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
