I'm not sure if this has been covered here, but I figured this post might help someone....
As a sideline to my CRT projector repairs, I have gotten into refurbing LCD computer monitors, from 15" to 22". I have a deal going with a couple of local computer stores that I get all of their bad trade in monitors... So I've been through about 200 now, and the vast majority of problems, dead monitors, intermittent backlight etc, are indeed due to bad capacitors. That topic is covered here to death, so I won't touch it.
About 50 monitors that I've been stockpiling here though have the 'the monitor works for about 3-5 seconds, then the backlight shuts off' problem. I scope and replace capacitors in the power supply, the whole power supply is stable, yet the backlight still doesn't work.
So,, on Friday I finally decided to attack these monitors, since I now have duplicates of many models, so I put a defective one beside a working one to compare the two.
As someone had written on another post here, the backlight driver design is pretty slick:
-If it detects a short/overload, it shuts down.
-if it detects an open circuit, it shuts down.
With a typical monitor, there are two backlights, one at the top and one at the bottom of the screen. These mini fluorescent tubes shine into a piece of plexiglass that's etched to refract the light towards the LCD screen.
Playing around with the working monitor, I found that disconnecting one of the backlights, would shut down, as one driver has an open circuit.
I then carefully pulled apart the working monitor and removed one of the working tubes and wiring assembly. This is finicky work, as you have to be really careful not to stress the very thin PC board ribbon cables going between the LCD panel and the LCD driver PC board.
Once I opened the LCD panel that was shutting down, I found the problem: One of the backlight tubes was broken, and once apart, I could smell the ozone created by the arcing HV, as the break was close to one end of the tube, and it appeared that the HV was arcing to the chassis/ground.
It appears that the length of the fluorescent tube is not important. It was a 19" widescreen panel that I pulled apart to get my 'test' light tube. I have since used it as a test light for other 15" to 22" panels, and very quickly I can determine whether the top or bottom tube is defective by simply disconnecting each internal light and connecting my known working tube to it.
In one case I've found Hv arcing from the wiring to the chassis/ground, but in most cases so far the light tube itself is either defective or broken.
Only 45 more monitors to go through....
As a sideline to my CRT projector repairs, I have gotten into refurbing LCD computer monitors, from 15" to 22". I have a deal going with a couple of local computer stores that I get all of their bad trade in monitors... So I've been through about 200 now, and the vast majority of problems, dead monitors, intermittent backlight etc, are indeed due to bad capacitors. That topic is covered here to death, so I won't touch it.
About 50 monitors that I've been stockpiling here though have the 'the monitor works for about 3-5 seconds, then the backlight shuts off' problem. I scope and replace capacitors in the power supply, the whole power supply is stable, yet the backlight still doesn't work.
So,, on Friday I finally decided to attack these monitors, since I now have duplicates of many models, so I put a defective one beside a working one to compare the two.
As someone had written on another post here, the backlight driver design is pretty slick:
-If it detects a short/overload, it shuts down.
-if it detects an open circuit, it shuts down.
With a typical monitor, there are two backlights, one at the top and one at the bottom of the screen. These mini fluorescent tubes shine into a piece of plexiglass that's etched to refract the light towards the LCD screen.
Playing around with the working monitor, I found that disconnecting one of the backlights, would shut down, as one driver has an open circuit.
I then carefully pulled apart the working monitor and removed one of the working tubes and wiring assembly. This is finicky work, as you have to be really careful not to stress the very thin PC board ribbon cables going between the LCD panel and the LCD driver PC board.
Once I opened the LCD panel that was shutting down, I found the problem: One of the backlight tubes was broken, and once apart, I could smell the ozone created by the arcing HV, as the break was close to one end of the tube, and it appeared that the HV was arcing to the chassis/ground.
It appears that the length of the fluorescent tube is not important. It was a 19" widescreen panel that I pulled apart to get my 'test' light tube. I have since used it as a test light for other 15" to 22" panels, and very quickly I can determine whether the top or bottom tube is defective by simply disconnecting each internal light and connecting my known working tube to it.
In one case I've found Hv arcing from the wiring to the chassis/ground, but in most cases so far the light tube itself is either defective or broken.
Only 45 more monitors to go through....
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