Dell 2408WFPb display inoperative

[magorkonig]

Greetings all,

The problem: blackout happened at the house and now monitor when plugged in will give a green power button for a second and then go to yellow. Power button does not function and there is no display using any input source. Using the great information I have found on this forum I have obtained the following test results:

Power board A
118.9VAC both sides of F601
118.7VAC into CN603

Power board B
23.4VDC both sides of F701
159.6Volts across large cap C605
Vcc pin on IC601 is 14.8VDC
D602 reads OL and 0.47 (ok)
Voltage across C716 is 18.3VDC stable
10 pin connector CN701: 0/0/0/23.4/23.4/0/23.4/2.80/empty/0 (all VDC)

Logic board
0VDC on either side of F1 (surface mounted but fuse is not open)

Since I have 23.4VDC on a number of pins to the logic board from CN701 I'm not sure if what I have is a power supply problem or something further downstream on the logic board. I have several donor boards from another monitor that I might be able to match and swap components but at this point I feel kind of blind.

I have many years of experience in automotive DC circuits and while I remember most of my schooling on component operation and testing I haven't had a lot of practice at the board level. I have a PDI 890 automotive meter who's only limitation is capacitor testing.

Thank you to all the contributors to this forum and any guidance you could offer.

[boyebaran]

Re: Dell 2408WFPb display inoperative

hi
please check F1 line (befor and after f1-which part connect to this fuse.).may you take a clear photo of around F1?
measure ohm F1 to ground .
always in switching power supply if you have problem so you dont have any dc output voltage.i think it is logic board problem.

[magorkonig]

Re: Dell 2408WFPb display inoperative

I have included the requested photo. I don't see anything mounted on the board on the left side of the fuse and the other side according to the etching looks like it's connected to a couple of caps (the large one above it and the tiny one beside it). The readings to ground are left side= 1.3kohms and right side= 4.8kohms. Maybe there is no voltage to that fuse because it isn't used??

[boyebaran]

Re: Dell 2408WFPb display inoperative

"The readings to ground are left side= 1.3kohms and right side= 4.8kohms:"
it is fuse and both side must be same ohm.
"is no voltage to that fuse because it isn't used??"
No.it have voltage.but seems that it is for external adapter.
please first read to topic below:
https://www.badcaps.net/forum/showthread.php?t=46546
https://www.badcaps.net/forum/showthread.php?t=29271

[magorkonig]

Re: Dell 2408WFPb display inoperative

So the whole family has been sick so it's been a while since I have worked on the monitor. I read through both posts you listed and interestingly enough when I plugged it in to do more testing following those posts it powered on. I though maybe all the disconnecting and reconnecting things may have resolved it. I put it back together and went to install it in the house and it was back to the failed state again. So it's back apart on my bench and all the previous readings I listed are the same.

[jetadm123]

Re: Dell 2408WFPb display inoperative

Interesting that you're getting 23V output at CN701 and only 160VDC across C605. When the the power supply is fully operational, the PFC circuit (IC651) should boost the voltage close to 400VDC across C605.

[magorkonig]

Re: Dell 2408WFPb display inoperative

So from I have read the standby voltage across C605 should be ~1.4 x input (120VAC) which is what it is. But as you state above, when operational it should be 400VDC. Given that not all the wires in connector CN701 have 24 volts could it be because C605 is not getting the stepped up voltage? And since the beginning as soon as you plug the monitor in the front power light is a solid amber color and doesn't change. Maybe something is preventing the power supply from coming out of standby mode. I'm just throwing out ideas. Please forgive my lack of knowledge on system operation.

[jetadm123]

Re: Dell 2408WFPb display inoperative

I think the schematic below is for your power supply. It was drawn up by some other member a while back. I suggest you check the Vcc voltage of IC651 (L6561D) to see if it meets the minimum startup voltage. Also, since you say the monitor did work a bit after you hooked it up, then there could possibly be a bad solder joint?

[magorkonig]

Re: Dell 2408WFPb display inoperative

Thanks for the schematic jetadm123. I checked the Vcc voltage per the diagram and when I measured pin 8 (Vcc) with the black probe on ground for the board I got -77.18VDC. When I checked the voltage using the IC ground pin (#6) it came up as 0.517VDC. I guess I need to know which is the correct way to read the voltage on Vcc line and depending on the answer I can assess my results. I will get out my magnifier and check all the solder joints again.

[jetadm123]

Re: Dell 2408WFPb display inoperative

You are measuring the L6561 Vcc on the "hot" side of the power supply. Therefore, your ground reference should be the negative lead of the main cap, C605. Pin 6 of the L6561 chip is also connected to the neg lead of C605. Be careful on the "hot" side as you are dealing with line voltage. Since you are reading .5 VDC for Vcc, something is clearly preventing the L6561 from starting. Suggest you check Q654 for shorts.

