Dell 2209WA turns off after a while

[Sanjuro]

Hi, I'm trying to fix my monitor but it would appear that its something more serious than just bad caps or cold joints.

Monitor history:
I used it for about 6 years and then the monitor was stored unused for about 1.5 years in my room but now I wanted to use it as a small TV so I connected it to the tuner. It worked for a couple of hours and then it started to turn off and on a couple of times and then it stayed off (blue LED indicator still ON). I plugged it in the next day and it worked again but after about 2h it started to go black again.

What I did:
1. I replaced some of the caps that I had and re-soldered the connectors to transformers but no change. The monitor still turns off after some time.
2. Connected it to a PC - still nothing


It looks like there's no 12/24V from the power supply. But I don't know how to make sure and what to do with this information.
I don't know if this helps but when the monitor was working as a PC screen it was set to brightness 12% but now as I wanted it to work as a TV in a well lit room I set it to 100%. After it started failing I tried to go back to 12% but it didn't change anything.


TL;DR
Monitor turns off after some time
Blue LED is ON
Replacing some caps did not help
missing 12/24V?


I'm attaching some pictures with the voltage measurements that I took (all other boards were disconnected)

Also links to the IC datasheets:
https://lib.chipdip.ru/923/DOC000923326.pdf
https://www.promelec.ru/pdf/SG6901.pdf

[Sanjuro]

Re: Dell 2209WA turns off after a while

Hi, again
So it would appear that there's no 24V because the transformer T850 is not getting the power.
Please, correct me if I'm wrong but the transformer should get the power from the Q850 transistor but it's not getting it? The IC that should be driving the Q850 is U850 but there is no power on the pin12 of it. Am I measuring and understanding it correctly?

Also, the service manual is getting me a bit confused. I've attached the part that I'm talking here. It says that there should be 354V on the pin8 of the U850 but I'm getting 0.3V there. I'm however getting ~379V but on pin8 of U950 which is a HV pin on that IC. Is this just a coincidence or is the manual wrong?



I've done some additional measurements so I'm attaching new images.
There's also:
around 16.5V at pin6 (VDD) of U950
C822 = fluctuating between 16.7V and 17.1V
C818 = around 0.65V
C812 = 2.84V

All measurements made without any other boards attached.

[budm]

Re: Dell 2209WA turns off after a while

1) When making Voltage measurement in the Hot side of the cirucit you need to use the negative leg of the main filter cap as the GND. REF. for your meter.
2) There will be no 24V or 400VDC PFC Boosted Voltage until the PS-ON is present. The 24V circuit needs that PFC Boosted Voltage to run. You should see about 400VDC when measured right between the two legs of the main filter cap when power switch is activated.
3) is the fusible resistor FR850 still OK?
4) That flow chart is for Dead power board (NO LED), you do have LED on per your post #2 so your power supply is NOT DEAD..

[Sanjuro]

Re: Dell 2209WA turns off after a while

1) Oh, I see. My reasoning was to try tracing from the negative leg and see where it disappears. I hope I didn't damage anything.

2) I connected all boards and I tried measuring it again:
on the main cap and I get:
switch off = 295V
switch on = momentarily 402V and then it settles at 380

24V (between pin1 and 4 on the connector that goes to the inverter)
switch off = 1mV
switch on = rises and settles at around 31.5mV

So still no 24V. Would there still be 24V here if the IC detected a problem with the inverter/CFL tube?

3) Yes, I just tested and FR850, FR950, FR851 are ok (almost the same ohm as connecting the leads to each other)

4) OK

Thank you for your reply.

[nihilianth]

Re: Dell 2209WA turns off after a while

Hey there,

this might not be too useful for you, but I've had the same issue with my Dell U2711 for some weeks now. It used to work for an hour, then started to flicker lightly for some minutes and then it went black. After some hours you could turn it on but it went black after a while again.

