Dell 2209wa - momentarily blinks every two seconds [ Video + Audio + Service Manual ]

[ohren]

Hello,

This is the second time I'm asking for advice in this forum. The first was about troubleshooting a PSU, which resulted in a successful repair. Naturally I want another fix of the Ikea effect.

Today I'm trying to diagnose a Dell 2209wa monitor manufactured 2008, and used 12000 h (according to factory menu). It worked fine all these years up until the other day. You'll find schematics in the attached pdf.

Please excuse possible erroneous terminology below -- I'm not that familiar with the domain.

Problem

Visible:
Screen is working normally if it's been unused for a while. After a few minutes the picture disappears (backlight goes out) for about one 60th - 120th of a second (estimated from video recording, attached in this post). This repeats about every 2 seconds.

It happens regardless of video source, whether there's any video input, or whether video cables are connected.

Audible:
Very faint buzzing coinciding with the blinking, primarily heard when pressing ear up against the monitor. Seems to be the loudest fairly close to the power cable socket.

I could predict that the screen would fail from the louder than normal buzzing that came from the monitor pre-failure. That loud buzzing is something I haven't noticed since it started blinking.


What I've done

• Inspected the power supply board. No visible problems. No bulging caps felt.
• Disassembled it down to the CCFLs. Couldn't inspect their sockets without desoldering them from the inverter board though, so didn't do that.
• Looked through the service manual (attached) and got the impression that the backlight will turn off if there is a problem with one of the lights, but I don't know enough to intuit the timescales of the processes described in the manual, and so I can't determine if it's at all a likely cause.
• Unable to determine if LCD also turns off, due to the very short time that it goes black (1/60th of a second).
• The power LED does not blink, but on the other hand it keeps shining long after unplugging the power so I disregard it as a clue.

What I haven't done

• Inspected the inverter board. It's soldered down to the lamp sockets and covered with a sheet of frosted mylar film held down with double sided sticky tape. The surface mounted electrolytic cap was visible, but I currently don't know how to read the signs of when those go bad.
• Removed the CCFLs. They look fragile. I don't know if they're locked in place or can just be pulled out. I can't see the sockets without desoldering the inverter board from all 12 sockets. The tubes look fine, nothing out of the ordinary.
• Measured voltages. I'm not that experienced so I imagine it will take a lot of time and energy for me to measure safely. I'd prefer to have some kind of plan, and some idea where to start, instead of making arbitrary measurements.

I'm wondering if someone has any intuition as to the cause and where I should concentrate my efforts? I imagine that the frequency of the blinking and the short timescale of the black screen would give some clues to someone with experience.

Can I exclude more parts by doing some additional tests?

Interesting pdf pages (mostly diagrams):

5. Power overview
6. Inverter overview
8. Inverter PWM circuit
16. DC_24V ,DC 12V and DC 5V Output Circuit and Feedback circuit
+ PCB layout pictures at the end

[ohren]

New info

• Lowering the backlight brightness results in backlight blinking with higher frequency

I noticed "You may edit your posts" at the bottom of the page but couldn't find where to do it.

[Per Hansson]

Originally Posted by ohren

I noticed "You may edit your posts" at the bottom of the page but couldn't find where to do it.

You can only edit your post for a short time after it has been posted.
This is mainly to prevent clever spam bots going in and editing in spam after a few weeks when no one will notice...

As for the fault one idea is to set a manual exposure on your camera if you can, and have it well exposed when the monitor is off.
Then power it on with a picture but also have a 500W light behind.
The idea is that if it is just the backlight that goes out you may be able to see the picture.
You can of course do this visually without a camera too...

[ohren]

Tack för the suggestions.

I first decided to treat it as a backlight issue, but after inspecting the tubes I could find nothing really objectionable. After measuring the 24 V going from the power supply to the inverter I concluded the voltage seems to drop in sync with the blinking, with no LCD connected. I'm therefore assuming that the LCD isn't relevant for isolating the problem further.

The challenge for me now is to understand whether the drop in voltage (only measured with a DMM since that's the best I can do) is the cause or the effect of the tubes shutting off. I can't intuit the probability that it would be an effect, so I can't rule it out.

Is there a way to figure out whether it's the cause (I don't have advanced equipment), or is it natural to assume it at this point and just buy caps?

[ohren]

Well, I'm considering replacing C862-C865 (can be found by searching for "C862" in the manual) which seem responsible for 24 V, unless someone has a suggestion for a better course of action. They don't look bad but that's the only idea I have.

They don't say low ESR on them. Should I aim for low ESR in the replacements anyway? I seem to recall a similar circuit when repairing a PSU which used low ESR there. Maybe I'm mistaken.

[R_J]

You say the 24v drops but to what? 23v, 22v, 0v? I would check the secondary resistance of each inverter transformer and compare the resistance, If one checks different than the others it is likely bad, also check the transistors that drive each transformer.

[ohren]

I didn't mention the voltage since I don't know if my DMM tells me the minimum voltage reached. It says ~23 V in sync with the blinking. Otherwise it's stable at basically 23.9-24.0.

Thanks for the suggestions, I'll check it out.

Sorry about the short reply. My first reply was thrown away by the forum.

[ohren]

Measured the transformers, but they all have the same values.

I had a bit of trouble figuring out how to measure the mosfets. Looked up how to do it, looked up the part, but must have missed something. I'm working under the assumption that you're referring to Q1-Q4 on page 8. I'll try again tomorrow.

Is it likely that they'd fail in a way that would cause this sort of problem? I really know very little about it, but imagined that if they'd fail it would result in some more constant state of failure.

[ohren]

Not sure if I did it right but I couldn't find an indication of any problem with any of the transistors.

[ohren]

I measured the four 470 uF capacitors on the 24 v output out of circuit with a cheap component tester:

[uF] - [ohm ESR]
430 - 0.06
437 - 0.10
439 - 0.10
437 - 0.09

I don't know... doesn't seem too bad to me. Capacitance is within 10% of spec. Is it still likely they're the problem?

Not sure what's my next step now. I'd try identifying a failing tube except that the platic holding them in place is incredibly brittle and will invariably break if I try to unplug one, and they all look in equally good condition, so I thought it'd be best to exhaust all other options first.