[magorkonig]

Re: Dell 2408WFPb display inoperative

Checked Q654 for shorts to ground on the base, emitter and collector. None were found. Continuity to surrounding components per the schematic were also ok. I will check the solder joints tonight and see if I can come up with something. By the diagram it looks like IC652 controls Q654 so I'll be sure to check the joints on that one.

[budm]

Re: Dell 2408WFPb display inoperative

Connector CN701 pin that is connected to the resistor R677 (1K), that pin is the PS-ON pin to turn on the PFC circuit, so it should have >2.5VDC from the main board to turn on Q656, which then turns on OPTO IC652, Q654 which supplies the VCC to VCC pin 8 of L6561 PFC IC.
Please note that I do not know how accurate the SCH is.

[stj]

Re: Dell 2408WFPb display inoperative

have the startup cap(s) been changed yet?
btw, from the foto i would re-solder the mains inlet connector.

[jetadm123]

Re: Dell 2408WFPb display inoperative

budm made a good point- the schematic I provided is not factory supplied and it's accuracy cannot be guaranteed. You should verify that the points you are testing actually match those on the schematic. You might want to verify that the 2.8V you read on CN701 is feeding R677. If yes, then check everything budm mentioned in his post.

As for the startup cap (c620?) stj mentioned, it should be connected to pin 8 of the L6561. It's value is generally something under 47uf. Try replacing it. If you don't have the correct value, then try heating up the cap with a hair dryer to see if affects the monitor startup.

[budm]

Re: Dell 2408WFPb display inoperative

Power board B
23.4VDC both sides of F701
159.6Volts across large cap C605
Vcc pin on IC601 is 14.8VDC
D602 reads OL and 0.47 (ok)
Voltage across C716 is 18.3VDC stable
10 pin connector CN701: 0/0/0/23.4/23.4/0/23.4/2.80/empty/0 (all VDC)
I do not suspect the startup caps (C606 (47/50V), C611 (47/50V) otherwise you will not have the 23.4V output, the PFC Voltage booster is not working right now.
Check Q504 Emitter DCV, it should have around 15V (about the same Voltage at VCC pin 6 of the IC 200A6).
Check the Q505 E-B DCV to make sure that transistor is ON to provide the VCC for the PFC IC.
Check the resistance between VCC and GND of PFC IC.
All of the above are based on the SCH we have.

[magorkonig]

Re: Dell 2408WFPb display inoperative

So the 2.8V i am reading in CN701 pin 3 (by the diagram) is not feeding R677. Pin 9 is the one feeding R677 and it is 0V.

stj: I have not replaced any parts yet.

budm: Resistance between Vcc and Gnd on the PFC IC is OL. I think we are on to something with the lack of voltage to pin 9 on CN701 to feed Q656 and so on. I have looked all over the power and logic boards to find Q504 and Q505 and have been unable to find them.

[budm]

Re: Dell 2408WFPb display inoperative

Run the board by itself and you can force on that Q656 with 3V battery to turn on that Transistor to see if the PFC will come alive.
BTW, the drawing shows the connector as 10 Pin but the drawing has only 9 pins, I guess pin 2 is blank so it is not drawn in..

[magorkonig]

Re: Dell 2408WFPb display inoperative

Sure enough, with a 3V batt on pins 1 and 9 of CN701 with it disconnected from the logic board got the PFC to come alive. I had 401VDC across large cap C605. Pulling logic board now to check under magnifier. Will update with any findings.

[creating]

Re: Dell 2408WFPb display inoperative

Sure enough, with a 3V batt on pins 1 and 9 of CN701 with it disconnected from the logic board got the PFC to come alive. I had 401VDC across large cap C605. Pulling logic board now to check under magnifier. Will update with any findings.

Good luck! Please do report back - watching this with interest as I'm suffering a very similar fault. My 2408b fails to start up usually showing just a momentary flash of the green power LED when connected to mains. However, unplugging/replugging 20-30 times it sometimes goes into state you describe with green going to the stuck amber state. However if I keep retrying then the monitor will usually start (though this is getting worse, used to take only 1 or 2 attempts). Once it starts, the monitor works fine in normal mode, standby, sleep until mains power is removed.

I also had the monitor apart and tested the 240V mains board (UK) and the secondary power board - all seemed ok and I was getting a solid 23.8V on the output side 100% of the time (although obviously under load). I couldn't see anything obvious on the logic board so gave up, put it back together and it fired up after a few attempts and I haven't switched it off since

[creating]

Re: Dell 2408WFPb display inoperative

^^ er, typo I can't edit... ** 23.8V 100% of the time (although obviously NOT under load).