I've taken a closer look at the pcbs yesterday, and I've noticed that one of the two ceramic capacitors on the secondary side was a little bit darker than the other (Those were two 101k 1kV rated (brown) ceramic caps). I've replaced both yesterday. So far, it was in use for a couple of hours at a time and hasn't shown any symptoms yet.

Cheers

[budm]

Re: Dell 2209WA turns off after a while

1) So are you getting 380DVC at both ends of FR850 with ref. to negative leg of the main cap?
2) Red body Fuses F851, 852 by the 24V output connector are OK?
3) Fusible resistor FR851 is for PFC circuit.
4) I cannot see where Fusible resistor FR950 is.
BTW, so right now the Monitor will work but will turn off (Backlights go off (no 24VDC) but power LED is still on) after a while still?
What is the PFC Voltage when monitor goes off?

"switch off = 295V" BTW, that is low for standby, the Voltage on the main cap during standby should be about 230VAC incoming AC x 1.414 = 325VDC.

[Sanjuro]

Re: Dell 2209WA turns off after a while

@budm
1) Yes

2) Yes - F851 and F852 are OK

3) OK

4) It's right below FR850. Sorry for the bad photos.


All the measurements that I've done was after the monitor went dark. I don't have the measurements from the working state.

BTW, so right now the Monitor will work but will turn off (Backlights go off (no 24VDC) but power LED is still on) after a while still?
What is the PFC Voltage when monitor goes off?

Well, I connected all the boards together today and it's still dark so I don't know if it's not intermittent anymore or if my measurements did something. The blue LED is still working fine though. It switches ON when I press the power button.
Maybe the time I spent doing measurements on "broken" state has finally finished the faulty component?

Could the low standby Voltage mean that the main cap is bad?


@nihilianth
Thx, I'll take a look.

[budm]

Re: Dell 2209WA turns off after a while

You need to find out if then PFC Boosted Voltage is still present or not when Monitor losts the backlights, if no PFC = No 24V = No backlights.

[Sanjuro]

Re: Dell 2209WA turns off after a while

I connected everything right now including a video source from tuner.
switch OFF = 308V on main cap
switch ON = 377V on main cap

Still no backlight and no 24V. But BlueLED ON.

[budm]

Re: Dell 2209WA turns off after a while

But the 24V comes on and the backlights come on and stay on for a while correct?
What happen if you disconnect the inverter board, will the 24V come on and stay on?

[Sanjuro]

Re: Dell 2209WA turns off after a while

Not any more. I connected everything together yesterday and it's not turning on even for a minute.
The blue LED is ON but I don't get the picture anymore. No backlight.
As I mention, maybe the time I spent doing measurements on "broken" state has finally finished the faulty component?


So the measurement below was made when ther's no backlight:
switch OFF = 308V on main cap
switch ON = 377V on main cap
no backlight and no 24V. But BlueLED ON


BTW, thank you for your patience with a newbie like me.

[Sanjuro]

Re: Dell 2209WA turns off after a while

Ok, so the monitor turned back on again today. Now it's just making a fool out of me.
But at least I know I didn't break it more.

[Sanjuro]

Re: Dell 2209WA turns off after a while

But the 24V comes on and the backlights come on and stay on for a while correct?
What happen if you disconnect the inverter board, will the 24V come on and stay on?

Since the monitor turned back on I tried what you suggested.
When I disconnect the inverter board the 24V still comes on and stays on.
This should rule out any problems with the inverter/CFL right?

Maybe I could use a hairdryer to warm the main PCB in different corners and see if it turns off? Or is this a bad idea?

PS. I just realized I wrote 3 posts in a row. Sorry about that.

[arthurfbs]

Re: Dell 2209WA turns off after a while

Hi,

I am trying to understand what AC to DC or/and DC/DC conversions happen in the Dell 2209WA circuit boards, would anybody be able to help me, please?

Looking at the manual it seems like there are 3 main output voltages: 5VDC, 12VDC, and 24VDC. Does anybody know how these conversions are made and what is the maximum power for each conversion?

Thank you very much!