Dell monitor

[dtvmcdonald]

One of my Dell monitors dies. Its essentially identical to the U2312HM I write this on. The symptom is that it works for about 2 seconds after turning on and then goes black.

This of course implies, first guess, a sick power supply going into protect mode.

But how do I get it open? The monitor easily comes apart from the stand, exposing four screws that attach the BPC to a large aluminum structure.
Removing those and the front and back of the BPC are still stuck together.
It must be some sort of "knifing" ... but where and how?

Of course there is no service info.

Doug McDonald

[Ed in Tx]

Looking at my Dell 2214 monitor here, looks like there must be a trick to getting into these things. I hope someone here knows how 'cause they will all have failing electrolytics someday.

Did you look on YouTube? Watched a guy use a knife to get a similar model and design started coming apart at the bottom then he went around the perimeter with fingers pulling it apart the rest of the way. I hate stuff built like that...

[dtvmcdonald]

Thanks! I didn't think of trying Youtube. Of course, its not in English.

[dtvmcdonald]

I tried the Youtube guy's method and that's how it comes apart. You still have to figure out how to disconnect one cable.

That done, I was able to find out that the main low voltage seems to be OK.
The backlight seems to be fed a >100 volt AC power at some high voltage
for the couple of seconds when the display is visible and correct, then shuts down.

There is only one HV cap in the power supply, 160 uF at 450 volts. It seems
to have about 200 volts DC across it, with maybe 15 volts AC at 60 Hz.
There is also about 200 volts AC, roughly square wave at 60 (or 120, I forgot to check) Hz from it to ground.

So it looks like there is something wrong with the backlight, be it power
or a problem in the backlight itself that is causing a protection shutdown.

Any comments? Like ... is this effectively fatal? This display has given many years of faithful service.

[Ed in Tx]

Hey you got it apart anyway!
Is that a CCFL or LED lit monitor? I didn't know LED monitors had voltages like that other than obviously the primary side of the power supply. Search for that symptom is all I can suggest.

[dtvmcdonald]

Its CCFL ... I got one pair of tubes out. They don't look too bad. It may be
the power board. I found a schematic for my other monitor, but its LED. I'll keep looking.

[MIPS]

With CCFL backlight replacement being part of my day gig, before you tear the LCD apart next time get a good high contrast wallpaper (I used the indian head test pattern) and then blast a light into the screen with a flashlight or desk lamp. You should get enough bounceback to tell if it's the backlights/inverter circuit or if the backlights AND the LCD are not functioning.

Backlights usually turning on then off are because most inverters now detect shorts or excessively heavy loading from tired tubes and will shut themselves off to protect themselves. I keep a variety of loose CCFL lamps handy so I can sub in and pinpoint which bulb is being the problem without dismantling the screen.

[CoogarXR]

Originally Posted by MIPS

With CCFL backlight replacement being part of my day gig, before you tear the LCD apart next time get a good high contrast wallpaper (I used the indian head test pattern) and then blast a light into the screen with a flashlight or desk lamp. You should get enough bounceback to tell if it's the backlights/inverter circuit or if the backlights AND the LCD are not functioning.

Backlights usually turning on then off are because most inverters now detect shorts or excessively heavy loading from tired tubes and will shut themselves off to protect themselves. I keep a variety of loose CCFL lamps handy so I can sub in and pinpoint which bulb is being the problem without dismantling the screen.

Back when I worked on flat screens a lot, I built a portable CCFL tube tester using an inverter board from a portable DVD player. It runs off a 9v battery. It has enough nuts to power just about any single CCFL tube. It won't completely light a super-long one (like a tube from a 50" TV), but it will light the ends, lol, which is enough to see if the tube is dead.