
I did not read your PDF yet.
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Voltage Regulator (LDO) testing:
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Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

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Last edited by budm; 02-19-2014 at 08:59 PM..
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Old 02-20-2014, 02:03 PM   #23
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

"Now C605 shows 78v at standby and 241v"
It is not your meter battery low?
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Old 02-21-2014, 07:29 AM   #24
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

It is a bit low, but I don't think the values would be any different. Besides, this was measured with the old trusty $400 American made Wavetek 2030 that has been faithfully serving me for the past 20+ years. Having said that, I will double check the C605 voltage with the old (1958) analogue multimeter that I inherited from my father. I like vintage tools
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Old 08-19-2014, 07:50 PM   #25
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Just picked up yet another 2408WFPb with the classic 2 seconds to black. By swapping parts quickly narrowed the issue down to the bad Inverter PCB.

All 14 transformers in the bad PCB reliably light up the test CCFL (as per the two excellent threads by budm) exactly as in the good PCB.

All 14 transformers in the bad PCB show 5% resistance variance in the secondary winding exactly as in the good PCB.

Sometimes on a rare occasion the bad PCB would light up the screen for a while but inevitably it would go black. Which leads me to suspect a cold solder joint or something stupid like that. I scrutinised every square centimeter of this PCB under the microscope but did not see anything.

Any ideas where to look? The pdf file with pictures is attached.
Attached Files
File Type: pdf Dell.2408.InverterPCB.pdf (5.96 MB, 38 views)
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Old 08-19-2014, 08:59 PM   #26
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

maybe a feedback problem.
if it has feedback through resistors - check the value.
i'v never seen an smd resistor rated for the kind of voltages they will get.
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Old 08-19-2015, 07:39 AM   #27
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Mega Necro Bump here.

So I think I have an idea what the 2 pin connector does.

Its a pair of op-amps that outputs a 2.4mV signal to the T-CON board. Its either an enable signal, an on signal, a status signal, who knows. Because it goes into the 60 pin TTL connector at the top, the 256 pin undocumented chip and all over that board.

My SPICE simulations show that its 2.4mV un-connected, but 550nV when connected to the T-CON.

Does anyone have a working monitor that they can test with? I'm working on an LED conversion for this monitor and will be replicating this circuit.

EDIT: Here is the circuit: http://i.imgur.com/ShUmDJq.png

Last edited by zombielinux; 08-19-2015 at 07:40 AM.. Reason: Added Link
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Old 08-14-2018, 10:04 PM   #28
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Had to take this monitor apart again. It has begun cutting out intermittently. Sometimes it would start up after 10-20 presses of the power button and sometimes would not start at all.

Quote:
Originally Posted by Davi.p View Post
"Now C605 shows 78v at standby and 241v when on"
It is not your meter battery low?
Turns out I had goofed up back then: I measured the voltage on C605 not across its leads but rather between its <+> lead and the chassis. Doh...

The voltage across the C605 is +160v in standby and +290v when the monitor is on. Curiously the second, working well, Dell 2408WFPb (I have three of those in the household) also shows +160v in standby and +290v when on.

Did some testing per this newer, rather useful thread:

The Pin9 (PS-ON, purple wire) on CN701 shows +3.25v after startup.

The L6561 (IC651) shows +14v between Pin8 (Vcc) and Pin6 (same as <-> lead of C605) in both monitors. However I still don't see the +400v on the C605. Ostensibly the PFC circuit is not working in both monitors.

Will be taking apart the third, also working well, Dell 2408WFPb just for the hell of it. Curious what is the voltage across the C605 in that one...

Last edited by HAHOMETP; 08-14-2018 at 10:06 PM..
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Old 07-05-2019, 10:01 AM   #29
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Default Re: Dell 2408WFPb / 2408WFP - 7 seconds to black

Quote:
Originally Posted by HAHOMETP View Post
Had to take this monitor apart again. It has begun cutting out intermittently. Sometimes it would start up after 10-20 presses of the power button and sometimes would not start at all.



Turns out I had goofed up back then: I measured the voltage on C605 not across its leads but rather between its <+> lead and the chassis. Doh...

The voltage across the C605 is +160v in standby and +290v when the monitor is on. Curiously the second, working well, Dell 2408WFPb (I have three of those in the household) also shows +160v in standby and +290v when on.

Did some testing per this newer, rather useful thread:

The Pin9 (PS-ON, purple wire) on CN701 shows +3.25v after startup.

The L6561 (IC651) shows +14v between Pin8 (Vcc) and Pin6 (same as <-> lead of C605) in both monitors. However I still don't see the +400v on the C605. Ostensibly the PFC circuit is not working in both monitors.

Will be taking apart the third, also working well, Dell 2408WFPb just for the hell of it. Curious what is the voltage across the C605 in that one...
Did you ever figure out what the problem was?

I have a similar problem with my 2408WFPb in that the monitor works properly after removing and reattaching the power cable several times. The problem reoccurs only when power is removed again.

I get 165/293Vdc on C605.

More info with pics: https://www.electronicspoint.com/for...eplace.291409/
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